EJB E-book

Enterprise JavaBeans (EJB) technology is the server-side component architecture for Java Platform, Enterprise Edition (Java EE). EJB technology enables rapid and simplified development of distributed, transactional, secure and portable applications based on Java technology.

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CMP Bean in NetBeans 5.5.1 using MS SQL Server 2005
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EJB [Stateless SessionBean] Steps
EJB [Stateless SessionBean]
Enhancements in Java 6
HeadFirst-EJB
Internationalization in Java
Java Beans
ava Mail using Swing
MDB using SJS App Server 8.2 in NetBeans 5.0
Manning.EJB.3.in.Action.Apr.2007
Mastering Enterprise Java Beans
MasteringEJB3rdEd
O'Reilly - Enterprise Java Beans
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Enterprise JavaBeans, 3rd Edition
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Java E-book

A high-level programming language developed by Sun Microsystems. Java was originally called OAK, and was designed for handheld devices and set-top boxes. Oak was unsuccessful so in 1995 Sun changed the name to Java and modified the language to take advantage of the burgeoning World Wide Web.

Java is an object-oriented language similar to C++, but simplified to eliminate language features that cause common programming errors. Java source code files (files with a .java extension) are compiled into a format called bytecode (files with a .class extension), which can then be executed by a Java interpreter. Compiled Java code can run on most computers because Java interpreters and runtime environments, known as Java Virtual Machines (VMs), exist for most operating systems, including UNIX, the Macintosh OS, and Windows. Bytecode can also be converted directly into machine language instructions by a just-in-time compiler (JIT).

Java is a general purpose programming language with a number of features that make the language well suited for use on the World Wide Web. Small Java applications are called Java applets and can be downloaded from a Web server and run on your computer by a Java-compatible Web browser, such as Netscape Navigator or Microsoft Internet Explorer.

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Easy way to study Java :: JInternalFrame

Java Swing Tutorial Explaining the JInternalFrame class. A JInternalFrame is confined to a visible area of a container it is placed in. JInternalFrame a top level swing component that has a contentpane.

• It can be iconified — in this case the icon remains in the main application container.
• It can be maximized — Frame consumes the main application
• It can be closed using standard popup window controls
• It can be layered

JInternalFrame Source Code

import javax.swing.JInternalFrame; import javax.swing.JDesktopPane; import javax.swing.JMenu; import javax.swing.JMenuItem; import javax.swing.JMenuBar; import javax.swing.JFrame; import java.awt.event.*; import java.awt.*; public class JInternalFrameDemo extends JFrame { JDesktopPane jdpDesktop; static int openFrameCount = 0; public JInternalFrameDemo() { super("JInternalFrame Usage Demo"); // Make the main window positioned as 50 pixels from each edge of the // screen. int inset = 50; Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize(); setBounds(inset, inset, screenSize.width - inset * 2, screenSize.height - inset * 2); // Add a Window Exit Listener addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); // Create and Set up the GUI. jdpDesktop = new JDesktopPane(); // A specialized layered pane to be used with JInternalFrames createFrame(); // Create first window setContentPane(jdpDesktop); setJMenuBar(createMenuBar()); // Make dragging faster by setting drag mode to Outline jdpDesktop.putClientProperty("JDesktopPane.dragMode", "outline"); } protected JMenuBar createMenuBar() { JMenuBar menuBar = new JMenuBar(); JMenu menu = new JMenu("Frame"); menu.setMnemonic(KeyEvent.VK_N); JMenuItem menuItem = new JMenuItem("New IFrame"); menuItem.setMnemonic(KeyEvent.VK_N); menuItem.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e) { createFrame(); } }); menu.add(menuItem); menuBar.add(menu); return menuBar; } protected void createFrame() { MyInternalFrame frame = new MyInternalFrame(); frame.setVisible(true); // Every JInternalFrame must be added to content pane using JDesktopPane jdpDesktop.add(frame); try { frame.setSelected(true); } catch (java.beans.PropertyVetoException e) { } } public static void main(String[] args) { JInternalFrameDemo frame = new JInternalFrameDemo(); frame.setVisible(true); } class MyInternalFrame extends JInternalFrame { static final int xPosition = 30, yPosition = 30; public MyInternalFrame() { super("IFrame #" + (++openFrameCount), true, // resizable true, // closable true, // maximizable true);// iconifiable setSize(300, 300); // Set the window's location. setLocation(xPosition * openFrameCount, yPosition * openFrameCount); } } }

Output

Java JInternalFrame Hierarchy

javax.swing
Class JInternalFrame
java.lang.Object
|
+–java.awt.Component
|
+–java.awt.Container
|
+–javax.swing.JComponent
|
+–javax.swing.JInternalFrame
All Implemented Interfaces:
Accessible, ImageObserver, MenuContainer, RootPaneContainer, Serializable, WindowConstants

JInternalFrame Constructor

JInternalFrame()
Creates a non-resizable, non-closable, non-maximizable, non-iconifiable JInternalