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| Version | User | Scope of changes |
|---|---|---|
| Feb 9 2007, 5:52 AM EST (current) | vishal_aish2000 | 26146 words added |
| Feb 9 2007, 5:46 AM EST | vishal_aish2000 |
Changes
Key: Additions Deletions
1)What is OOPs?
Ans: Object oriented programming organizes a program around its data,i.e.,objects and a set of well defined interfaces to that data.An object-oriented program can be characterized as data controlling access to code.
2)what is the difference between Procedural and OOPs?
Ans: a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOPs program, unit of program is object, which is nothing but combination of data and code. b) In procedural program,data is exposed to the whole program whereas in OOPs program,it is accessible with in the object and which in turn assures the security of the code.
3)What are Encapsulation, Inheritance and Polymorphism?
Ans: Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse. Inheritance is the process by which one object acquires the properties of another object. Polymorphism is the feature that allows one interface to be used for general class actions.
4)What is the difference between Assignment and Initialization?
Ans: Assignment can be done as many times as desired whereas initialization can be done only once.
5)What are Class, Constructor and Primitive data types?
Ans: Class is a template for multiple objects with similar features and it is a blue print for objects. It defines a type of object according to the data the object can hold and the operations the object can perform. Constructor is a special kind of method that determines how an object is initialized when created. Primitive data types are 8 types and they are: byte, short, int, long float, double boolean char 6)What is an Object and how do you allocate memory to it?
Ans: Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data. When an object is created using new operator, memory is allocated to it.
7)What is the difference between constructor and method?
Ans: Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.
8)What are methods and how are they defined? Ans: Methods are functions that operate on instances of classes in which they are defined. Objects can communicate with each other using methods and can call methods in other classes. Method definition has four parts. They are name of the method, type of object or primitive type the method returns, a list of parameters and the body of the method. A method's signature is a combination of the first three parts mentioned above.
9)What is the use of bin and lib in JDK?
Ans: Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.
10)What is casting?
Ans: Casting is used to convert the value of one type to another.
11)How many ways can an argument be passed to a subroutine and explain them?
Ans: An argument can be passed in two ways. They are passing by value and passing by reference. Passing by value: This method copies the value of an argument into the formal parameter of the subroutine. Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to the parameter.
12)What is the difference between an argument and a parameter?
Ans: While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.
13)What are different types of access modifiers?
Ans: public: Any thing declared as public can be accessed from anywhere. private: Any thing declared as private can't be seen outside of its class. protected: Any thing declared as protected can be accessed by classes in the same package and subclasses in the other packages. default modifier : Can be accessed only to classes in the same package.
14)What is final, finalize() and finally?
Ans: final : final keyword can be used for class, method and variables. A final class cannot be subclassed and it prevents other programmers from subclassing a secure class to invoke insecure methods. A final method can' t be overridden A final variable can't change from its initialized value.
finalize( ) : finalize( ) method is used just before an object is destroyed and can be called just prior to garbage collection.
finally : finally, a key word used in exception handling, creates a block of code that will be executed after a try/catch block has completed and before the code following the try/catch block. The finally block will execute whether or not an exception is thrown. For example, if a method opens a file upon exit, then you will not want the code that closes the file to be bypassed by the exception-handling mechanism. This finally keyword is designed to address this contingency.
15)What is UNICODE?
Ans: Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.
16)What is Garbage Collection and how to call it explicitly?
Ans: When an object is no longer referred to by any variable, java automatically reclaims memory used by that object. This is known as garbage collection. System.gc() method may be used to call it explicitly.
17)What is finalize() method ?
Ans: finalize () method is used just before an object is destroyed and can be called just prior to garbage collection.
18)What are Transient and Volatile Modifiers?
Ans: Transient: The transient modifier applies to variables only and it is not stored as part of its object's Persistent state. Transient variables are not serialized. Volatile: Volatile modifier applies to variables only and it tells the compiler that the variable modified by volatile can be changed unexpectedly by other parts of the program.
19)What is method overloading and method overriding?
Ans: Method overloading: When a method in a class having the same method name with different arguments is said to be method overloading. Method overriding : When a method in a class having the same method name with same arguments is said to be method overriding.
20)What is difference between overloading and overriding?
Ans: a) In overloading, there is a relationship between methods available in the same class whereas in overriding, there is relationship between a superclass method and subclass method.
b) Overloading does not block inheritance from the superclass whereas overriding blocks inheritance from the superclass.
c) In overloading, separate methods share the same name whereas in overriding,subclass method replaces the superclass.
d) Overloading must have different method signatures whereas overriding must have same signature.
21) What is meant by Inheritance and what are its advantages?
Ans: Inheritance is the process of inheriting all the features from a class. The advantages of inheritance are reusability of code and accessibility of variables and methods of the super class by subclasses.
22)What is the difference between this() and super()?
Ans: this() can be used to invoke a constructor of the same class whereas super() can be used to invoke a super class constructor.
23)What is the difference between superclass and subclass?
Ans: A super class is a class that is inherited whereas sub class is a class that does the inheriting.
24) What modifiers may be used with top-level class?
Ans: public, abstract and final can be used for top-level class.
25)What are inner class and anonymous class?
Ans: Inner class : classes defined in other classes, including those defined in methods are called inner classes. An inner class can have any accessibility including private.
Anonymous class : Anonymous class is a class defined inside a method without a name and is instantiated and declared in the same place and cannot have explicit constructors.
26)What is a package?
Ans: A package is a collection of classes and interfaces that provides a high-level layer of access protection and name space management.
27) What is a reflection package?
Ans: java.lang.reflect package has the ability to analyze itself in runtime.
28) What is interface and its use?
Ans: Interface is similar to a class which may contain method's signature only but not bodies and it is a formal set of method and constant declarations that must be defined by the class that implements it. Interfaces are useful for: a)Declaring methods that one or more classes are expected to implement b)Capturing similarities between unrelated classes without forcing a class relationship. c)Determining an object's programming interface without revealing the actual body of the class.
29) What is an abstract class? Ans: An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately incomplete.
30) What is the difference between Integer and int?
Ans: a) Integer is a class defined in the java.lang package, whereas int is a primitive data type defined in the Java language itself. Java does not automatically convert from one to the other. b) Integer can be used as an argument for a method that requires an object, whereas int can be used for calculations.
31) What is a cloneable interface and how many methods does it contain? Ans- It is not having any method because it is a TAGGED or MARKER interface.
32) What is the difference between abstract class and interface? Ans: a) All the methods declared inside an interface are abstract whereas abstract class must have at least one abstract method and others may be concrete or abstract. b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that keyword for the methods. c) Abstract class must have subclasses whereas interface can't have subclasses.
33) Can you have an inner class inside a method and what variables can you access?
Ans: Yes, we can have an inner class inside a method and final variables can be accessed.
34) What is the difference between String and String Buffer?
Ans: a) String objects are constants and immutable whereas StringBuffer objects are not. b) String class supports constant strings whereas StringBuffer class supports growable and modifiable strings.
35) What is the difference between Array and vector?
Ans: Array is a set of related data type and static whereas vector is a growable array of objects and dynamic.
36) What is the difference between exception and error?
Ans: The exception class defines mild error conditions that your program encounters. Ex: Arithmetic Exception, FilenotFound exception Exceptions can occur when -- try to open the file, which does not exist -- the network connection is disrupted -- operands being manipulated are out of prescribed ranges -- the class file you are interested in loading is missing The error class defines serious error conditions that you should not attempt to recover from. In most cases it is advisable to let the program terminate when such an error is encountered. Ex: Running out of memory error, Stack overflow error.
37) What is the difference between process and thread?
Ans: Process is a program in execution whereas thread is a separate path of execution in a program.
38) What is multithreading and what are the methods for inter-thread communication and what is the class in which these methods are defined?
Ans: Multithreading is the mechanism in which more than one thread run independent of each other within the process. wait (), notify () and notifyAll() methods can be used for inter-thread communication and these methods are in Object class. wait( ) : When a thread executes a call to wait( ) method, it surrenders the object lock and enters into a waiting state. notify( ) or notifyAll( ) : To remove a thread from the waiting state, some other thread must make a call to notify( ) or notifyAll( ) method on the same object.
39) What is the class and interface in java to create thread and which is the most advantageous method?
Ans: Thread class and Runnable interface can be used to create threads and using Runnable interface is the most advantageous method to create threads because we need not extend thread class here.
40) What are the states associated in the thread?
Ans: Thread contains ready, running, waiting and dead states.
41) What is synchronization?
Ans: Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time.
42) When you will synchronize a piece of your code?
Ans: When you expect your code will be accessed by different threads and these threads may change a particular data causing data corruption.
43) What is deadlock?
Ans: When two threads are waiting each other and can't precede the program is said to be deadlock.
44) What is daemon thread and which method is used to create the daemon thread?
Ans: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.
45) Are there any global variables in Java, which can be accessed by other part of your program?
Ans: No, it is not the main method in which you define variables. Global variables is not possible because concept of encapsulation is eliminated here.
46)What is an applet?
Ans: Applet is a dynamic and interactive program that runs inside a web page displayed by a java capable browser.
47)What is the difference between applications and applets?
Ans: a)Application must be run on local machine whereas applet needs no explicit installation on local machine. b)Application must be run explicitly within a java-compatible virtual machine whereas applet loads and runs itself automatically in a java-enabled browser. d)Application starts execution with its main method whereas applet starts execution with its init method. e)Application can run with or without graphical user interface whereas applet must run within a graphical user interface.
48)How does applet recognize the height and width?
Ans:Using getParameters() method.
49)When do you use codebase in applet?
Ans:When the applet class file is not in the same directory, codebase is used.
50)What is the lifecycle of an applet?
Ans:init( ) method - Can be called when an applet is first loaded start( ) method - Can be called each time an applet is started paint( ) method - Can be called when the applet is minimized or maximized stop( ) method - Can be used when the browser moves off the applet's page destroy( ) method - Can be called when the browser is finished with the applet
51)How do you set security in applets?
Ans: using setSecurityManager() method
52) What is an event and what are the models available for event handling?
Ans: An event is an event object that describes a state of change in a source. In other words, event occurs when an action is generated, like pressing button, clicking mouse, selecting a list, etc. There are two types of models for handling events and they are: a) event-inheritance model and b) event-delegation model
53) What are the advantages of the model over the event-inheritance model?
Ans: The event-delegation model has two advantages over the event-inheritance model. They are: a)It enables event handling by objects other than the ones that generate the events. This allows a clean separation between a component's design and its use. b)It performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to be repeatedly process unhandled events as is the case of the event-inheritance.
54)What is source and listener ?
Ans: source : A source is an object that generates an event. This occurs when the internal state of that object changes in some way. listener : A listener is an object that is notified when an event occurs. It has two major requirements. First, it must have been registered with one or more sources to receive notifications about specific types of events. Second, it must implement methods to receive and process these notifications.
55) What is adapter class?
Ans: An adapter class provides an empty implementation of all methods in an event listener interface. Adapter classes are useful when you want to receive and process only some of the events that are handled by a particular event listener interface. You can define a new class to act listener by extending one of the adapter classes and implementing only those events in which you are interested. For example, the MouseMotionAdapter class has two methods, mouseDragged( )and mouseMoved(). The signatures of these empty are exactly as defined in the MouseMotionListener interface. If you are interested in only mouse drag events, then you could simply extend MouseMotionAdapter and implement mouseDragged( ) .
56)What is meant by controls and what are different types of controls in AWT?
Ans: Controls are components that allow a user to interact with your application and the AWT supports the following types of controls: Labels, Push Buttons, Check Boxes, Choice Lists, Lists, Scrollbars, Text Components. These controls are subclasses of Component.
57) What is the difference between choice and list?
Ans: A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices and only one item may be selected from a choice. A List may be displayed in such a way that several list items are visible and it supports the selection of one or more list items.
58) What is the difference between scrollbar and scrollpane?
Ans: A Scrollbar is a Component, but not a Container whereas Scrollpane is a Conatiner and handles its own events and perform its own scrolling.
59) What is a layout manager and what are different types of layout managers available in java.awt?
Ans: A layout manager is an object that is used to organize components in a container. The different layouts are available are FlowLayout, BorderLayout, CardLayout, GridLayout and GridBagLayout.
60) How are the elements of different layouts organized?
Ans: FlowLayout: The elements of a FlowLayout are organized in a top to bottom, left to right fashion. BorderLayout: The elements of a BorderLayout are organized at the borders (North, South, East and West) and the center of a container. CardLayout: The elements of a CardLayout are stacked, on top of the other, like a deck of cards. GridLayout: The elements of a GridLayout are of equal size and are laid out using the square of a grid. GridBagLayout: The elements of a GridBagLayout are organized according to a grid. However, the elements are of different size and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.
61) Which containers use a Border layout as their default layout?
Ans: Window, Frame and Dialog classes use a BorderLayout as their layout.
62) Which containers use a Flow layout as their default layout?
Ans: Panel and Applet classes use the FlowLayout as their default layout.
63) What are wrapper classes?
Ans: Wrapper classes are classes that allow primitive types to be accessed as objects.
64) What are Vector, Hashtable, LinkedList and Enumeration?
Ans: Vector : The Vector class provides the capability to implement a growable array of objects. Hashtable : The Hashtable class implements a Hashtable data structure. A Hashtable indexes and stores objects in a dictionary using hash codes as the object's keys. Hash codes are integer values that identify objects. LinkedList: Removing or inserting elements in the middle of an array can be done using LinkedList. A LinkedList stores each object in a separate link whereas an array stores object references in consecutive locations. Enumeration: An object that implements the Enumeration interface generates a series of elements, one at a time. It has two methods, namely hasMoreElements( ) and nextElement( ). HasMoreElemnts( ) tests if this enumeration has more elements and nextElement method returns successive elements of the series.
65) What is the difference between set and list?
Ans: Set stores elements in an unordered way but does not contain duplicate elements, whereas list stores elements in an ordered way but may contain duplicate elements.
66) What is a stream and what are the types of Streams and classes of the Streams?
Ans: A Stream is an abstraction that either produces or consumes information. There are two types of Streams and they are: Byte Streams: Provide a convenient means for handling input and output of bytes. Character Streams: Provide a convenient means for handling input & output of characters. Byte Streams classes: Are defined by using two abstract classes, namely InputStream and OutputStream. Character Streams classes: Are defined by using two abstract classes, namely Reader and Writer.
67) What is the difference between Reader/Writer and InputStream/Output Stream?
Ans: The Reader/Writer class is character-oriented and the InputStream/OutputStream class is byte-oriented.
68) What is an I/O filter?
Ans: An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
69) What is serialization and deserialization?
Ans: Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.
70) What is JDBC?
Ans: JDBC is a set of Java API for executing SQL statements. This API consists of a set of classes and interfaces to enable programs to write pure Java Database applications.
71) What are drivers available?
Ans: a) JDBC-ODBC Bridge driver b) Native API Partly-Java driver c) JDBC-Net Pure Java driver d) Native-Protocol Pure Java driver
72) What is the difference between JDBC and ODBC?
Ans: a) OBDC is for Microsoft and JDBC is for Java applications. b) ODBC can't be directly used with Java because it uses a C interface. c) ODBC makes use of pointers which have been removed totally from Java. d) ODBC mixes simple and advanced features together and has complex options for simple queries. But JDBC is designed to keep things simple while allowing advanced capabilities when required. e) ODBC requires manual installation of the ODBC driver manager and driver on all client machines. JDBC drivers are written in Java and JDBC code is automatically installable, secure, and portable on all platforms. f) JDBC API is a natural Java interface and is built on ODBC. JDBC retains some of the basic features of ODBC.
73) What are the types of JDBC Driver Models and explain them?
Ans: There are two types of JDBC Driver Models and they are: a) Two tier model and b) Three tier model Two tier model: In this model, Java applications interact directly with the database. A JDBC driver is required to communicate with the particular database management system that is being accessed. SQL statements are sent to the database and the results are given to user. This model is referred to as client/server configuration where user is the client and the machine that has the database is called as the server. Three tier model: A middle tier is introduced in this model. The functions of this model are: a) Collection of SQL statements from the client and handing it over to the database, b) Receiving results from database to the client and c) Maintaining control over accessing and updating of the above.
74) What are the steps involved for making a connection with a database or how do you connect to a database?
Ans:
a) Loading the driver : To load the driver, Class.forName( ) method is used. Class.forName("sun.jdbc.odbc.JdbcOdbcDriver"); When the driver is loaded, it registers itself with the java.sql.DriverManager class as an available database driver.
b) Making a connection with database : To open a connection to a given database, DriverManager.getConnection( ) method is used. Connection con = DriverManager.getConnection ("jdbc:odbc:somedb", "user", "password");
c) Executing SQL statements : To execute a SQL query, java.sql.statements class is used. createStatement( ) method of Connection to obtain a new Statement object. Statement stmt = con.createStatement( ); A query that returns data can be executed using the executeQuery( ) method of Statement. This method executes the statement and returns a java.sql.ResultSet that encapsulates the retrieved data: ResultSet rs = stmt.executeQuery("SELECT * FROM some table");
d) Process the results : ResultSet returns one row at a time. Next( ) method of ResultSet object can be called to move to the next row. The getString( ) and getObject( ) methods are used for retrieving column values: while(rs.next( ) ) { String event = rs.getString("event"); Object count = (Integer) rs.getObject("count");
75) What type of driver did you use in project?
Ans: JDBC-ODBC Bridge driver (is a driver that uses native(C language) libraries and makes calls to an existing ODBC driver to access a database engine).
76) What are the types of statements in JDBC?
Ans: Statement -- To be used createStatement() method for executing single SQL statement PreparedStatement -- To be used preparedStatement() method for executing same SQL statement over and over CallableStatement -- To be used prepareCall( ) method for multiple SQL statements over and over
77) What is stored procedure?
Ans: Stored procedure is a group of SQL statements that forms a logical unit and performs a particular task. Stored Procedures are used to encapsulate a set of operations or queries to execute on database. Stored procedures can be compiled and executed with different parameters and results and may have any combination of input/output parameters.
78) How to create and call stored procedures?
Ans: To create stored procedures: Create procedure procedurename (specify in, out and in out parameters) BEGIN Any multiple SQL statement; END; To call stored procedures: CallableStatement csmt = con.prepareCall("{call procedure name(?,?)}"); csmt.registerOutParameter(column no., data type); csmt.setInt(column no., column name) csmt.execute( );
79) What is servlet?
Ans: Servlets are modules that extend request/response-oriented servers, such as java-enabled web servers. For example, a servlet might be responsible for taking data in an HTML order-entry form and applying the business logic used to update a company's order database.
80) What are the classes and interfaces for servlets?
Ans: There are two packages in servlets and they are javax.servlet and javax.servlet.http. Javax.servlet contains: Interfaces Classes Servlet Generic Servlet ServletRequest ServletInputStream ServletResponse ServletOutputStream ServletConfig ServletException ServletContext UnavailableException SingleThreadModel Javax.servlet.http contains: Interfaces Classes HttpServletRequest Cookie HttpServletResponse HttpServlet HttpSession HttpSessionBindingEvent HttpSessionCintext HttpUtils HttpSeesionBindingListener
81) What is the difference between an applet and a servlet?
Ans: a) Servlets are to servers what applets are to browsers. b) Applets must have graphical user interfaces whereas servlets have no graphical user interfaces.
82) What is the difference between doPost and doGet methods?
Ans: a) doGet() method is used to get information, while doPost( ) method is used for posting information. b) doGet() requests can't send large amount of information and is limited to 240-255 characters. However, doPost( )requests passes all of its data, of unlimited length. c) A doGet( ) request is appended to the request URL in a query string and this allows the exchange is visible to the client, whereas a doPost() request passes directly over the socket connection as part of its HTTP request body and the exchange are invisible to the client.
83) What is the life cycle of a servlet?
Ans: Each Servlet has the same life cycle: a) A server loads and initializes the servlet by init () method. b) The servlet handles zero or more client's requests through service( ) method. c) The server removes the servlet through destroy() method.
84) Who is loading the init() method of servlet?
Ans: Web server
85) What are the different servers available for developing and deploying Servlets?
Ans: a) Java Web Server b) JRun g) Apache Server h) Netscape Information Server i) Web Logic
86) How many ways can we track client and what are they?
Ans: The servlet API provides two ways to track client state and they are: a) Using Session tracking and b) Using Cookies.
87) What is session tracking and how do you track a user session in servlets?
Ans: Session tracking is a mechanism that servlets use to maintain state about a series requests from the same user across some period of time. The methods used for session tracking are: a) User Authentication - occurs when a web server restricts access to some of its resources to only those clients that log in using a recognized username and password b) Hidden form fields - fields are added to an HTML form that are not displayed in the client's browser. When the form containing the fields is submitted, the fields are sent back to the server c) URL rewriting - every URL that the user clicks on is dynamically modified or rewritten to include extra information. The extra information can be in the form of extra path information, added parameters or some custom, server-specific URL change. d) Cookies - a bit of information that is sent by a web server to a browser and which can later be read back from that browser. e) HttpSession- places a limit on the number of sessions that can exist in memory. This limit is set in the session.maxresidents property
88) What is Server-Side Includes (SSI)?
Ans: Server-Side Includes allows embedding servlets within HTML pages using a special servlet tag. In many servlets that support servlets, a page can be processed by the server to include output from servlets at certain points inside the HTML page. This is accomplished using a special internal SSINCLUDE, which processes the servlet tags. SSINCLUDE servlet will be invoked whenever a file with an. shtml extension is requested. So HTML files that include server-side includes must be stored with an .shtml extension.
89) What are cookies and how will you use them?
Ans: Cookies are a mechanism that a servlet uses to have a client hold a small amount of state-information associated with the user. a) Create a cookie with the Cookie constructor: public Cookie(String name, String value) b) A servlet can send a cookie to the client by passing a Cookie object to the addCookie() method of HttpServletResponse: public void HttpServletResponse.addCookie(Cookie cookie) c) A servlet retrieves cookies by calling the getCookies() method of HttpServletRequest: public Cookie[ ] HttpServletRequest.getCookie( ).
90) Is it possible to communicate from an applet to servlet and how many ways and how?
Ans: Yes, there are three ways to communicate from an applet to servlet and they are:
a) HTTP Communication(Text-based and object-based)
b) Socket Communication
c) RMI Communication (You can say, by using URL object open the connection to server and get the InputStream from URLConnection object). Steps involved for applet-servlet communication:
1) Get the server URL. URL url = new URL();
2) Connect to the host URLConnection Con = url.openConnection();
3) Initialize the connection Con.setUseCatches(false): Con.setDoOutput(true); Con.setDoInput(true);
4) Data will be written to a byte array buffer so that we can tell the server the length of the data. ByteArrayOutputStream byteout = new ByteArrayOutputStream();
5) Create the OutputStream to be used to write the data to the buffer. DataOutputStream out = new DataOutputStream(byteout);
91) What is connection pooling?
Ans: With servlets, opening a database connection is a major bottleneck because we are creating and tearing down a new connection for every page request and the time taken to create connection will be more. Creating a connection pool is an ideal approach for a complicated servlet. With a connection pool, we can duplicate only the resources we need to duplicate rather than the entire servlet. A connection pool can also intelligently manage the size of the pool and make sure each connection remains valid. A number of connection pool packages are currently available. Some like DbConnectionBroker are freely available from Java Exchange Works by creating an object that dispenses connections and connection Ids on request. The ConnectionPool class maintains a Hastable, using Connection objects as keys and Boolean values as stored values. The Boolean value indicates whether a connection is in use or not. A program calls getConnection( ) method of the ConnectionPool for getting Connection object it can use; it calls returnConnection( ) to give the connection back to the pool.
92) Why should we go for interservlet communication?
Ans: Servlets running together in the same server communicate with each other in several ways. The three major reasons to use interservlet communication are: a) Direct servlet manipulation - allows to gain access to the other currently loaded servlets and perform certain tasks (through the ServletContext object) b) Servlet reuse - allows the servlet to reuse the public methods of another servlet. c) Servlet collaboration - requires to communicate with each other by sharing specific information (through method invocation)
93) Is it possible to call servlet with parameters in the URL?
Ans: Yes. You can call a servlet with parameters in the syntax as (?Param1 = xxx || m2 = yyy). 94) What is Servlet chaining? Ans: Servlet chaining is a technique in which two or more servlets can cooperate in servicing a single request. In servlet chaining, one servlet's output is piped to the next servlet's input. This process continues until the last servlet is reached. Its output is then sent back to the client.
95) How do servlets handle multiple simultaneous requests?
Ans: The server has multiple threads that are available to handle requests. When a request comes in, it is assigned to a thread, which calls a service method (for example: doGet(), doPost( ) and service( ) ) of the servlet. For this reason, a single servlet object can have its service methods called by many threads at once.
96) What is the difference between TCP/IP and UDP?
Ans: TCP/IP is a two-way communication between the client and the server and it is a reliable and there is a confirmation regarding reaching the message to the destination. It is like a phone call. UDP is a one-way communication only between the client and the server and it is not a reliable and there is no confirmation regarding reaching the message to the destination. It is like a postal mail.
97) What is Inet address?
Ans: Every computer connected to a network has an IP address. An IP address is a number that uniquely identifies each computer on the Net. An IP address is a 32-bit number.
98) What is Domain Naming Service(DNS)?
Ans: It is very difficult to remember a set of numbers(IP address) to connect to the Internet. The Domain Naming Service(DNS) is used to overcome this problem. It maps one particular IP address to a string of characters. For example, www.mascom.com implies com is the domain name reserved for US commercial sites, moscom is the name of the company and www is the name of the specific computer, which is mascom's server.
99) What is URL?
Ans: URL stands for Uniform Resource Locator and it points to resource files on the Internet. URL has four components: http://www.Pentafour.com:80/index.html http - protocol name, Pentafour - IP address or host name, 80 - port number and index.html - file path.
100) What is RMI and steps involved in developing an RMI object?
Ans: Remote Method Invocation (RMI) allows java object that executes on one machine and to invoke the method of a Java object to execute on another machine. The steps involved in developing an RMI object are: a) Define the interfaces b) Implementing these interfaces c) Compile the interfaces and their implementations with the java compiler d) Compile the server implementation with RMI compiler e) Run the RMI registry f) Run the application
101) What is RMI architecture?
Ans: - RMI architecture consists of four layers and each layer performs specific functions: a) Application layer ---- contains the actual object definition b) Proxy layer ---- consists of stub and skeleton c) Remote Reference layer ---- gets the stream of bytes from the transport layer and sends it to the proxy layer d) Transportation layer ---- responsible for handling the actual machine-to-machine communication
102) what is UnicastRemoteObject?
Ans: All remote objects must extend UnicastRemoteObject, which provides functionality that is needed to make objects available from remote machines.
103) Explain the methods, rebind( ) and lookup() in Naming class?
Ans: rebind( ) of the Naming class(found in java.rmi) is used to update the RMI registry on the server machine. Naming. rebind("AddSever", AddServerImpl); lookup( ) of the Naming class accepts one argument, the rmi URL and returns a reference to an object of type AddServerImpl.
104) What is a Java Bean?
Ans: A Java Bean is a software component that has been designed to be reusable in a variety of different environments.
105) What is a Jar file?
Ans: Jar file allows to efficiently deploying a set of classes and their associated resources. The elements in a jar file are compressed, which makes downloading a Jar file much faster than separately downloading several uncompressed files. The package java.util.zip contains classes that read and write jar files.
106) What is BDK?
Ans: BDK, Bean Development Kit is a tool that enables to create, configure and connect a set of set of Beans and it can be used to test Beans without writing a code.
107) What is JSP?
Ans: JSP is a dynamic scripting capability for web pages that allows Java as well as a few special tags to be embedded into a web file (HTML/XML, etc). The suffix traditionally ends with .jsp to indicate to the web server that the file is a JSP files. JSP is a server side technology - you can't do any client side validation with it. The advantages are: a) The JSP assists in making the HTML more functional. Servlets on the other hand allow outputting of HTML but it is a tedious process. b) It is easy to make a change and then let the JSP capability of the web server you are using deal with compiling it into a servlet and running it.
108) What are JSP scripting elements?
Ans: JSP scripting elements lets to insert Java code into the servlet that will be generated from the current JSP page.
There are three forms:
a) Expressions of the form <%= expression %> that are evaluated and inserted into the output,
b) Scriptlets of the form <% code %> that are inserted into the servlet's service method, and
c) Declarations of the form <%! Code %> that are inserted into the body of the servlet class, outside of any existing methods.
109) What are JSP Directives?
Ans: A JSP directive affects the overall structure of the servlet class. It usually has the following form: <%@ directive attribute="value" %> However, you can also combine multiple attribute settings for a single directive, as follows: <%@ directive attribute1="value1" attribute 2="value2" ... attributeN ="valueN" %> There are two main types of directive: page, which lets to do things like import classes, customize the servlet superclass, and the like; and include, which lets to insert a file into the servlet class at the time the JSP file is translated into a servlet
110) What are Predefined variables or implicit objects?
Ans: To simplify code in JSP expressions and scriptlets, we can use eight automatically defined variables, sometimes called implicit objects. They are request, response, out, session, application, config, pageContext, and page.
111) What are JSP ACTIONS?
Ans: JSP actions use constructs in XML syntax to control the behavior of the servlet engine. You can dynamically insert a file, reuse JavaBeans components, forward the user to another page, or generate HTML for the Java plugin. Available actions include: ? jsp:include - Include a file at the time the page is requested. ?
jsp:useBean - Find or instantiate a JavaBean. ?
jsp:setProperty - Set the property of a JavaBean. ?
jsp:getProperty - Insert the property of a JavaBean into the output. ?
jsp:forward - Forward the requester to a newpage. ?
Jsp: plugin - Generate browser-specific code that makes an OBJECT or EMBED
112) How do you pass data (including JavaBeans) to a JSP from a servlet?
Ans: (1) Request Lifetime: Using this technique to pass beans, a request dispatcher (using either "include" or forward") can be called. This bean will disappear after processing this request has been completed. Servlet: request.setAttribute("theBean", myBean); RequestDispatcher rd = getServletContext().getRequestDispatcher("thepage.jsp"); rd.forward(request, response); JSP PAGE: <jsp: useBean id="theBean" scope="request" class="....." />
(2) Session Lifetime: Using this technique to pass beans that are relevant to a particular session (such as in individual user login) over a number of requests. This bean will disappear when the session is invalidated or it times out, or when you remove it. Servlet: HttpSession session = request.getSession(true); session.putValue("theBean", myBean); /* You can do a request dispatcher here, or just let the bean be visible on the next request */ JSP Page: <jsp:useBean id="theBean" scope="session" class="..." />
3) Application Lifetime: Using this technique to pass beans that are relevant to all servlets and JSP pages in a particular app, for all users. For example, I use this to make a JDBC connection pool object available to the various servlets and JSP pages in my apps. This bean will disappear when the servlet engine is shut down, or when you remove it. Servlet: GetServletContext(). setAttribute("theBean", myBean); JSP PAGE: <jsp:useBean id="theBean" scope="application" class="..." />
113) How can I set a cookie in JSP?
Ans: response.setHeader("Set-Cookie", "cookie string"); To give the response-object to a bean, write a method setResponse (HttpServletResponse response) - to the bean, and in jsp-file: <% bean.setResponse (response); %> 114) How can I delete a cookie with JSP? Ans: Say that I have a cookie called "foo," that I set a while ago & I want it to go away. I simply: <% Cookie killCookie = new Cookie("foo", null); KillCookie.setPath("/"); killCookie.setMaxAge(0); response.addCookie(killCookie); %>
115) How are Servlets and JSP Pages related?
Ans: JSP pages are focused around HTML (or XML) with Java codes and JSP tags inside them. When a web server that has JSP support is asked for a JSP page, it checks to see if it has already compiled the page into a servlet. Thus, JSP pages become servlets and are transformed into pure Java and then compiled, loaded into the server and executed CORE JAVA
JSP
SERVLET
EJB
JMS
DATABASE
Java API
The Java API is what makes Java attractive and useful. It consists of a set of packages that are distributed with the JDK as class libraries. These packages provide a common interface for developing Java programs on all Java platforms. The Java API furnishes all the capabilities needed to develop console programs, window programs, client and server networking software, applets, and other applications. It is the difference that takes Java from being a really good programming language to making it a very powerful and efficient programming environment.
The Java API consists of eight major development packages and a supporting debug package; Packages are collections of related objects. For example, there are separate packages for developing window programs, applets, and networking software.
Java packages contain classes, interfaces, and exceptions
Classes form the basis for object-oriented software development in Java. They contain variables and methods. Variables are data containers. Methods implement operations on the class and its variables.
Interfaces are collections of related methods. An example of an interface is the DataInput interfacee of the java.io package. It specifies a collection of methods that must be defined for a class to implement the DataInput interface. Exceptions are events that alter the normal course of program execution. Java applets
An applet is a mini-program that will run only under a Web browser. The applet is downloaded automatically as part of a Web page. When the applet is activated it executes a program. This is part of its beauty – it provides you with a way to automatically distribute the client software from the server at the time the user needs the client software, and no sooner. They get the latest version of the client software without fail and without difficult re-installation. Because of the way
Java is designed, the programmer needs to create only a single program, and that program automatically works with all computers that have browsers with built-in Java interpreters.
Applets are Java programs that execute within the context of a Web page. They interact with the user while his Web page is active and stop their execution when his Web page is no longer active. Applets are valuable because they are simple to use. A user only needs to open an applet's Web page to download and execute an applet.
Java applets, are run from inside a browser. A reference to an applet is embedded in a Web page using a special HTML tag. When a reader, using a Java-enabled browser, loads a Web page with an applet in it, the browser downloads that applet from a Web server and executes it on the local system Because Java applets run inside a Java browser, they have access to the structure the browser provides: an existing window, an event-handling and graphics context, and the surrounding user interface.
One advantage a Java applet has over a scripted program is that it’s in compiled form, so the source code isn’t available to the client.
Applet security is a major concern among Web users and applet developers. From a user's perspective, an exploitable applet security flaw could result in sensitive data being modified or disclosed, or their computer being rendered inoperable. From a developer's perspective, strong applet security is necessary to make Web users comfortable with using applets. However, too high a level of security limits their applets' capabilities. Struts
Struts is a open source framework which make building of the web applications easier based on the java Servlet and JavaServer pages technologies.
Struts framework was created by Craig R. McClanahan and donated to the Apache Software Foundation in 2000. The Project now has several committers, and many developers are contributing to overall to the framework.
Developing web application using struts frame work is fairly complex, but it eases things after it is setup. It encourages software development following the MVC design pattern. Many web applications are JSP-only or Servlets-only. With JSP and Servlets, Java code is embedded in the HTML code and the Java code calls println methods to generate the HTML code respectively. Both approaches have their advantages and drawbacks; Struts gathers their strengths to get the best of their association. Struts is based on Model-View-Controller (MVC) design paradigm, it is an implementation of JSP Model 2 Architecture. For more of Model-View-Controller (MVC) click here.
Consists of 8 Top-Level Packages and approx 250 Classes and Interfaces.
Struts is a set of cooperating classes, servlets, and JSP tags that make up a reusable MVC 2 design. This definition implies that Struts is a framework, rather than a library, but Struts also contains an extensive tag library and utility classes that work independently of the framework.
The overview of struts
Client browser
An HTTP request from the client browser creates an event. The Web container will respond with an HTTP response.
Controller
The controller is responsible for intercepting and translating user input into actions to
be performed by the model. The controller is responsible for selecting the next view based on user input and the outcome of model operations.The Controller receives the request from the browser, and makes the decision where to send the request. With Struts, the Controller is a command design pattern implemented as a servlet. The struts-config.xml file configures the Controller.
Business logic
The business logic updates the state of the model and helps control the flow of the application. With Struts this is done with an Action class as a thin wrapper to the actual business logic.
Model
A model represents an application’s data and contains the logic for accessing and manipulating that data. Any data that is part of the persistent state of the application should reside in the model objects. The business objects update the application state. ActionForm bean represents the Model state at a session or request level, and not at a persistent level. Model services are accessed by the controller for either querying or effecting a change in the model state. The model notifies the view when a state change occurs in the model.The JSP file reads information from the ActionForm bean using JSP tags.
View
The view is responsible for rendering the state of the model. The presentation semantics are encapsulated within the view, therefore model data can be adapted for several different kinds of clients.The view modifies itself when a change in the model is communicated to the view. A view forwards user input to the controller.The view is simply a JSP file. There is no flow logic, no business logic, and no model information -- just tags. Tags are one of the things that make Struts unique compared to other frameworks like Velocity. What is RmiJdbc ? RmiJdbc is a type 3 JDBC driver that can network any local JDBC API.
It is just a bridge, not a driver dedicated to a specific database: RmiJdbc works in conjunction with a local JDBC driver on the server side (often a type 1 or type 2 driver, that does not offer remote access facilities).
RmiJdbc is made of a server-side component (daemon), and client classes that implement the java.sql.* interfaces. All JDBC classes (like Connection, ResultSet, etc...) are distributed as RMI objects, so that you can distribute as you like the access to any database supporting the JDBC API.
Using RmiJdbc is a good idea... · to access remotely an ODBC datasource (suing the JDBC/ODBC bridge) · to network a database that only offers a local JDBC API (like InstantDB) Using RmiJdbc is a bad idea... · along with a database that already has a good type 4 driver (like Oracle, PostgreSQL or MySQL) - better use the specific product. · when using JDBc from an applet: RMI is not properly supported by Microsoft Internet Explorer, and using JDBC from an applet is not really a good idea indeed. Some specific usages: · If you write your own database with a JDBC API, you can use RmiJdbc to network it · If you need serializable JDBC classes, the RmiJdbc client classes are serializable. RmiJdbc Story RmiJdbc was born in September 1997, at the Inria research center located in Montbonnot, France. The product was financed by GIE Dyade, a joint venture between groupe Bull and Inria.
Like many open-source initiatives, RmiJdbc was first a personal work; but its success quickly made it an "official" project. It was ther release under the GPL (Gnu Public License).
At the end of 1999, I was thinking of hosting & managing RmiJdbc by myself: RmiJdbc had no more official existence at Inria (I had left a few months ago to create my own company), although it was still hosted on their servers.
The ObjectWeb team proposed me to host RmiJdbc and make it one of their projects: I immediately accepted the proposal, and RmiJdbc joined ObjectWeb in June 2000.
In may 2001, the RmiJdbc license moved from GPL to LGPL (Lesser Gnu Public License), a license we at ObjectWeb consider more open.
Since the beginning of the story, some major contributions came from developers all over the world: for example, JDBC 3.0 is supported, RmiJdbc can be run on a single port, SSL can be used to secure RmiJdbc - and most of this is due to external contributors, as well as many bug fixes and enhancements.
In the future, I will do my best to maintain RmiJdbc, and integrate later contributions as efficiently as I can. I hope you will find RmiJdbc friendly and useful, A Bit of History Of JVM The Java programming language is a general-purpose object-oriented concurrent language. Its syntax is similar to C and C++, but it omits many of the features that make C and C++ complex, confusing, and unsafe. The Java platform was initially developed to address the problems of building software for networked consumer devices. It was designed to support multiple host architectures and to allow secure delivery of software components. To meet these requirements, compiled code had to survive transport across networks, operate on any client, and assure the client that it was safe to run. The popularization of the World Wide Web made these attributes much more interesting. The Internet demonstrated how media-rich content could be made accessible in simple ways. Web browsers such as Mosaic enabled millions of people to roam the Net and made Web surfing part of popular culture. At last there was a medium where what you saw and heard was essentially the same whether you were using a Mac, PC, or UNIX machine, and whether you were connected to a high-speed network or a slow modem. Web enthusiasts soon discovered that the content supported by the Web's HTML document format was too limited. HTML extensions, such as forms, only highlighted those limitations, while making it clear that no browser could include all the features users wanted. Extensibility was the answer. Sun's HotJava browser showcases the interesting properties of the Java programming language and platform by making it possible to embed programs inside HTML pages. These programs are transparently downloaded into the HotJava browser along with the HTML pages in which they appear. Before being accepted by the browser, the programs are carefully checked to make sure they are safe. Like HTML pages, compiled programs are network- and host-independent. The programs behave the same way regardless of where they come from or what kind of machine they are being loaded into and run on. A Web browser incorporating the Java or Java 2 platform is no longer limited to a predetermined set of capabilities. Visitors to Web pages incorporating dynamic content can be assured that their machines cannot be damaged by that content. Programmers can write a program once, and it will run on any machine supplying a Java or Java 2 runtime environment. The Java Virtual Machine The Java virtual machine is the cornerstone of the Java and Java 2 platforms. It is the component of the technology responsible for its hardware- and operating system- independence, the small size of its compiled code, and its ability to protect users from malicious programs. The Java virtual machine is an abstract computing machine. Like a real computing machine, it has an instruction set and manipulates various memory areas at run time. It is reasonably common to implement a programming language using a virtual machine; the best-known virtual machine may be the P-Code machine of UCSD Pascal. The first prototype implementation of the Java virtual machine, done at Sun Microsystems, Inc., emulated the Java virtual machine instruction set in software hosted by a handheld device that resembled a contemporary Personal Digital Assistant (PDA). Sun's current Java virtual machine implementations, components of its JavaTM 2 SDK and JavaTM 2 Runtime Environment products, emulate the Java virtual machine on Win32 and Solaris hosts in much more sophisticated ways. However, the Java virtual machine does not assume any particular implementation technology, host hardware, or host operating system. It is not inherently interpreted, but can just as well be implemented by compiling its instruction set to that of a silicon CPU. It may also be implemented in microcode or directly in silicon. The Java virtual machine knows nothing of the Java programming language, only of a particular binary format, the class file format. A class file contains Java virtual machine instructions (or bytecodes) and a symbol table, as well as other ancillary information. For the sake of security, the Java virtual machine imposes strong format and structural constraints on the code in a class file. However, any language with functionality that can be expressed in terms of a valid class file can be hosted by the Java virtual machine. Attracted by a generally available, machine-independent platform, implementors of other languages are turning to the Java virtual machine as a delivery vehicle for their languages.
Ans: Object oriented programming organizes a program around its data,i.e.,objects and a set of well defined interfaces to that data.An object-oriented program can be characterized as data controlling access to code.
2)what is the difference between Procedural and OOPs?
Ans: a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOPs program, unit of program is object, which is nothing but combination of data and code. b) In procedural program,data is exposed to the whole program whereas in OOPs program,it is accessible with in the object and which in turn assures the security of the code.
3)What are Encapsulation, Inheritance and Polymorphism?
Ans: Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse. Inheritance is the process by which one object acquires the properties of another object. Polymorphism is the feature that allows one interface to be used for general class actions.
4)What is the difference between Assignment and Initialization?
Ans: Assignment can be done as many times as desired whereas initialization can be done only once.
5)What are Class, Constructor and Primitive data types?
Ans: Class is a template for multiple objects with similar features and it is a blue print for objects. It defines a type of object according to the data the object can hold and the operations the object can perform. Constructor is a special kind of method that determines how an object is initialized when created. Primitive data types are 8 types and they are: byte, short, int, long float, double boolean char 6)What is an Object and how do you allocate memory to it?
Ans: Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data. When an object is created using new operator, memory is allocated to it.
7)What is the difference between constructor and method?
Ans: Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.
8)What are methods and how are they defined? Ans: Methods are functions that operate on instances of classes in which they are defined. Objects can communicate with each other using methods and can call methods in other classes. Method definition has four parts. They are name of the method, type of object or primitive type the method returns, a list of parameters and the body of the method. A method's signature is a combination of the first three parts mentioned above.
9)What is the use of bin and lib in JDK?
Ans: Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.
10)What is casting?
Ans: Casting is used to convert the value of one type to another.
11)How many ways can an argument be passed to a subroutine and explain them?
Ans: An argument can be passed in two ways. They are passing by value and passing by reference. Passing by value: This method copies the value of an argument into the formal parameter of the subroutine. Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to the parameter.
12)What is the difference between an argument and a parameter?
Ans: While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.
13)What are different types of access modifiers?
Ans: public: Any thing declared as public can be accessed from anywhere. private: Any thing declared as private can't be seen outside of its class. protected: Any thing declared as protected can be accessed by classes in the same package and subclasses in the other packages. default modifier : Can be accessed only to classes in the same package.
14)What is final, finalize() and finally?
Ans: final : final keyword can be used for class, method and variables. A final class cannot be subclassed and it prevents other programmers from subclassing a secure class to invoke insecure methods. A final method can' t be overridden A final variable can't change from its initialized value.
finalize( ) : finalize( ) method is used just before an object is destroyed and can be called just prior to garbage collection.
finally : finally, a key word used in exception handling, creates a block of code that will be executed after a try/catch block has completed and before the code following the try/catch block. The finally block will execute whether or not an exception is thrown. For example, if a method opens a file upon exit, then you will not want the code that closes the file to be bypassed by the exception-handling mechanism. This finally keyword is designed to address this contingency.
15)What is UNICODE?
Ans: Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.
16)What is Garbage Collection and how to call it explicitly?
Ans: When an object is no longer referred to by any variable, java automatically reclaims memory used by that object. This is known as garbage collection. System.gc() method may be used to call it explicitly.
17)What is finalize() method ?
Ans: finalize () method is used just before an object is destroyed and can be called just prior to garbage collection.
18)What are Transient and Volatile Modifiers?
Ans: Transient: The transient modifier applies to variables only and it is not stored as part of its object's Persistent state. Transient variables are not serialized. Volatile: Volatile modifier applies to variables only and it tells the compiler that the variable modified by volatile can be changed unexpectedly by other parts of the program.
19)What is method overloading and method overriding?
Ans: Method overloading: When a method in a class having the same method name with different arguments is said to be method overloading. Method overriding : When a method in a class having the same method name with same arguments is said to be method overriding.
20)What is difference between overloading and overriding?
Ans: a) In overloading, there is a relationship between methods available in the same class whereas in overriding, there is relationship between a superclass method and subclass method.
b) Overloading does not block inheritance from the superclass whereas overriding blocks inheritance from the superclass.
c) In overloading, separate methods share the same name whereas in overriding,subclass method replaces the superclass.
d) Overloading must have different method signatures whereas overriding must have same signature.
21) What is meant by Inheritance and what are its advantages?
Ans: Inheritance is the process of inheriting all the features from a class. The advantages of inheritance are reusability of code and accessibility of variables and methods of the super class by subclasses.
22)What is the difference between this() and super()?
Ans: this() can be used to invoke a constructor of the same class whereas super() can be used to invoke a super class constructor.
23)What is the difference between superclass and subclass?
Ans: A super class is a class that is inherited whereas sub class is a class that does the inheriting.
24) What modifiers may be used with top-level class?
Ans: public, abstract and final can be used for top-level class.
25)What are inner class and anonymous class?
Ans: Inner class : classes defined in other classes, including those defined in methods are called inner classes. An inner class can have any accessibility including private.
Anonymous class : Anonymous class is a class defined inside a method without a name and is instantiated and declared in the same place and cannot have explicit constructors.
26)What is a package?
Ans: A package is a collection of classes and interfaces that provides a high-level layer of access protection and name space management.
27) What is a reflection package?
Ans: java.lang.reflect package has the ability to analyze itself in runtime.
28) What is interface and its use?
Ans: Interface is similar to a class which may contain method's signature only but not bodies and it is a formal set of method and constant declarations that must be defined by the class that implements it. Interfaces are useful for: a)Declaring methods that one or more classes are expected to implement b)Capturing similarities between unrelated classes without forcing a class relationship. c)Determining an object's programming interface without revealing the actual body of the class.
29) What is an abstract class? Ans: An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately incomplete.
30) What is the difference between Integer and int?
Ans: a) Integer is a class defined in the java.lang package, whereas int is a primitive data type defined in the Java language itself. Java does not automatically convert from one to the other. b) Integer can be used as an argument for a method that requires an object, whereas int can be used for calculations.
31) What is a cloneable interface and how many methods does it contain? Ans- It is not having any method because it is a TAGGED or MARKER interface.
32) What is the difference between abstract class and interface? Ans: a) All the methods declared inside an interface are abstract whereas abstract class must have at least one abstract method and others may be concrete or abstract. b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that keyword for the methods. c) Abstract class must have subclasses whereas interface can't have subclasses.
33) Can you have an inner class inside a method and what variables can you access?
Ans: Yes, we can have an inner class inside a method and final variables can be accessed.
34) What is the difference between String and String Buffer?
Ans: a) String objects are constants and immutable whereas StringBuffer objects are not. b) String class supports constant strings whereas StringBuffer class supports growable and modifiable strings.
35) What is the difference between Array and vector?
Ans: Array is a set of related data type and static whereas vector is a growable array of objects and dynamic.
36) What is the difference between exception and error?
Ans: The exception class defines mild error conditions that your program encounters. Ex: Arithmetic Exception, FilenotFound exception Exceptions can occur when -- try to open the file, which does not exist -- the network connection is disrupted -- operands being manipulated are out of prescribed ranges -- the class file you are interested in loading is missing The error class defines serious error conditions that you should not attempt to recover from. In most cases it is advisable to let the program terminate when such an error is encountered. Ex: Running out of memory error, Stack overflow error.
37) What is the difference between process and thread?
Ans: Process is a program in execution whereas thread is a separate path of execution in a program.
38) What is multithreading and what are the methods for inter-thread communication and what is the class in which these methods are defined?
Ans: Multithreading is the mechanism in which more than one thread run independent of each other within the process. wait (), notify () and notifyAll() methods can be used for inter-thread communication and these methods are in Object class. wait( ) : When a thread executes a call to wait( ) method, it surrenders the object lock and enters into a waiting state. notify( ) or notifyAll( ) : To remove a thread from the waiting state, some other thread must make a call to notify( ) or notifyAll( ) method on the same object.
39) What is the class and interface in java to create thread and which is the most advantageous method?
Ans: Thread class and Runnable interface can be used to create threads and using Runnable interface is the most advantageous method to create threads because we need not extend thread class here.
40) What are the states associated in the thread?
Ans: Thread contains ready, running, waiting and dead states.
41) What is synchronization?
Ans: Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time.
42) When you will synchronize a piece of your code?
Ans: When you expect your code will be accessed by different threads and these threads may change a particular data causing data corruption.
43) What is deadlock?
Ans: When two threads are waiting each other and can't precede the program is said to be deadlock.
44) What is daemon thread and which method is used to create the daemon thread?
Ans: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.
45) Are there any global variables in Java, which can be accessed by other part of your program?
Ans: No, it is not the main method in which you define variables. Global variables is not possible because concept of encapsulation is eliminated here.
46)What is an applet?
Ans: Applet is a dynamic and interactive program that runs inside a web page displayed by a java capable browser.
47)What is the difference between applications and applets?
Ans: a)Application must be run on local machine whereas applet needs no explicit installation on local machine. b)Application must be run explicitly within a java-compatible virtual machine whereas applet loads and runs itself automatically in a java-enabled browser. d)Application starts execution with its main method whereas applet starts execution with its init method. e)Application can run with or without graphical user interface whereas applet must run within a graphical user interface.
48)How does applet recognize the height and width?
Ans:Using getParameters() method.
49)When do you use codebase in applet?
Ans:When the applet class file is not in the same directory, codebase is used.
50)What is the lifecycle of an applet?
Ans:init( ) method - Can be called when an applet is first loaded start( ) method - Can be called each time an applet is started paint( ) method - Can be called when the applet is minimized or maximized stop( ) method - Can be used when the browser moves off the applet's page destroy( ) method - Can be called when the browser is finished with the applet
51)How do you set security in applets?
Ans: using setSecurityManager() method
52) What is an event and what are the models available for event handling?
Ans: An event is an event object that describes a state of change in a source. In other words, event occurs when an action is generated, like pressing button, clicking mouse, selecting a list, etc. There are two types of models for handling events and they are: a) event-inheritance model and b) event-delegation model
53) What are the advantages of the model over the event-inheritance model?
Ans: The event-delegation model has two advantages over the event-inheritance model. They are: a)It enables event handling by objects other than the ones that generate the events. This allows a clean separation between a component's design and its use. b)It performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to be repeatedly process unhandled events as is the case of the event-inheritance.
54)What is source and listener ?
Ans: source : A source is an object that generates an event. This occurs when the internal state of that object changes in some way. listener : A listener is an object that is notified when an event occurs. It has two major requirements. First, it must have been registered with one or more sources to receive notifications about specific types of events. Second, it must implement methods to receive and process these notifications.
55) What is adapter class?
Ans: An adapter class provides an empty implementation of all methods in an event listener interface. Adapter classes are useful when you want to receive and process only some of the events that are handled by a particular event listener interface. You can define a new class to act listener by extending one of the adapter classes and implementing only those events in which you are interested. For example, the MouseMotionAdapter class has two methods, mouseDragged( )and mouseMoved(). The signatures of these empty are exactly as defined in the MouseMotionListener interface. If you are interested in only mouse drag events, then you could simply extend MouseMotionAdapter and implement mouseDragged( ) .
56)What is meant by controls and what are different types of controls in AWT?
Ans: Controls are components that allow a user to interact with your application and the AWT supports the following types of controls: Labels, Push Buttons, Check Boxes, Choice Lists, Lists, Scrollbars, Text Components. These controls are subclasses of Component.
57) What is the difference between choice and list?
Ans: A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices and only one item may be selected from a choice. A List may be displayed in such a way that several list items are visible and it supports the selection of one or more list items.
58) What is the difference between scrollbar and scrollpane?
Ans: A Scrollbar is a Component, but not a Container whereas Scrollpane is a Conatiner and handles its own events and perform its own scrolling.
59) What is a layout manager and what are different types of layout managers available in java.awt?
Ans: A layout manager is an object that is used to organize components in a container. The different layouts are available are FlowLayout, BorderLayout, CardLayout, GridLayout and GridBagLayout.
60) How are the elements of different layouts organized?
Ans: FlowLayout: The elements of a FlowLayout are organized in a top to bottom, left to right fashion. BorderLayout: The elements of a BorderLayout are organized at the borders (North, South, East and West) and the center of a container. CardLayout: The elements of a CardLayout are stacked, on top of the other, like a deck of cards. GridLayout: The elements of a GridLayout are of equal size and are laid out using the square of a grid. GridBagLayout: The elements of a GridBagLayout are organized according to a grid. However, the elements are of different size and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.
61) Which containers use a Border layout as their default layout?
Ans: Window, Frame and Dialog classes use a BorderLayout as their layout.
62) Which containers use a Flow layout as their default layout?
Ans: Panel and Applet classes use the FlowLayout as their default layout.
63) What are wrapper classes?
Ans: Wrapper classes are classes that allow primitive types to be accessed as objects.
64) What are Vector, Hashtable, LinkedList and Enumeration?
Ans: Vector : The Vector class provides the capability to implement a growable array of objects. Hashtable : The Hashtable class implements a Hashtable data structure. A Hashtable indexes and stores objects in a dictionary using hash codes as the object's keys. Hash codes are integer values that identify objects. LinkedList: Removing or inserting elements in the middle of an array can be done using LinkedList. A LinkedList stores each object in a separate link whereas an array stores object references in consecutive locations. Enumeration: An object that implements the Enumeration interface generates a series of elements, one at a time. It has two methods, namely hasMoreElements( ) and nextElement( ). HasMoreElemnts( ) tests if this enumeration has more elements and nextElement method returns successive elements of the series.
65) What is the difference between set and list?
Ans: Set stores elements in an unordered way but does not contain duplicate elements, whereas list stores elements in an ordered way but may contain duplicate elements.
66) What is a stream and what are the types of Streams and classes of the Streams?
Ans: A Stream is an abstraction that either produces or consumes information. There are two types of Streams and they are: Byte Streams: Provide a convenient means for handling input and output of bytes. Character Streams: Provide a convenient means for handling input & output of characters. Byte Streams classes: Are defined by using two abstract classes, namely InputStream and OutputStream. Character Streams classes: Are defined by using two abstract classes, namely Reader and Writer.
67) What is the difference between Reader/Writer and InputStream/Output Stream?
Ans: The Reader/Writer class is character-oriented and the InputStream/OutputStream class is byte-oriented.
68) What is an I/O filter?
Ans: An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
69) What is serialization and deserialization?
Ans: Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.
70) What is JDBC?
Ans: JDBC is a set of Java API for executing SQL statements. This API consists of a set of classes and interfaces to enable programs to write pure Java Database applications.
71) What are drivers available?
Ans: a) JDBC-ODBC Bridge driver b) Native API Partly-Java driver c) JDBC-Net Pure Java driver d) Native-Protocol Pure Java driver
72) What is the difference between JDBC and ODBC?
Ans: a) OBDC is for Microsoft and JDBC is for Java applications. b) ODBC can't be directly used with Java because it uses a C interface. c) ODBC makes use of pointers which have been removed totally from Java. d) ODBC mixes simple and advanced features together and has complex options for simple queries. But JDBC is designed to keep things simple while allowing advanced capabilities when required. e) ODBC requires manual installation of the ODBC driver manager and driver on all client machines. JDBC drivers are written in Java and JDBC code is automatically installable, secure, and portable on all platforms. f) JDBC API is a natural Java interface and is built on ODBC. JDBC retains some of the basic features of ODBC.
73) What are the types of JDBC Driver Models and explain them?
Ans: There are two types of JDBC Driver Models and they are: a) Two tier model and b) Three tier model Two tier model: In this model, Java applications interact directly with the database. A JDBC driver is required to communicate with the particular database management system that is being accessed. SQL statements are sent to the database and the results are given to user. This model is referred to as client/server configuration where user is the client and the machine that has the database is called as the server. Three tier model: A middle tier is introduced in this model. The functions of this model are: a) Collection of SQL statements from the client and handing it over to the database, b) Receiving results from database to the client and c) Maintaining control over accessing and updating of the above.
74) What are the steps involved for making a connection with a database or how do you connect to a database?
Ans:
a) Loading the driver : To load the driver, Class.forName( ) method is used. Class.forName("sun.jdbc.odbc.JdbcOdbcDriver"); When the driver is loaded, it registers itself with the java.sql.DriverManager class as an available database driver.
b) Making a connection with database : To open a connection to a given database, DriverManager.getConnection( ) method is used. Connection con = DriverManager.getConnection ("jdbc:odbc:somedb", "user", "password");
c) Executing SQL statements : To execute a SQL query, java.sql.statements class is used. createStatement( ) method of Connection to obtain a new Statement object. Statement stmt = con.createStatement( ); A query that returns data can be executed using the executeQuery( ) method of Statement. This method executes the statement and returns a java.sql.ResultSet that encapsulates the retrieved data: ResultSet rs = stmt.executeQuery("SELECT * FROM some table");
d) Process the results : ResultSet returns one row at a time. Next( ) method of ResultSet object can be called to move to the next row. The getString( ) and getObject( ) methods are used for retrieving column values: while(rs.next( ) ) { String event = rs.getString("event"); Object count = (Integer) rs.getObject("count");
75) What type of driver did you use in project?
Ans: JDBC-ODBC Bridge driver (is a driver that uses native(C language) libraries and makes calls to an existing ODBC driver to access a database engine).
76) What are the types of statements in JDBC?
Ans: Statement -- To be used createStatement() method for executing single SQL statement PreparedStatement -- To be used preparedStatement() method for executing same SQL statement over and over CallableStatement -- To be used prepareCall( ) method for multiple SQL statements over and over
77) What is stored procedure?
Ans: Stored procedure is a group of SQL statements that forms a logical unit and performs a particular task. Stored Procedures are used to encapsulate a set of operations or queries to execute on database. Stored procedures can be compiled and executed with different parameters and results and may have any combination of input/output parameters.
78) How to create and call stored procedures?
Ans: To create stored procedures: Create procedure procedurename (specify in, out and in out parameters) BEGIN Any multiple SQL statement; END; To call stored procedures: CallableStatement csmt = con.prepareCall("{call procedure name(?,?)}"); csmt.registerOutParameter(column no., data type); csmt.setInt(column no., column name) csmt.execute( );
79) What is servlet?
Ans: Servlets are modules that extend request/response-oriented servers, such as java-enabled web servers. For example, a servlet might be responsible for taking data in an HTML order-entry form and applying the business logic used to update a company's order database.
80) What are the classes and interfaces for servlets?
Ans: There are two packages in servlets and they are javax.servlet and javax.servlet.http. Javax.servlet contains: Interfaces Classes Servlet Generic Servlet ServletRequest ServletInputStream ServletResponse ServletOutputStream ServletConfig ServletException ServletContext UnavailableException SingleThreadModel Javax.servlet.http contains: Interfaces Classes HttpServletRequest Cookie HttpServletResponse HttpServlet HttpSession HttpSessionBindingEvent HttpSessionCintext HttpUtils HttpSeesionBindingListener
81) What is the difference between an applet and a servlet?
Ans: a) Servlets are to servers what applets are to browsers. b) Applets must have graphical user interfaces whereas servlets have no graphical user interfaces.
82) What is the difference between doPost and doGet methods?
Ans: a) doGet() method is used to get information, while doPost( ) method is used for posting information. b) doGet() requests can't send large amount of information and is limited to 240-255 characters. However, doPost( )requests passes all of its data, of unlimited length. c) A doGet( ) request is appended to the request URL in a query string and this allows the exchange is visible to the client, whereas a doPost() request passes directly over the socket connection as part of its HTTP request body and the exchange are invisible to the client.
83) What is the life cycle of a servlet?
Ans: Each Servlet has the same life cycle: a) A server loads and initializes the servlet by init () method. b) The servlet handles zero or more client's requests through service( ) method. c) The server removes the servlet through destroy() method.
84) Who is loading the init() method of servlet?
Ans: Web server
85) What are the different servers available for developing and deploying Servlets?
Ans: a) Java Web Server b) JRun g) Apache Server h) Netscape Information Server i) Web Logic
86) How many ways can we track client and what are they?
Ans: The servlet API provides two ways to track client state and they are: a) Using Session tracking and b) Using Cookies.
87) What is session tracking and how do you track a user session in servlets?
Ans: Session tracking is a mechanism that servlets use to maintain state about a series requests from the same user across some period of time. The methods used for session tracking are: a) User Authentication - occurs when a web server restricts access to some of its resources to only those clients that log in using a recognized username and password b) Hidden form fields - fields are added to an HTML form that are not displayed in the client's browser. When the form containing the fields is submitted, the fields are sent back to the server c) URL rewriting - every URL that the user clicks on is dynamically modified or rewritten to include extra information. The extra information can be in the form of extra path information, added parameters or some custom, server-specific URL change. d) Cookies - a bit of information that is sent by a web server to a browser and which can later be read back from that browser. e) HttpSession- places a limit on the number of sessions that can exist in memory. This limit is set in the session.maxresidents property
88) What is Server-Side Includes (SSI)?
Ans: Server-Side Includes allows embedding servlets within HTML pages using a special servlet tag. In many servlets that support servlets, a page can be processed by the server to include output from servlets at certain points inside the HTML page. This is accomplished using a special internal SSINCLUDE, which processes the servlet tags. SSINCLUDE servlet will be invoked whenever a file with an. shtml extension is requested. So HTML files that include server-side includes must be stored with an .shtml extension.
89) What are cookies and how will you use them?
Ans: Cookies are a mechanism that a servlet uses to have a client hold a small amount of state-information associated with the user. a) Create a cookie with the Cookie constructor: public Cookie(String name, String value) b) A servlet can send a cookie to the client by passing a Cookie object to the addCookie() method of HttpServletResponse: public void HttpServletResponse.addCookie(Cookie cookie) c) A servlet retrieves cookies by calling the getCookies() method of HttpServletRequest: public Cookie[ ] HttpServletRequest.getCookie( ).
90) Is it possible to communicate from an applet to servlet and how many ways and how?
Ans: Yes, there are three ways to communicate from an applet to servlet and they are:
a) HTTP Communication(Text-based and object-based)
b) Socket Communication
c) RMI Communication (You can say, by using URL object open the connection to server and get the InputStream from URLConnection object). Steps involved for applet-servlet communication:
1) Get the server URL. URL url = new URL();
2) Connect to the host URLConnection Con = url.openConnection();
3) Initialize the connection Con.setUseCatches(false): Con.setDoOutput(true); Con.setDoInput(true);
4) Data will be written to a byte array buffer so that we can tell the server the length of the data. ByteArrayOutputStream byteout = new ByteArrayOutputStream();
5) Create the OutputStream to be used to write the data to the buffer. DataOutputStream out = new DataOutputStream(byteout);
91) What is connection pooling?
Ans: With servlets, opening a database connection is a major bottleneck because we are creating and tearing down a new connection for every page request and the time taken to create connection will be more. Creating a connection pool is an ideal approach for a complicated servlet. With a connection pool, we can duplicate only the resources we need to duplicate rather than the entire servlet. A connection pool can also intelligently manage the size of the pool and make sure each connection remains valid. A number of connection pool packages are currently available. Some like DbConnectionBroker are freely available from Java Exchange Works by creating an object that dispenses connections and connection Ids on request. The ConnectionPool class maintains a Hastable, using Connection objects as keys and Boolean values as stored values. The Boolean value indicates whether a connection is in use or not. A program calls getConnection( ) method of the ConnectionPool for getting Connection object it can use; it calls returnConnection( ) to give the connection back to the pool.
92) Why should we go for interservlet communication?
Ans: Servlets running together in the same server communicate with each other in several ways. The three major reasons to use interservlet communication are: a) Direct servlet manipulation - allows to gain access to the other currently loaded servlets and perform certain tasks (through the ServletContext object) b) Servlet reuse - allows the servlet to reuse the public methods of another servlet. c) Servlet collaboration - requires to communicate with each other by sharing specific information (through method invocation)
93) Is it possible to call servlet with parameters in the URL?
Ans: Yes. You can call a servlet with parameters in the syntax as (?Param1 = xxx || m2 = yyy). 94) What is Servlet chaining? Ans: Servlet chaining is a technique in which two or more servlets can cooperate in servicing a single request. In servlet chaining, one servlet's output is piped to the next servlet's input. This process continues until the last servlet is reached. Its output is then sent back to the client.
95) How do servlets handle multiple simultaneous requests?
Ans: The server has multiple threads that are available to handle requests. When a request comes in, it is assigned to a thread, which calls a service method (for example: doGet(), doPost( ) and service( ) ) of the servlet. For this reason, a single servlet object can have its service methods called by many threads at once.
96) What is the difference between TCP/IP and UDP?
Ans: TCP/IP is a two-way communication between the client and the server and it is a reliable and there is a confirmation regarding reaching the message to the destination. It is like a phone call. UDP is a one-way communication only between the client and the server and it is not a reliable and there is no confirmation regarding reaching the message to the destination. It is like a postal mail.
97) What is Inet address?
Ans: Every computer connected to a network has an IP address. An IP address is a number that uniquely identifies each computer on the Net. An IP address is a 32-bit number.
98) What is Domain Naming Service(DNS)?
Ans: It is very difficult to remember a set of numbers(IP address) to connect to the Internet. The Domain Naming Service(DNS) is used to overcome this problem. It maps one particular IP address to a string of characters. For example, www.mascom.com implies com is the domain name reserved for US commercial sites, moscom is the name of the company and www is the name of the specific computer, which is mascom's server.
99) What is URL?
Ans: URL stands for Uniform Resource Locator and it points to resource files on the Internet. URL has four components: http://www.Pentafour.com:80/index.html http - protocol name, Pentafour - IP address or host name, 80 - port number and index.html - file path.
100) What is RMI and steps involved in developing an RMI object?
Ans: Remote Method Invocation (RMI) allows java object that executes on one machine and to invoke the method of a Java object to execute on another machine. The steps involved in developing an RMI object are: a) Define the interfaces b) Implementing these interfaces c) Compile the interfaces and their implementations with the java compiler d) Compile the server implementation with RMI compiler e) Run the RMI registry f) Run the application
101) What is RMI architecture?
Ans: - RMI architecture consists of four layers and each layer performs specific functions: a) Application layer ---- contains the actual object definition b) Proxy layer ---- consists of stub and skeleton c) Remote Reference layer ---- gets the stream of bytes from the transport layer and sends it to the proxy layer d) Transportation layer ---- responsible for handling the actual machine-to-machine communication
102) what is UnicastRemoteObject?
Ans: All remote objects must extend UnicastRemoteObject, which provides functionality that is needed to make objects available from remote machines.
103) Explain the methods, rebind( ) and lookup() in Naming class?
Ans: rebind( ) of the Naming class(found in java.rmi) is used to update the RMI registry on the server machine. Naming. rebind("AddSever", AddServerImpl); lookup( ) of the Naming class accepts one argument, the rmi URL and returns a reference to an object of type AddServerImpl.
104) What is a Java Bean?
Ans: A Java Bean is a software component that has been designed to be reusable in a variety of different environments.
105) What is a Jar file?
Ans: Jar file allows to efficiently deploying a set of classes and their associated resources. The elements in a jar file are compressed, which makes downloading a Jar file much faster than separately downloading several uncompressed files. The package java.util.zip contains classes that read and write jar files.
106) What is BDK?
Ans: BDK, Bean Development Kit is a tool that enables to create, configure and connect a set of set of Beans and it can be used to test Beans without writing a code.
107) What is JSP?
Ans: JSP is a dynamic scripting capability for web pages that allows Java as well as a few special tags to be embedded into a web file (HTML/XML, etc). The suffix traditionally ends with .jsp to indicate to the web server that the file is a JSP files. JSP is a server side technology - you can't do any client side validation with it. The advantages are: a) The JSP assists in making the HTML more functional. Servlets on the other hand allow outputting of HTML but it is a tedious process. b) It is easy to make a change and then let the JSP capability of the web server you are using deal with compiling it into a servlet and running it.
108) What are JSP scripting elements?
Ans: JSP scripting elements lets to insert Java code into the servlet that will be generated from the current JSP page.
There are three forms:
a) Expressions of the form <%= expression %> that are evaluated and inserted into the output,
b) Scriptlets of the form <% code %> that are inserted into the servlet's service method, and
c) Declarations of the form <%! Code %> that are inserted into the body of the servlet class, outside of any existing methods.
109) What are JSP Directives?
Ans: A JSP directive affects the overall structure of the servlet class. It usually has the following form: <%@ directive attribute="value" %> However, you can also combine multiple attribute settings for a single directive, as follows: <%@ directive attribute1="value1" attribute 2="value2" ... attributeN ="valueN" %> There are two main types of directive: page, which lets to do things like import classes, customize the servlet superclass, and the like; and include, which lets to insert a file into the servlet class at the time the JSP file is translated into a servlet
110) What are Predefined variables or implicit objects?
Ans: To simplify code in JSP expressions and scriptlets, we can use eight automatically defined variables, sometimes called implicit objects. They are request, response, out, session, application, config, pageContext, and page.
111) What are JSP ACTIONS?
Ans: JSP actions use constructs in XML syntax to control the behavior of the servlet engine. You can dynamically insert a file, reuse JavaBeans components, forward the user to another page, or generate HTML for the Java plugin. Available actions include: ? jsp:include - Include a file at the time the page is requested. ?
jsp:useBean - Find or instantiate a JavaBean. ?
jsp:setProperty - Set the property of a JavaBean. ?
jsp:getProperty - Insert the property of a JavaBean into the output. ?
jsp:forward - Forward the requester to a newpage. ?
Jsp: plugin - Generate browser-specific code that makes an OBJECT or EMBED
112) How do you pass data (including JavaBeans) to a JSP from a servlet?
Ans: (1) Request Lifetime: Using this technique to pass beans, a request dispatcher (using either "include" or forward") can be called. This bean will disappear after processing this request has been completed. Servlet: request.setAttribute("theBean", myBean); RequestDispatcher rd = getServletContext().getRequestDispatcher("thepage.jsp"); rd.forward(request, response); JSP PAGE: <jsp: useBean id="theBean" scope="request" class="....." />
(2) Session Lifetime: Using this technique to pass beans that are relevant to a particular session (such as in individual user login) over a number of requests. This bean will disappear when the session is invalidated or it times out, or when you remove it. Servlet: HttpSession session = request.getSession(true); session.putValue("theBean", myBean); /* You can do a request dispatcher here, or just let the bean be visible on the next request */ JSP Page: <jsp:useBean id="theBean" scope="session" class="..." />
3) Application Lifetime: Using this technique to pass beans that are relevant to all servlets and JSP pages in a particular app, for all users. For example, I use this to make a JDBC connection pool object available to the various servlets and JSP pages in my apps. This bean will disappear when the servlet engine is shut down, or when you remove it. Servlet: GetServletContext(). setAttribute("theBean", myBean); JSP PAGE: <jsp:useBean id="theBean" scope="application" class="..." />
113) How can I set a cookie in JSP?
Ans: response.setHeader("Set-Cookie", "cookie string"); To give the response-object to a bean, write a method setResponse (HttpServletResponse response) - to the bean, and in jsp-file: <% bean.setResponse (response); %> 114) How can I delete a cookie with JSP? Ans: Say that I have a cookie called "foo," that I set a while ago & I want it to go away. I simply: <% Cookie killCookie = new Cookie("foo", null); KillCookie.setPath("/"); killCookie.setMaxAge(0); response.addCookie(killCookie); %>
115) How are Servlets and JSP Pages related?
Ans: JSP pages are focused around HTML (or XML) with Java codes and JSP tags inside them. When a web server that has JSP support is asked for a JSP page, it checks to see if it has already compiled the page into a servlet. Thus, JSP pages become servlets and are transformed into pure Java and then compiled, loaded into the server and executed CORE JAVA
| Q: | What is the difference between an Interface and an Abstract class? |
| A: | An Abstract class declares have at least one instance method that is declared abstract which will be implemented by the subclasses. An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior. |
| Q: | What is the purpose of garbage collection in Java, and when is it used? |
| A: | The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used. |
| Q: | Describe synchronization in respect to multithreading. |
| A: | With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors. |
| Q: | Explain different way of using thread? |
| A: | The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help. |
| Q: | What are pass by reference and passby value? |
| A: | Pass By Reference means the passing the address itself rather than passing the value. Pass by Value means passing a copy of the value to be passed. |
| Q: | What is HashMap and Map? |
| A: | Map is Interface and Hashmap is class that implements that. |
| Q: | Difference between HashMap and HashTable? |
| A: | The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn’t allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is non synchronized and Hashtable is synchronized. |
| Q: | Difference between Vector and ArrayList? |
| A: | Vector is synchronized whereas arraylist is not. |
| Q: | Difference between Swing and Awt? |
| A: | AWT are heavy-weight components. Swings are light-weight components. Hence swing works faster than AWT. |
| Q: | What is the difference between a constructor and a method? |
| A: | A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator. |
| Q: | What is an Iterators? |
| A: | Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator. |
| Q: | State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers. |
| A: | public : Public class is visible in other packages, field is visible everywhere (class must be public too) private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature. protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature. default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package. |
| Q: | What is an abstract class? |
| A: | Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated. |
| Q: | What is static in java? |
| A: | Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass. |
| Q: | What is final? |
| A: | A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant). |
| Q: | What if the main method is declared as private? |
| A: | The program compiles properly but at runtime it will give "Main method not public." message. |
| Q: | What if the static modifier is removed from the signature of the main method? |
| A: | Program compiles. But at runtime throws an error "NoSuchMethodError". |
| Q: | What if I write static public void instead of public static void? |
| A: | Program compiles and runs properly. |
| Q: | What if I do not provide the String array as the argument to the method? |
| A: | Program compiles but throws a runtime error "NoSuchMethodError". |
| Q: | What is the first argument of the String array in main method? |
| A: | The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name. |
| Q: | If I do not provide any arguments on the command line, then the String array of Main method will be empty of null? |
| A: | It is empty. But not null. |
| A: | Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length. |
| Q: | What environment variables do I need to set on my machine in order to be able to run Java programs? |
| A: | CLASSPATH and PATH are the two variables. |
| Q: | Can an application have multiple classes having main method? |
| A: | Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method. |
| Q: | Can I have multiple main methods in the same class? |
| A: | No the program fails to compile. The compiler says that the main method is already defined in the class. |
| Q: | Do I need to import java.lang package any time? Why ? |
| A: | No. It is by default loaded internally by the JVM. |
| Q: | Can I import same package/class twice? Will the JVM load the package twice at runtime? |
| A: | One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class. |
| Q: | What are Checked and UnChecked Exception? |
| A: | A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be. |
| Q: | What is Overriding? |
| A: | When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass. When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private. |
| Q: | What are different types of inner classes? |
| A: | Nested top-level classes, Member classes, Local classes, Anonymous classes Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety. Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class. Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable. Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor. |
| Q: | Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile? |
| A: | Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol symbol : class ABCD location: package io import java.io.ABCD; |
| Q: | Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*? |
| A: | No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage. |
| Q: | What is the difference between declaring a variable and defining a variable? |
| A: | In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization. e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions. |
| Q: | What is the default value of an object reference declared as an instance variable? |
| A: | null unless we define it explicitly. |
| Q: | Can a top level class be private or protected? |
| A: | No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected. |
| Q: | What type of parameter passing does Java support? |
| A: | In Java the arguments are always passed by value . |
| Q: | Primitive data types are passed by reference or pass by value? |
| A: | Primitive data types are passed by value. |
| Q: | Objects are passed by value or by reference? |
| A: | Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object . |
| Q: | What is serialization? |
| A: | Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream. |
| Q: | How do I serialize an object to a file? |
| A: | The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file. |
| Q: | Which methods of Serializable interface should I implement? |
| A: | The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods. |
| Q: | How can I customize the seralization process? i.e. how can one have a control over the serialization process? |
| A: | Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process. |
| Q: | What is the common usage of serialization? |
| A: | Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed. |
| Q: | What is Externalizable interface? |
| A: | Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods. |
| Q: | What happens to the object references included in the object? |
| A: | The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect. |
| Q: | What one should take care of while serializing the object? |
| A: | One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException. |
| Q: | What happens to the static fields of a class during serialization? Are these fields serialized as a part of each serialized object? |
| A: | Yes the static fields do get serialized. If the static field is an object then it must have implemented Serializable interface. The static fields are serialized as a part of every object. But the commonness of the static fields across all the instances is maintained even after serialization. |
| Q: | How are Observer and Observable used? |
| A: | Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects. |
| Q: | What is synchronization and why is it important? |
| A: | With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors. |
| Q: | How does Java handle integer overflows and underflows? |
| A: | It uses those low order bytes of the result that can fit into the size of the type allowed by the operation. |
| Q: | Does garbage collection guarantee that a program will not run out of memory? |
| A: | Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection . |
| Q: | What is the difference between preemptive scheduling and time slicing? |
| A: | Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors. |
| Q: | When a thread is created and started, what is its initial state? |
| A: | A thread is in the ready state after it has been created and started. |
| Q: | What is the purpose of finalization? |
| A: | The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected. |
| Q: | What is the Locale class? |
| A: | The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region. |
| Q: | What is the difference between a while statement and a do statement? |
| A: | A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once. |
| Q: | What is the difference between static and non-static variables? |
| A: | A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance. |
| Q: | How are this() and super() used with constructors? |
| A: | Othis() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor. |
| Q: | What are synchronized methods and synchronized statements? |
| A: | Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement. |
| Q: | Does Java provide any construct to find out the size of an object? |
| A: | No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java. |
| Q: | Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*? |
| A: | Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution. To put it in code... long start = System.currentTimeMillis (); method (); long end = System.currentTimeMillis (); System.out.println ("Time taken for execution is " + (end - start)); Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amout of processing. |
| Q: | What are wrapper classes? |
| A: | Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc. |
| Q: | Why do we need wrapper classes? |
| A: | It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object. |
| Q: | What are checked exceptions? |
| A: | Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions. |
| Q: | What are runtime exceptions? |
| A: | Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time. |
| Q: | What is the difference between error and an exception? |
| A: | An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.). |
| Q: | How to create custom exceptions? |
| A: | Your class should extend class Exception, or some more specific type thereof. |
| Q: | If I want an object of my class to be thrown as an exception object, what should I do? |
| A: | The class should extend from Exception class. Or you can extend your class from some more precise exception type also. |
| Q: | If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object? |
| A: | One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well. |
| Q: | What happens to an unhandled exception? |
| A: | One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well. |
| Q: | How does an exception permeate through the code? |
| A: | An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates. |
| Q: | What are the different ways to handle exceptions? |
| A: | There are two ways to handle exceptions, 1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and 2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions. |
| Q: | Q: What is the basic difference between the 2 approaches to exception handling...1> try catch block and 2> specifying the candidate exceptions in the throws clause? When should you use which approach? |
| A: | In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use. |
| Q: | Is it necessary that each try block must be followed by a catch block? |
| A: | It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method. |
| Q: | If I write return at the end of the try block, will the finally block still execute? |
| A: | Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return. |
| Q: | If I write System.exit (0); at the end of the try block, will the finally block still execute? |
| A: | No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes. |
| Q: | What is a output comment? |
| A: | A comment that is sent to the client in the viewable page source.The JSP engine handles an output comment as uninterpreted HTML text, returning the comment in the HTML output sent to the client. You can see the comment by viewing the page source from your Web browser. JSP Syntax <!-- comment [ <%= expression %> ] --> Example 1 <!-- This is a commnet sent to client on <%= (new java.util.Date()).toLocaleString() %> --> Displays in the page source: <!-- This is a commnet sent to client on January 24, 2004 --> |
| Q: | What is a Hidden Comment? |
| A: | A comments that documents the JSP page but is not sent to the client. The JSP engine ignores a hidden comment, and does not process any code within hidden comment tags. A hidden comment is not sent to the client, either in the displayed JSP page or the HTML page source. The hidden comment is useful when you want to hide or "comment out" part of your JSP page. You can use any characters in the body of the comment except the closing --%> combination. If you need to use --%> in your comment, you can escape it by typing --%\>. JSP Syntax <%-- comment --%> Examples <%@ page language="java" %> <html> <head><title>A Hidden Comment </title></head> <body> <%-- This comment will not be visible to the colent in the page source --%> </body> </html> |
| Q: | What is a Expression? |
| A: | An expression tag contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file. Because the value of an expression is converted to a String, you can use an expression within text in a JSP file. Like <%= someexpression %> <%= (new java.util.Date()).toLocaleString() %> You cannot use a semicolon to end an expression |
| Q: | What is a Declaration? |
| A: | A declaration declares one or more variables or methods for use later in the JSP source file. A declaration must contain at least one complete declarative statement. You can declare any number of variables or methods within one declaration tag, as long as they are separated by semicolons. The declaration must be valid in the scripting language used in the JSP file. <%! somedeclarations %> <%! int i = 0; %> <%! int a, b, c; %> |
| Q: | What is a Scriptlet? |
| A: | A scriptlet can contain any number of language statements, variable or method declarations, or expressions that are valid in the page scripting language.Within scriptlet tags, you can 1.Declare variables or methods to use later in the file (see also Declaration). 2.Write expressions valid in the page scripting language (see also Expression). 3.Use any of the JSP implicit objects or any object declared with a <jsp:useBean> tag. You must write plain text, HTML-encoded text, or other JSP tags outside the scriptlet. Scriptlets are executed at request time, when the JSP engine processes the client request. If the scriptlet produces output, the output is stored in the out object, from which you can display it. |
| Q: | What are implicit objects? List them? | |
| A: | Certain objects that are available for the use in JSP documents without being declared first. These objects are parsed by the JSP engine and inserted into the generated servlet. The implicit objects re listed below
|
| Q: | Difference between forward and sendRedirect? |
| A: | When you invoke a forward request, the request is sent to another resource on the server, without the client being informed that a different resource is going to process the request. This process occurs completly with in the web container. When a sendRedirtect method is invoked, it causes the web container to return to the browser indicating that a new URL should be requested. Because the browser issues a completly new request any object that are stored as request attributes before the redirect occurs will be lost. This extra round trip a redirect is slower than forward. |
| Q: | What are the different scope valiues for the <jsp:useBean>? |
| A: | The different scope values for <jsp:useBean> are 1. page 2. request 3.session 4.application |
| Q: | Explain the life-cycle mehtods in JSP? |
| A: | THe generated servlet class for a JSP page implements the HttpJspPage interface of the javax.servlet.jsp package. Hte HttpJspPage interface extends the JspPage interface which inturn extends the Servlet interface of the javax.servlet package. the generated servlet class thus implements all the methods of the these three interfaces. The JspPage interface declares only two mehtods - jspInit() and jspDestroy() that must be implemented by all JSP pages regardless of the client-server protocol. However the JSP specification has provided the HttpJspPage interfaec specifically for the JSp pages serving HTTP requests. This interface declares one method _jspService(). The jspInit()- The container calls the jspInit() to initialize te servlet instance.It is called before any other method, and is called only once for a servlet instance. The _jspservice()- The container calls the _jspservice() for each request, passing it the request and the response objects. The jspDestroy()- The container calls this when it decides take the instance out of service. It is the last method called n the servlet instance. |
| Q: | How do I prevent the output of my JSP or Servlet pages from being cached by the browser? |
| A: | You will need to set the appropriate HTTP header attributes to prevent the dynamic content output by the JSP page from being cached by the browser. Just execute the following scriptlet at the beginning of your JSP pages to prevent them from being cached at the browser. You need both the statements to take care of some of the older browser versions. <% response.setHeader("Cache-Control","no-store"); //HTTP 1.1 response.setHeader("Pragma\","no-cache"); //HTTP 1.0 response.setDateHeader ("Expires", 0); //prevents caching at the proxy server %> |
| Q: | How does JSP handle run-time exceptions? |
| A: | You can use the errorPage attribute of the page directive to have uncaught run-time exceptions automatically forwarded to an error processing page. For example: <%@ page errorPage=\"error.jsp\" %> redirects the browser to the JSP page error.jsp if an uncaught exception is encountered during request processing. Within error.jsp, if you indicate that it is an error-processing page, via the directive: <%@ page isErrorPage=\"true\" %> Throwable object describing the exception may be accessed within the error page via the exception implicit object. Note: You must always use a relative URL as the value for the errorPage attribute. |
| Q: | How can I implement a thread-safe JSP page? What are the advantages and Disadvantages of using it? |
| A: | You can make your JSPs thread-safe by having them implement the SingleThreadModel interface. This is done by adding the directive <%@ page isThreadSafe="false" %> within your JSP page. With this, instead of a single instance of the servlet generated for your JSP page loaded in memory, you will have N instances of the servlet loaded and initialized, with the service method of each instance effectively synchronized. You can typically control the number of instances (N) that are instantiated for all servlets implementing SingleThreadModel through the admin screen for your JSP engine. More importantly, avoid using the tag for variables. If you do use this tag, then you should set isThreadSafe to true, as mentioned above. Otherwise, all requests to that page will access those variables, causing a nasty race condition. SingleThreadModel is not recommended for normal use. There are many pitfalls, including the example above of not being able to use <%! %>. You should try really hard to make them thread-safe the old fashioned way: by making them thread-safe . |
| Q: | How do I use a scriptlet to initialize a newly instantiated bean? |
| A: | A jsp:useBean action may optionally have a body. If the body is specified, its contents will be automatically invoked when the specified bean is instantiated. Typically, the body will contain scriptlets or jsp:setProperty tags to initialize the newly instantiated bean, although you are not restricted to using those alone. The following example shows the “today” property of the Foo bean initialized to the current date when it is instantiated. Note that here, we make use of a JSP expression within the jsp:setProperty action. <jsp:useBean id="foo" class="com.Bar.Foo" > <jsp:setProperty name="foo" property="today" value="<%=java.text.DateFormat.getDateInstance().format(new java.util.Date()) %>" / > <%-- scriptlets calling bean setter methods go here --%> </jsp:useBean > |
| Q: | How can I prevent the word "null" from appearing in my HTML input text fields when I populate them with a resultset that has null values? |
| A: | You could make a simple wrapper function, like <%! String blanknull(String s) { return (s == null) ? \"\" : s; } %> then use it inside your JSP form, like <input type="text" name="lastName" value="<%=blanknull(lastName)% >" > |
| Q: | What's a better approach for enabling thread-safe servlets and JSPs? SingleThreadModel Interface or Synchronization? |
| A: | Although the SingleThreadModel technique is easy to use, and works well for low volume sites, it does not scale well. If you anticipate your users to increase in the future, you may be better off implementing explicit synchronization for your shared data. The key however, is to effectively minimize the amount of code that is synchronzied so that you take maximum advantage of multithreading. Also, note that SingleThreadModel is pretty resource intensive from the server\'s perspective. The most serious issue however is when the number of concurrent requests exhaust the servlet instance pool. In that case, all the unserviced requests are queued until something becomes free - which results in poor performance. Since the usage is non-deterministic, it may not help much even if you did add more memory and increased the size of the instance pool. |
| Q: | How can I enable session tracking for JSP pages if the browser has disabled cookies? |
| A: | We know that session tracking uses cookies by default to associate a session identifier with a unique user. If the browser does not support cookies, or if cookies are disabled, you can still enable session tracking using URL rewriting. URL rewriting essentially includes the session ID within the link itself as a name/value pair. However, for this to be effective, you need to append the session ID for each and every link that is part of your servlet response. Adding the session ID to a link is greatly simplified by means of of a couple of methods: response.encodeURL() associates a session ID with a given URL, and if you are using redirection, response.encodeRedirectURL() can be used by giving the redirected URL as input. Both encodeURL() and encodeRedirectedURL() first determine whether cookies are supported by the browser; if so, the input URL is returned unchanged since the session ID will be persisted as a cookie. Consider the following example, in which two JSP files, say hello1.jsp and hello2.jsp, interact with each other. Basically, we create a new session within hello1.jsp and place an object within this session. The user can then traverse to hello2.jsp by clicking on the link present within the page. Within hello2.jsp, we simply extract the object that was earlier placed in the session and display its contents. Notice that we invoke the encodeURL() within hello1.jsp on the link used to invoke hello2.jsp; if cookies are disabled, the session ID is automatically appended to the URL, allowing hello2.jsp to still retrieve the session object. Try this example first with cookies enabled. Then disable cookie support, restart the brower, and try again. Each time you should see the maintenance of the session across pages. Do note that to get this example to work with cookies disabled at the browser, your JSP engine has to support URL rewriting. hello1.jsp <%@ page session=\"true\" %> <% Integer num = new Integer(100); session.putValue("num",num); String url =response.encodeURL("hello2.jsp"); %> <a href=\'<%=url%>\'>hello2.jsp</a> hello2.jsp <%@ page session="true" %> <% Integer i= (Integer )session.getValue("num"); out.println("Num value in session is " + i.intValue()); %> |
| Q: | Explain the life cycle methods of a Servlet. |
| A: | The javax.servlet.Servlet interface defines the three methods known as life-cycle method. public void init(ServletConfig config) throws ServletException public void service( ServletRequest req, ServletResponse res) throws ServletException, IOException public void destroy() First the servlet is constructed, then initialized wih the init() method. Any request from client are handled initially by the service() method before delegating to the doXxx() methods in the case of HttpServlet. The servlet is removed from service, destroyed with the destroy() methid, then garbaged collected and finalized. |
| Q: | What is the difference between the getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface and javax.servlet.ServletContext interface? |
| A: | The getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface accepts parameter the path to the resource to be included or forwarded to, which can be relative to the request of the calling servlet. If the path begins with a "/" it is interpreted as relative to the current context root. The getRequestDispatcher(String path) method of javax.servlet.ServletContext interface cannot accepts relative paths. All path must sart with a "/" and are interpreted as relative to curent context root. |
| Q: | Explain the directory structure of a web application. |
| A: | The directory structure of a web application consists of two parts. A private directory called WEB-INF A public resource directory which contains public resource folder. WEB-INF folder consists of 1. web.xml 2. classes directory 3. lib directory |
| Q: | What are the common mechanisms used for session tracking? |
| A: | Cookies SSL sessions URL- rewriting |
| Q: | Explain ServletContext. |
| A: | ServletContext interface is a window for a servlet to view it's environment. A servlet can use this interface to get information such as initialization parameters for the web applicationor servlet container's version. Every web application has one and only one ServletContext and is accessible to all active resource of that application. |
| Q: | What is preinitialization of a servlet? |
| A: | A container doesnot initialize the servlets ass soon as it starts up, it initializes a servlet when it receives a request for that servlet first time. This is called lazy loading. The servlet specification defines the <load-on-startup> element, which can be specified in the deployment descriptor to make the servlet container load and initialize the servlet as soon as it starts up. The process of loading a servlet before any request comes in is called preloading or preinitializing a servlet. |
| Q: | What is the difference between Difference between doGet() and doPost()? |
| A: | A doGet() method is limited with 2k of data to be sent, and doPost() method doesn't have this limitation. A request string for doGet() looks like the following: http://www.allapplabs.com/svt1?p1=v1&p2=v2&...&pN=vN doPost() method call doesn't need a long text tail after a servlet name in a request. All parameters are stored in a request itself, not in a request string, and it's impossible to guess the data transmitted to a servlet only looking at a request string. |
| Q: | What is the difference between HttpServlet and GenericServlet? |
| A: | A GenericServlet has a service() method aimed to handle requests. HttpServlet extends GenericServlet and adds support for doGet(), doPost(), doHead() methods (HTTP 1.0) plus doPut(), doOptions(), doDelete(), doTrace() methods (HTTP 1.1). Both these classes are abstract. |
| Q: | What are the different kinds of enterprise beans? | |
| A: | Different kind of enterrise beans are Stateless session bean, Stateful session bean, Entity bean, Message-driven bean........... | |
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| Q: | What is Session Bean? | |
| A: | A session bean is a non-persistent object that implements some business logic running on the server. One way to think of a session object........... | |
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| Q: | What is Entity Bean? | |
| A: | The entity bean is used to represent data in the database. It provides an object-oriented interface to ........... | |
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| Q: | What are the methods of Entity Bean? | |
| A: | An entity bean consists of 4 groups of methods, create methods........... | |
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| Q: | What is the difference between Container-Managed Persistent (CMP) bean and Bean-Managed Persistent(BMP) ? | |
| A: | Container-managed persistence (CMP) and bean-managed persistence (BMP). With CMP, the container manages the persistence of the entity bean............ | |
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| Q: | What are the callback methods in Entity beans? | |
| A: | Callback methods allows the container to notify the bean of events in its life cycle. The callback methods are defined in the javax.ejb.EntityBean interface............ | |
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| Q: | What is software architecture of EJB? | |
| A: | Session and Entity EJBs consist of 4 and 5 parts respectively, a remote interface........... | |
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| Q: | Can Entity Beans have no create() methods? | |
| A: | Yes. In some cases the data is inserted NOT using Java application,........... | |
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| Q: | What is bean managed transaction? | |
| A: | If a developer doesn't want a Container to manage transactions, it's possible to implement all database operations manually........... | |
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| Q: | What are transaction isolation levels in EJB? |
| A: | Transaction_read_uncommitted , Transaction_read_committed , Transaction_repeatable_read........... |
| Q: | How EJB Invocation happens? |
| A: | Step 1: Retrieve Home Object reference from Naming Service via JNDI. step 2: Return Home Object reference to the client. step 3: Create me a new EJB Object through Home Object interface. step 4: Create EJB Object from the Ejb Object step 5: Return EJB Object reference to the client. step 6: Invoke business method using EJB Object reference. step 7: Delegate request to Bean (Enterprise Bean). |
| Q: | Is it possible to share an HttpSession between a JSP and EJB? What happens when I change a value in the HttpSession from inside an EJB? |
| A: | You can pass the HttpSession as parameter to an EJB method, only if all objects in session are serializable.This has to be consider as ?passed-by-value", that means that it?s read-only in the EJB. If anything is altered from inside the EJB, it won?t be reflected back to the HttpSession of the Servlet Container.The ?pass-by-reference? can be used between EJBs Remote Interfaces, as they are remote references. While it IS possible to pass an HttpSession as a parameter to an EJB object, it is considered to be ?bad practice ? in terms of object oriented design. This is because you are creating an unnecessary coupling between back-end objects (ejbs) and front-end objects (HttpSession). Create a higher-level of abstraction for your ejb?s api. Rather than passing the whole, fat, HttpSession (which carries with it a bunch of http semantics), create a class that acts as a value object (or structure) that holds all the data you need to pass back and forth between front-end/back-end. Consider the case where your ejb needs to support a non-http-based client. This higher level of abstraction will be flexible enough to support it. |
| Q: | The EJB container implements the EJBHome and EJBObject classes. For every request from a unique client, does the container create a separate instance of the generated EJBHome and EJBObject classes? |
| A: | The EJB container maintains an instance pool. The container uses these instances for the EJB Home reference irrespective of the client request. while refering the EJB Object classes the container creates a separate instance for each client request. The instance pool maintainence is up to the implementation of the container. If the container provides one, it is available otherwise it is not mandatory for the provider to implement it. Having said that, yes most of the container providers implement the pooling functionality to increase the performance of the application server. The way it is implemented is again up to the implementer. |
| Q: | Can the primary key in the entity bean be a Java primitive type such as int? |
| A: | The primary key can't be a primitive type--use the primitive wrapper classes, instead. For example, you can use java.lang.Integer as the primary key class, but not int (it has to be a class, not a primitive) |
| Q: | Can you control when passivation occurs? |
| A: | The developer, according to the specification, cannot directly control when passivation occurs. Although for Stateful Session Beans, the container cannot passivate an instance that is inside a transaction. So using transactions can be a a strategy to control passivation. The ejbPassivate() method is called during passivation, so the developer has control over what to do during this exercise and can implement the require optimized logic. Some EJB containers, such as BEA WebLogic, provide the ability to tune the container to minimize passivation calls. Taken from the WebLogic 6.0 DTD -"The passivation-strategy can be either "default" or "transaction". With the default setting the container will attempt to keep a working set of beans in the cache. With the "transaction" setting, the container will passivate the bean after every transaction (or method call for a non-transactional invocation). |
| Q: | What is the advantage of using Entity bean for database operations, over directly using JDBC API to do database operations? When would I use one over the other? |
| A: | Entity Beans actually represents the data in a database. It is not that Entity Beans replaces JDBC API. There are two types of Entity Beans Container Managed and Bean Mananged. In Container Managed Entity Bean - Whenever the instance of the bean is created the container automatically retrieves the data from the DB/Persistance storage and assigns to the object variables in bean for user to manipulate or use them. For this the developer needs to map the fields in the database to the variables in deployment descriptor files (which varies for each vendor). In the Bean Managed Entity Bean - The developer has to specifically make connection, retrive values, assign them to the objects in the ejbLoad() which will be called by the container when it instatiates a bean object. Similarly in the ejbStore() the container saves the object values back the the persistance storage. ejbLoad and ejbStore are callback methods and can be only invoked by the container. Apart from this, when you use Entity beans you dont need to worry about database transaction handling, database connection pooling etc. which are taken care by the ejb container. But in case of JDBC you have to explicitly do the above features. what suresh told is exactly perfect. ofcourse, this comes under the database transations, but i want to add this. the great thing about the entity beans of container managed, whenever the connection is failed during the transaction processing, the database consistancy is mantained automatically. the container writes the data stored at persistant storage of the entity beans to the database again to provide the database consistancy. where as in jdbc api, we, developers has to do manually. |
| Q: | What is EJB QL? |
| A: | EJB QL is a Query Language provided for navigation across a network of enterprise beans and dependent objects defined by means of container managed persistence. EJB QL is introduced in the EJB 2.0 specification. The EJB QL query language defines finder methods for entity beans with container managed persistenceand is portable across containers and persistence managers. EJB QL is used for queries of two types of finder methods: Finder methods that are defined in the home interface of an entity bean and which return entity objects. Select methods, which are not exposed to the client, but which are used by the Bean Provider to select persistent values that are maintained by the Persistence Manager or to select entity objects that are related to the entity bean on which the query is defined. |
| Q: | Brief description about local interfaces? |
| A: | EEJB was originally designed around remote invocation using the Java Remote Method Invocation (RMI) mechanism, and later extended to support to standard CORBA transport for these calls using RMI/IIOP. This design allowed for maximum flexibility in developing applications without consideration for the deployment scenario, and was a strong feature in support of a goal of component reuse in J2EE. Many developers are using EJBs locally -- that is, some or all of their EJB calls are between beans in a single container. With this feedback in mind, the EJB 2.0 expert group has created a local interface mechanism. The local interface may be defined for a bean during development, to allow streamlined calls to the bean if a caller is in the same container. This does not involve the overhead involved with RMI like marshalling etc. This facility will thus improve the performance of applications in which co-location is planned. Local interfaces also provide the foundation for container-managed relationships among entity beans with container-managed persistence. |
| Q: | What are the special design care that must be taken when you work with local interfaces? |
| A: | EIt is important to understand that the calling semantics of local interfaces are different from those of remote interfaces. For example, remote interfaces pass parameters using call-by-value semantics, while local interfaces use call-by-reference. This means that in order to use local interfaces safely, application developers need to carefully consider potential deployment scenarios up front, then decide which interfaces can be local and which remote, and finally, develop the application code with these choices in mind. While EJB 2.0 local interfaces are extremely useful in some situations, the long-term costs of these choices, especially when changing requirements and component reuse are taken into account, need to be factored into the design decision. |
| Q: | What happens if remove( ) is never invoked on a session bean? |
| A: | In case of a stateless session bean it may not matter if we call or not as in both cases nothing is done. The number of beans in cache is managed by the container. In case of stateful session bean, the bean may be kept in cache till either the session times out, in which case the bean is removed or when there is a requirement for memory in which case the data is cached and the bean is sent to free pool. |
| Q: | What is the difference between Message Driven Beans and Stateless Session beans? |
| A: | In several ways, the dynamic creation and allocation of message-driven bean instances mimics the behavior of stateless session EJB instances, which exist only for the duration of a particular method call. However, message-driven beans are different from stateless session EJBs (and other types of EJBs) in several significant ways: Message-driven beans process multiple JMS messages asynchronously, rather than processing a serialized sequence of method calls. Message-driven beans have no home or remote interface, and therefore cannot be directly accessed by internal or external clients. Clients interact with message-driven beans only indirectly, by sending a message to a JMS Queue or Topic. Note: Only the container directly interacts with a message-driven bean by creating bean instances and passing JMS messages to those instances as necessary. The Container maintains the entire lifecycle of a message-driven bean; instances cannot be created or removed as a result of client requests or other API calls. |
| Q: | How can I call one EJB from inside of another EJB? |
| A: | EJBs can be clients of other EJBs. It just works. Use JNDI to locate the Home Interface of the other bean, then acquire an instance reference, and so forth. |
| Q: | What is an EJB Context? |
| A: | EJBContext is an interface that is implemented by the container, and it is also a part of the bean-container contract. Entity beans use a subclass of EJBContext called EntityContext. Session beans use a subclass called SessionContext. These EJBContext objects provide the bean class with information about its container, the client using the bean and the bean itself. They also provide other functions. See the API docs and the spec for more details. |
| Q: | What is JMS? |
| A: | JMS is an acronym used for Java Messaging Service. It is Java's answer to creating software using asynchronous messaging. It is one of the official specifications of the J2EE technologies and is a key technology. |
| Q: | How JMS is different from RPC? |
| A: | In RPC the method invoker waits for the method to finish execution and return the control back to the invoker. Thus it is completely synchronous in nature. While in JMS the message sender just sends the message to the destination and continues it's own processing. The sender does not wait for the receiver to respond. This is asynchronous behavior. |
| Q: | What are the advantages of JMS? |
| A: | JMS is asynchronous in nature. Thus not all the pieces need to be up all the time for the application to function as a whole. Even if the receiver is down the MOM will store the messages on it's behalf and will send them once it comes back up. Thus at least a part of application can still function as there is no blocking. |
| Q: | Are you aware of any major JMS products available in the market? |
| A: | IBM's MQ Series is one of the most popular product used as Message Oriented Middleware. Some of the other products are SonicMQ, iBus etc. Weblogic application server also comes with built in support for JMS messaging. |
| Q: | What are the different types of messages available in the JMS API? |
| A: | Message, TextMessage, BytesMessage, StreamMessage, ObjectMessage, MapMessage are the different messages available in the JMS API. |
| Q: | What are the different messaging paradigms JMS supports? |
| A: | Publish and Subscribe i.e. pub/suc and Point to Point i.e. p2p. |
| Q: | What is the difference between topic and queue? |
| A: | A topic is typically used for one to many messaging i.e. it supports publish subscribe model of messaging. While queue is used for one-to-one messaging i.e. it supports Point to Point Messaging. |
| Q: | What is the role of JMS in enterprise solution development? |
| A: | JMS is typically used in the following scenarios 1. Enterprise Application Integration: - Where a legacy application is integrated with a new application via messaging. 2. B2B or Business to Business: - Businesses can interact with each other via messaging because JMS allows organizations to cooperate without tightly coupling their business systems. 3. Geographically dispersed units: - JMS can ensure safe exchange of data amongst the geographically dispersed units of an organization. 4. One to many applications: - The applications that have to push data in packet to huge number of clients in a one-to-many fashion are good candidates for the use JMS. Typical such applications are Auction Sites, Stock Quote Services etc. |
| Q: | What is the use of Message object? |
| A: | Message is a light weight message having only header and properties and no payload. Thus if the received are to be notified abt an event, and no data needs to be exchanged then using Message can be very efficient. |
| Q: | What is the basic difference between Publish Subscribe model and P2P model? |
| A: | Publish Subscribe model is typically used in one-to-many situation. It is unreliable but very fast. P2P model is used in one-to-one situation. It is highly reliable. |
| Q: | What is the use of BytesMessage? |
| A: | BytesMessage contains an array of primitive bytes in it's payload. Thus it can be used for transfer of data between two applications in their native format which may not be compatible with other Message types. It is also useful where JMS is used purely as a transport between two systems and the message payload is opaque to the JMS client. Whenever you store any primitive type, it is converted into it's byte representation and then stored in the payload. There is no boundary line between the different data types stored. Thus you can even read a long as short. This would result in erroneous data and hence it is advisable that the payload be read in the same order and using the same type in which it was created by the sender. |
| Q: | What is the use of StreamMessage? |
| A: | StreamMessage carries a stream of Java primitive types as it's payload. It contains some conveient methods for reading the data stored in the payload. However StreamMessage prevents reading a long value as short, something that is allwed in case of BytesMessage. This is so because the StreamMessage also writes the type information alonwgith the value of the primitive type and enforces a set of strict conversion rules which actually prevents reading of one primitive type as another. |
| Q: | What is the use of TextMessage? |
| A: | TextMessage contains instance of java.lang.String as it's payload. Thus it is very useful for exchanging textual data. It can also be used for exchanging complex character data such as an XML document. |
| Q: | What is the use of ObjectMessage? |
| A: | ObjectMessage contains a Serializable java object as it's payload. Thus it allows exchange of Java objects between applications. This in itself mandates that both the applications be Java applications. The consumer of the message must typecast the object received to it's appropriate type. Thus the consumer should before hand know the actual type of the object sent by the sender. Wrong type casting would result in ClassCastException. Moreover the class definition of the object set in the payload should be available on both the machine, the sender as well as the consumer. If the class definition is not available in the consumer machine, an attempt to type cast would result in ClassNotFoundException. Some of the MOMs might support dynamic loading of the desired class over the network, but the JMS specification does not mandate this behavior and would be a value added service if provided by your vendor. And relying on any such vendor specific functionality would hamper the portability of your application. Most of the time the class need to be put in the classpath of both, the sender and the consumer, manually by the developer. |
| Q: | What is the use of MapMessage? |
| A: | A MapMessage carries name-value pair as it's payload. Thus it's payload is similar to the java.util.Properties object of Java. The values can be Java primitives or their wrappers. |
| Q: | What is the difference between BytesMessage and StreamMessage?? |
| A: | BytesMessage stores the primitive data types by converting them to their byte representation. Thus the message is one contiguous stream of bytes. While the StreamMessage maintains a boundary between the different data types stored because it also stores the type information along with the value of the primitive being stored. BytesMessage allows data to be read using any type. Thus even if your payload contains a long value, you can invoke a method to read a short and it will return you something. It will not give you a semantically correct data but the call will succeed in reading the first two bytes of data. This is strictly prohibited in the StreamMessage. It maintains the type information of the data being stored and enforces strict conversion rules on the data being read. |
| Q: | What is SQL? |
| A: | SQL stands for 'Structured Query Language'. |
| Q: | What is SELECT statement? |
| A: | The SELECT statement lets you select a set of values from a table in a database. The values selected from the database table would depend on the various conditions that are specified in the SQL query. |
| Q: | How can you compare a part of the name rather than the entire name? |
| A: | SELECT * FROM people WHERE empname LIKE '%ab%' Would return a recordset with records consisting empname the sequence 'ab' in empname . |
| Q: | What is the INSERT statement? |
| A: | The INSERT statement lets you insert information into a database. |
| Q: | How do you delete a record from a database? |
| A: | Use the DELETE statement to remove records or any particular column values from a database. |
| Q: | How could I get distinct entries from a table? |
| A: | The SELECT statement in conjunction with DISTINCT lets you select a set of distinct values from a table in a database. The values selected from the database table would of course depend on the various conditions that are specified in the SQL query. Example SELECT DISTINCT empname FROM emptable |
| Q: | How to get the results of a Query sorted in any order? |
| A: | You can sort the results and return the sorted results to your program by using ORDER BY keyword thus saving you the pain of carrying out the sorting yourself. The ORDER BY keyword is used for sorting. SELECT empname, age, city FROM emptable ORDER BY empname |
| Q: | How can I find the total number of records in a table? |
| A: | You could use the COUNT keyword , example SELECT COUNT(*) FROM emp WHERE age>40 |
| Q: | What is GROUP BY? |
| A: | The GROUP BY keywords have been added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called. Without the GROUP BY functionality, finding the sum for each individual group of column values was not possible. |
| Q: | What is the difference among "dropping a table", "truncating a table" and "deleting all records" from a table. |
| A: | Dropping : (Table structure + Data are deleted), Invalidates the dependent objects ,Drops the indexes Truncating: (Data alone deleted), Performs an automatic commit, Faster than delete Delete : (Data alone deleted), Doesn’t perform automatic commit |
| Q: | What are the Large object types suported by Oracle? |
| A: | Blob and Clob. |
| Q: | Difference between a "where" clause and a "having" clause. |
| A: | Having clause is used only with group functions whereas Where is not used with. |
| Q: | What's the difference between a primary key and a unique key? |
| A: | Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only. |
| Q: | What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors? |
| A: | Cursors allow row-by-row prcessing of the resultsets. Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. See books online for more information. Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip, where as a normal SELECT query makes only one rowundtrip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Furthere, there are restrictions on the SELECT statements that can be used with some types of cursors. Most of the times, set based operations can be used instead of cursors. |
| Q: | What are triggers? How to invoke a trigger on demand? |
| A: | Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table. Triggers can't be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens on the table on which they are defined. Triggers are generally used to implement business rules, auditing. Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster. |
| Q: | What is a join and explain different types of joins. |
| A: | Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table. Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS. |
| Q: | What is a self join? |
| A: | Self join is just like any other join, except that two instances of the same table will be joined in the query. |
The Java API is what makes Java attractive and useful. It consists of a set of packages that are distributed with the JDK as class libraries. These packages provide a common interface for developing Java programs on all Java platforms. The Java API furnishes all the capabilities needed to develop console programs, window programs, client and server networking software, applets, and other applications. It is the difference that takes Java from being a really good programming language to making it a very powerful and efficient programming environment.
The Java API consists of eight major development packages and a supporting debug package; Packages are collections of related objects. For example, there are separate packages for developing window programs, applets, and networking software.
Java packages contain classes, interfaces, and exceptions
Classes form the basis for object-oriented software development in Java. They contain variables and methods. Variables are data containers. Methods implement operations on the class and its variables.
Interfaces are collections of related methods. An example of an interface is the DataInput interfacee of the java.io package. It specifies a collection of methods that must be defined for a class to implement the DataInput interface. Exceptions are events that alter the normal course of program execution. Java applets
An applet is a mini-program that will run only under a Web browser. The applet is downloaded automatically as part of a Web page. When the applet is activated it executes a program. This is part of its beauty – it provides you with a way to automatically distribute the client software from the server at the time the user needs the client software, and no sooner. They get the latest version of the client software without fail and without difficult re-installation. Because of the way
Java is designed, the programmer needs to create only a single program, and that program automatically works with all computers that have browsers with built-in Java interpreters.
Applets are Java programs that execute within the context of a Web page. They interact with the user while his Web page is active and stop their execution when his Web page is no longer active. Applets are valuable because they are simple to use. A user only needs to open an applet's Web page to download and execute an applet.
Java applets, are run from inside a browser. A reference to an applet is embedded in a Web page using a special HTML tag. When a reader, using a Java-enabled browser, loads a Web page with an applet in it, the browser downloads that applet from a Web server and executes it on the local system Because Java applets run inside a Java browser, they have access to the structure the browser provides: an existing window, an event-handling and graphics context, and the surrounding user interface.
One advantage a Java applet has over a scripted program is that it’s in compiled form, so the source code isn’t available to the client.
Applet security is a major concern among Web users and applet developers. From a user's perspective, an exploitable applet security flaw could result in sensitive data being modified or disclosed, or their computer being rendered inoperable. From a developer's perspective, strong applet security is necessary to make Web users comfortable with using applets. However, too high a level of security limits their applets' capabilities. Struts
Struts is a open source framework which make building of the web applications easier based on the java Servlet and JavaServer pages technologies.
Struts framework was created by Craig R. McClanahan and donated to the Apache Software Foundation in 2000. The Project now has several committers, and many developers are contributing to overall to the framework.
Developing web application using struts frame work is fairly complex, but it eases things after it is setup. It encourages software development following the MVC design pattern. Many web applications are JSP-only or Servlets-only. With JSP and Servlets, Java code is embedded in the HTML code and the Java code calls println methods to generate the HTML code respectively. Both approaches have their advantages and drawbacks; Struts gathers their strengths to get the best of their association. Struts is based on Model-View-Controller (MVC) design paradigm, it is an implementation of JSP Model 2 Architecture. For more of Model-View-Controller (MVC) click here.
Consists of 8 Top-Level Packages and approx 250 Classes and Interfaces.
Struts is a set of cooperating classes, servlets, and JSP tags that make up a reusable MVC 2 design. This definition implies that Struts is a framework, rather than a library, but Struts also contains an extensive tag library and utility classes that work independently of the framework.
The overview of struts
Client browser
An HTTP request from the client browser creates an event. The Web container will respond with an HTTP response.
Controller
The controller is responsible for intercepting and translating user input into actions to
be performed by the model. The controller is responsible for selecting the next view based on user input and the outcome of model operations.The Controller receives the request from the browser, and makes the decision where to send the request. With Struts, the Controller is a command design pattern implemented as a servlet. The struts-config.xml file configures the Controller.
Business logic
The business logic updates the state of the model and helps control the flow of the application. With Struts this is done with an Action class as a thin wrapper to the actual business logic.
Model
A model represents an application’s data and contains the logic for accessing and manipulating that data. Any data that is part of the persistent state of the application should reside in the model objects. The business objects update the application state. ActionForm bean represents the Model state at a session or request level, and not at a persistent level. Model services are accessed by the controller for either querying or effecting a change in the model state. The model notifies the view when a state change occurs in the model.The JSP file reads information from the ActionForm bean using JSP tags.
View
The view is responsible for rendering the state of the model. The presentation semantics are encapsulated within the view, therefore model data can be adapted for several different kinds of clients.The view modifies itself when a change in the model is communicated to the view. A view forwards user input to the controller.The view is simply a JSP file. There is no flow logic, no business logic, and no model information -- just tags. Tags are one of the things that make Struts unique compared to other frameworks like Velocity. What is RmiJdbc ? RmiJdbc is a type 3 JDBC driver that can network any local JDBC API.
It is just a bridge, not a driver dedicated to a specific database: RmiJdbc works in conjunction with a local JDBC driver on the server side (often a type 1 or type 2 driver, that does not offer remote access facilities).
RmiJdbc is made of a server-side component (daemon), and client classes that implement the java.sql.* interfaces. All JDBC classes (like Connection, ResultSet, etc...) are distributed as RMI objects, so that you can distribute as you like the access to any database supporting the JDBC API.
Using RmiJdbc is a good idea... · to access remotely an ODBC datasource (suing the JDBC/ODBC bridge) · to network a database that only offers a local JDBC API (like InstantDB) Using RmiJdbc is a bad idea... · along with a database that already has a good type 4 driver (like Oracle, PostgreSQL or MySQL) - better use the specific product. · when using JDBc from an applet: RMI is not properly supported by Microsoft Internet Explorer, and using JDBC from an applet is not really a good idea indeed. Some specific usages: · If you write your own database with a JDBC API, you can use RmiJdbc to network it · If you need serializable JDBC classes, the RmiJdbc client classes are serializable. RmiJdbc Story RmiJdbc was born in September 1997, at the Inria research center located in Montbonnot, France. The product was financed by GIE Dyade, a joint venture between groupe Bull and Inria.
Like many open-source initiatives, RmiJdbc was first a personal work; but its success quickly made it an "official" project. It was ther release under the GPL (Gnu Public License).
At the end of 1999, I was thinking of hosting & managing RmiJdbc by myself: RmiJdbc had no more official existence at Inria (I had left a few months ago to create my own company), although it was still hosted on their servers.
The ObjectWeb team proposed me to host RmiJdbc and make it one of their projects: I immediately accepted the proposal, and RmiJdbc joined ObjectWeb in June 2000.
In may 2001, the RmiJdbc license moved from GPL to LGPL (Lesser Gnu Public License), a license we at ObjectWeb consider more open.
Since the beginning of the story, some major contributions came from developers all over the world: for example, JDBC 3.0 is supported, RmiJdbc can be run on a single port, SSL can be used to secure RmiJdbc - and most of this is due to external contributors, as well as many bug fixes and enhancements.
In the future, I will do my best to maintain RmiJdbc, and integrate later contributions as efficiently as I can. I hope you will find RmiJdbc friendly and useful, A Bit of History Of JVM The Java programming language is a general-purpose object-oriented concurrent language. Its syntax is similar to C and C++, but it omits many of the features that make C and C++ complex, confusing, and unsafe. The Java platform was initially developed to address the problems of building software for networked consumer devices. It was designed to support multiple host architectures and to allow secure delivery of software components. To meet these requirements, compiled code had to survive transport across networks, operate on any client, and assure the client that it was safe to run. The popularization of the World Wide Web made these attributes much more interesting. The Internet demonstrated how media-rich content could be made accessible in simple ways. Web browsers such as Mosaic enabled millions of people to roam the Net and made Web surfing part of popular culture. At last there was a medium where what you saw and heard was essentially the same whether you were using a Mac, PC, or UNIX machine, and whether you were connected to a high-speed network or a slow modem. Web enthusiasts soon discovered that the content supported by the Web's HTML document format was too limited. HTML extensions, such as forms, only highlighted those limitations, while making it clear that no browser could include all the features users wanted. Extensibility was the answer. Sun's HotJava browser showcases the interesting properties of the Java programming language and platform by making it possible to embed programs inside HTML pages. These programs are transparently downloaded into the HotJava browser along with the HTML pages in which they appear. Before being accepted by the browser, the programs are carefully checked to make sure they are safe. Like HTML pages, compiled programs are network- and host-independent. The programs behave the same way regardless of where they come from or what kind of machine they are being loaded into and run on. A Web browser incorporating the Java or Java 2 platform is no longer limited to a predetermined set of capabilities. Visitors to Web pages incorporating dynamic content can be assured that their machines cannot be damaged by that content. Programmers can write a program once, and it will run on any machine supplying a Java or Java 2 runtime environment. The Java Virtual Machine The Java virtual machine is the cornerstone of the Java and Java 2 platforms. It is the component of the technology responsible for its hardware- and operating system- independence, the small size of its compiled code, and its ability to protect users from malicious programs. The Java virtual machine is an abstract computing machine. Like a real computing machine, it has an instruction set and manipulates various memory areas at run time. It is reasonably common to implement a programming language using a virtual machine; the best-known virtual machine may be the P-Code machine of UCSD Pascal. The first prototype implementation of the Java virtual machine, done at Sun Microsystems, Inc., emulated the Java virtual machine instruction set in software hosted by a handheld device that resembled a contemporary Personal Digital Assistant (PDA). Sun's current Java virtual machine implementations, components of its JavaTM 2 SDK and JavaTM 2 Runtime Environment products, emulate the Java virtual machine on Win32 and Solaris hosts in much more sophisticated ways. However, the Java virtual machine does not assume any particular implementation technology, host hardware, or host operating system. It is not inherently interpreted, but can just as well be implemented by compiling its instruction set to that of a silicon CPU. It may also be implemented in microcode or directly in silicon. The Java virtual machine knows nothing of the Java programming language, only of a particular binary format, the class file format. A class file contains Java virtual machine instructions (or bytecodes) and a symbol table, as well as other ancillary information. For the sake of security, the Java virtual machine imposes strong format and structural constraints on the code in a class file. However, any language with functionality that can be expressed in terms of a valid class file can be hosted by the Java virtual machine. Attracted by a generally available, machine-independent platform, implementors of other languages are turning to the Java virtual machine as a delivery vehicle for their languages.
| 1. What is a transient variable? A transient variable is a variable that may not be serialized. 2. Which containers use a border Layout as their default layout? The window, Frame and Dialog classes use a border layout as their default layout. 3. Why do threads block on I/O? Threads block on I/O (that is enters the waiting state) so that other threads may execute while the I/O Operation is performed. 4. How are Observer and Observable used? Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects. 5. What is synchronization and why is it important? With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors. 6. Can a lock be acquired on a class? Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object. 7. What's new with the stop(), suspend() and resume() methods in JDK 1.2? The stop(), suspend() and resume() methods have been deprecated in JDK 1.2. 8. Is null a keyword? The null value is not a keyword. 9. What is the preferred size of a component? The preferred size of a component is the minimum component size that will allow the component to display normally. 10. What method is used to specify a container's layout? The setLayout() method is used to specify a container's layout. 11. Which containers use a FlowLayout as their default layout? The Panel and Applet classes use the FlowLayout as their default layout. 12. What state does a thread enter when it terminates its processing? When a thread terminates its processing, it enters the dead state. 13. What is the Collections API? The Collections API is a set of classes and interfaces that support operations on collections of objects. 14. which characters may be used as the second character of an identifier, but not as the first character of an identifier? The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier. 15. What is the List interface? The List interface provides support for ordered collections of objects. 16. How does Java handle integer overflows and underflows? It uses those low order bytes of the result that can fit into the size of the type allowed by the operation. 17. What is the Vector class? The Vector class provides the capability to implement a growable array of objects 18. What modifiers may be used with an inner class that is a member of an outer class? A (non-local) inner class may be declared as public, protected, private, static, final, or abstract. 19. What is an Iterator interface? The Iterator interface is used to step through the elements of a Collection. 20. What is the difference between the >> and >>> operators? The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out. 21. Which method of the Component class is used to set the position and size of a component? setBounds() 22. How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters? Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns. 23 What is the difference between yielding and sleeping? When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state. 24. Which java.util classes and interfaces support event handling? The EventObject class and the EventListener interface support event processing. 25. Is sizeof a keyword? The sizeof operator is not a keyword. 26. What are wrapper classes? Wrapper classes are classes that allow primitive types to be accessed as objects. 27. Does garbage collection guarantee that a program will not run out of memory? Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection. 28. What restrictions are placed on the location of a package statement within a source code file? A package statement must appear as the first line in a source code file (excluding blank lines and comments). 29. Can an object's finalize() method be invoked while it is reachable? An object's finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object's finalize() method may be invoked by other objects. 30. What is the immediate superclass of the Applet class? Panel 31. What is the difference between preemptive scheduling and time slicing? Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors. 32. Name three Component subclasses that support painting. The Canvas, Frame, Panel, and Applet classes support painting. 33. What value does readLine() return when it has reached the end of a file? The readLine() method returns null when it has reached the end of a file. 34. What is the immediate superclass of the Dialog class? Window. 35. What is clipping? Clipping is the process of confining paint operations to a limited area or shape. 36. What is a native method? A native method is a method that is implemented in a language other than Java. 37. Can a for statement loop indefinitely? Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ; 38. What are order of precedence and associativity, and how are they used? Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left 39. When a thread blocks on I/O, what state does it enter? A thread enters the waiting state when it blocks on I/O. 40. To what value is a variable of the String type automatically initialized? The default value of a String type is null. 41. What is the catch or declare rule for method declarations? If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause. 42. What is the difference between a MenuItem and a CheckboxMenuItem? The CheckboxMenuItem class extends the MenuItem class to support a menu item that may be checked or unchecked. 43. What is a task's priority and how is it used in scheduling? A task's priority is an integer value that identifies the relative order in which it should be executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks. 44. What class is the top of the AWT event hierarchy? The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy. 45. When a thread is created and started, what is its initial state? A thread is in the ready state after it has been created and started. 46. Can an anonymous class be declared as implementing an interface and extending a class? An anonymous class may implement an interface or extend a superclass, but may not be declared to do both. 47. What is the range of the short type? The range of the short type is -(2^15) to 2^15 - 1. 48. What is the range of the char type? The range of the char type is 0 to 2^16 - 1. 49. In which package are most of the AWT events that support the event-delegation model defined? Most of the AWT-related events of the event-delegation model are defined in the java.awt.event package. The AWTEvent class is defined in the java.awt package. 50. What is the immediate superclass of Menu? MenuItem 51. What is the purpose of finalization? The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected. 52. Which class is the immediate superclass of the MenuComponent class. Object 53. What invokes a thread's run() method? After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed. 54. What is the difference between the Boolean & operator and the && operator? If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped. 55. Name three subclasses of the Component class. Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or TextComponent 56. What is the GregorianCalendar class? The GregorianCalendar provides support for traditional Western calendars. 57. Which Container method is used to cause a container to be laid out and redisplayed? validate() 58. What is the purpose of the Runtime class? The purpose of the Runtime class is to provide access to the Java runtime system. 59. How many times may an object's finalize() method be invoked by the garbage collector? An object's finalize() method may only be invoked once by the garbage collector. 60. What is the purpose of the finally clause of a try-catch-finally statement? The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught. 61. What is the argument type of a program's main() method? A program's main() method takes an argument of the String[] type. 62. Which Java operator is right associative? The = operator is right associative. 63. What is the Locale class? The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.64. Can a double value be cast to a byte? Yes, a double value can be cast to a byte. 65. What is the difference between a break statement and a continue statement? A break statement results in the termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement. 66. What must a class do to implement an interface? It must provide all of the methods in the interface and identify the interface in its implements clause. 67. What method is invoked to cause an object to begin executing as a separate thread? The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread. 68. Name two subclasses of the TextComponent class. TextField and TextArea 69. What is the advantage of the event-delegation model over the earlier event-inheritance model? The event-delegation model has two advantages over the event-inheritance model. First, it enables event handling to be handled by objects other than the ones that generate the events (or their containers). This allows a clean separation between a component's design and its use. The other advantage of the event-delegation model is that it performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to repeatedly process unhandled events, as is the case of the event-inheritance model. 70. Which containers may have a MenuBar? Frame 71. How are commas used in the initialization and iteration parts of a for statement? Commas are used to separate multiple statements within the initialization and iteration parts of a for statement. 72. What is the purpose of the wait(), notify(), and notifyAll() methods? The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread executes an object's wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object's notify() or notifyAll() methods. 73. What is an abstract method? An abstract method is a method whose implementation is deferred to a subclass. 74. How are Java source code files named? A Java source code file takes the name of a public class or interface that is defined within the file. A source code file may contain at most one public class or interface. If a public class or interface is defined within a source code file, then the source code file must take the name of the public class or interface. If no public class or interface is defined within a source code file, then the file must take on a name that is different than its classes and interfaces. Source code files use the .java extension. 75. What is the relationship between the Canvas class and the Graphics class? A Canvas object provides access to a Graphics object via its paint() method. 76. What are the high-level thread states? The high-level thread states are ready, running, waiting, and dead. 77. What value does read() return when it has reached the end of a file? The read() method returns -1 when it has reached the end of a file. 78. Can a Byte object be cast to a double value? No, an object cannot be cast to a primitive value. 79. What is the difference between a static and a non-static inner class? A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances. 80. What is the difference between the String and StringBuffer classes? String objects are constants. StringBuffer objects are not. 81. If a variable is declared as private, where may the variable be accessed? A private variable may only be accessed within the class in which it is declared. 82. What is an object's lock and which objects have locks? An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object. 83. What is the Dictionary class? The Dictionary class provides the capability to store key-value pairs. 84. How are the elements of a BorderLayout organized? The elements of a BorderLayout are organized at the borders (North, South, East, and West) and the center of a container. 85. What is the % operator? It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand. 86. When can an object reference be cast to an interface reference? An object reference be cast to an interface reference when the object implements the referenced interface. 87. What is the difference between a Window and a Frame? The Frame class extends Window to define a main application window that can have a menu bar. 88. Which class is extended by all other classes? The Object class is extended by all other classes. 89. Can an object be garbage collected while it is still reachable? A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected.. 90. Is the ternary operator written x : y ? z or x ? y : z ? It is written x ? y : z. 91. What is the difference between the Font and FontMetrics classes? The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object. 92. How is rounding performed under integer division? The fractional part of the result is truncated. This is known as rounding toward zero. 93. What happens when a thread cannot acquire a lock on an object? If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available. 94. What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy? The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented. 95. What classes of exceptions may be caught by a catch clause? A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types. 96. If a class is declared without any access modifiers, where may the class be accessed? A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package. 97. What is the SimpleTimeZone class? The SimpleTimeZone class provides support for a Gregorian calendar. 98. What is the Map interface? The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.99. Does a class inherit the constructors of its superclass? A class does not inherit constructors from any of its super classes. 100. For which statements does it make sense to use a label? The only statements for which it makes sense to use a label are those statements that can enclose a break or continue statement. 101. What is the purpose of the System class? The purpose of the System class is to provide access to system resources. 102. Which TextComponent method is used to set a TextComponent to the read-only state? setEditable() 103. How are the elements of a CardLayout organized? The elements of a CardLayout are stacked, one on top of the other, like a deck of cards. 104. Is &&= a valid Java operator? No, it is not. 105. Name the eight primitive Java types. The eight primitive types are byte, char, short, int, long, float, double, and boolean. 106. Which class should you use to obtain design information about an object? The Class class is used to obtain information about an object's design. 107. What is the relationship between clipping and repainting? When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting. 108. Is "abc" a primitive value? The String literal "abc" is not a primitive value. It is a String object. 109. What is the relationship between an event-listener interface and an event-adapter class? An event-listener interface defines the methods that must be implemented by an event handler for a particular kind of event. An event adapter provides a default implementation of an event-listener interface. 110. What restrictions are placed on the values of each case of a switch statement? During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value. 111. What modifiers may be used with an interface declaration? An interface may be declared as public or abstract. 112. Is a class a subclass of itself? A class is a subclass of itself. 113. What is the highest-level event class of the event-delegation model? The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy. 114. What event results from the clicking of a button? The ActionEvent event is generated as the result of the clicking of a button. 115. How can a GUI component handle its own events? A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener. 116. What is the difference between a while statement and a do statement? A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once. 117. How are the elements of a GridBagLayout organized? The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes. 118. What advantage do Java's layout managers provide over traditional windowing systems? Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems. 119. What is the Collection interface? The Collection interface provides support for the implementation of a mathematical bag - an unordered collection of objects that may contain duplicates. 120. What modifiers can be used with a local inner class? A local inner class may be final or abstract. 121. What is the difference between static and non-static variables? A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance. 122. What is the difference between the paint() and repaint() methods? The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread. 123. What is the purpose of the File class? The File class is used to create objects that provide access to the files and directories of a local file system. 124. Can an exception be rethrown? Yes, an exception can be rethrown. 125. Which Math method is used to calculate the absolute value of a number? The abs() method is used to calculate absolute values. 126. How does multithreading take place on a computer with a single CPU? The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially. 127. When does the compiler supply a default constructor for a class? The compiler supplies a default constructor for a class if no other constructors are provided. 128. When is the finally clause of a try-catch-finally statement executed? The finally clause of the try-catch-finally statement is always executed unless the thread of execution terminates or an exception occurs within the execution of the finally clause. 129. Which class is the immediate superclass of the Container class? Component 130. If a method is declared as protected, where may the method be accessed? A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared. 131. How can the Checkbox class be used to create a radio button? By associating Checkbox objects with a CheckboxGroup. 132. Which non-Unicode letter characters may be used as the first character of an identifier? The non-Unicode letter characters $ and _ may appear as the first character of an identifier 133. What restrictions are placed on method overloading? Two methods may not have the same name and argument list but different return types. 134. What happens when you invoke a thread's interrupt method while it is sleeping or waiting? When a task's interrupt() method is executed, the task enters the ready state. The next time the task enters the running state, an InterruptedException is thrown. 135. What is casting? There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference. 136. What is the return type of a program's main() method? A program's main() method has a void return type. 137. Name four Container classes. Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane 138. What is the difference between a Choice and a List? A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices. Only one item may be selected from a Choice. A List may be displayed in such a way that several List items are visible. A List supports the selection of one or more List items. 139. What class of exceptions are generated by the Java run-time system? The Java runtime system generates RuntimeException and Error exceptions. 140. What class allows you to read objects directly from a stream? The ObjectInputStream class supports the reading of objects from input streams. 141. What is the difference between a field variable and a local variable? A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method. 142. Under what conditions is an object's finalize() method invoked by the garbage collector? The garbage collector invokes an object's finalize() method when it detects that the object has become unreachable. 143. How are this () and super () used with constructors? this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor. 144. What is the relationship between a method's throws clause and the exceptions that can be thrown during the method's execution? A method's throws clause must declare any checked exceptions that are not caught within the body of the method. 145. What is the difference between the JDK 1.02 event model and the event-delegation model introduced with JDK 1.1? The JDK 1.02 event model uses an event inheritance or bubbling approach. In this model, components are required to handle their own events. If they do not handle a particular event, the event is inherited by (or bubbled up to) the component's container. The container then either handles the event or it is bubbled up to its container and so on, until the highest-level container has been tried. In the event-delegation model, specific objects are designated as event handlers for GUI components. These objects implement event-listener interfaces. The event-delegation model is more efficient than the event-inheritance model because it eliminates the processing required to support the bubbling of unhandled events. 146. How is it possible for two String objects with identical values not to be equal under the == operator? The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory. 147. Why are the methods of the Math class static? So they can be invoked as if they are a mathematical code library. 148. What Checkbox method allows you to tell if a Checkbox is checked? getState() 149. What state is a thread in when it is executing? An executing thread is in the running state. 150. What are the legal operands of the instanceof operator? The left operand is an object reference or null value and the right operand is a class, interface, or array type. 151. How are the elements of a GridLayout organized? The elements of a GridBad layout are of equal size and are laid out using the squares of a grid. 152. What an I/O filter? An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another. 153. If an object is garbage collected, can it become reachable again? Once an object is garbage collected, it ceases to exist. It can no longer become reachable again. 154. What is the Set interface? The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements. 155. What classes of exceptions may be thrown by a throw statement? A throw statement may throw any expression that may be assigned to the Throwable type. 156. What are E and PI? E is the base of the natural logarithm and PI is mathematical value pi. 157. Are true and false keywords? The values true and false are not keywords. 158. What is a void return type? A void return type indicates that a method does not return a value. 159. What is the purpose of the enableEvents() method? The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods. 160. What is the difference between the File and RandomAccessFile classes? The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file. 161. What happens when you add a double value to a String? The result is a String object.162. What is your platform's default character encoding? If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1.. 163. Which package is always imported by default? The java.lang package is always imported by default. 164. What interface must an object implement before it can be written to a stream as an object? An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object. 165. How are this and super used? this is used to refer to the current object instance. super is used to refer to the variables and methods of the superclass of the current object instance. 166. What is the purpose of garbage collection? The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed and reused. 167. What is a compilation unit? A compilation unit is a Java source code file. 168. What interface is extended by AWT event listeners? All AWT event listeners extend the java.util.EventListener interface. 169. What restrictions are placed on method overriding? Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides. The overriding method may not throw any exceptions that may not be thrown by the overridden method. 170. How can a dead thread be restarted? A dead thread cannot be restarted. 171. What happens if an exception is not caught? An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup being invoked, which eventually results in the termination of the program in which it is thrown. 172. What is a layout manager? A layout manager is an object that is used to organize components in a container. 173. Which arithmetic operations can result in the throwing of an ArithmeticException? Integer / and % can result in the throwing of an ArithmeticException. 174. What are three ways in which a thread can enter the waiting state? A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method. 175. Can an abstract class be final? An abstract class may not be declared as final.176. What is the ResourceBundle class? The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run. 177. What happens if a try-catch-finally statement does not have a catch clause to handle an exception that is thrown within the body of the try statement? The exception propagates up to the next higher level try-catch statement (if any) or results in the program's termination. 178. What is numeric promotion? Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place. In numerical promotion, byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required. 179. What is the difference between a Scrollbar and a ScrollPane? A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling. 180. What is the difference between a public and a non-public class? A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package. 181. To what value is a variable of the boolean type automatically initialized? The default value of the boolean type is false. 182. Can try statements be nested? Try statements may be tested. 183. What is the difference between the prefix and postfix forms of the ++ operator? The prefix form performs the increment operation and returns the value of the increment operation. The postfix form returns the current value all of the expression and then performs the increment operation on that value. 184. What is the purpose of a statement block? A statement block is used to organize a sequence of statements as a single statement group. 185. What is a Java package and how is it used? A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces. 186. What modifiers may be used with a top-level class? A top-level class may be public, abstract, or final. 187. What are the Object and Class classes used for? The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program. 188. How does a try statement determine which catch clause should be used to handle an exception? When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored. 189. Can an unreachable object become reachable again? An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.190. When is an object subject to garbage collection? An object is subject to garbage collection when it becomes unreachable to the program in which it is used. 191. What method must be implemented by all threads? All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface. 192. What methods are used to get and set the text label displayed by a Button object? getLabel() and setLabel() 193. Which Component subclass is used for drawing and painting? Canvas 194. What are synchronized methods and synchronized statements? Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement195. What are the two basic ways in which classes that can be run as threads may be defined? A thread class may be declared as a subclass of Thread, or it may implement the Runnable interface. 196. What are the problems faced by Java programmers who don't use layout managers? Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system. 197. What is the difference between an if statement and a switch statement? The if statement is used to select among two alternatives. It uses a boolean expression to decide which alternative should be executed. The switch statement is used to select among multiple alternatives. It uses an int expression to determine which alternative should be executed. |
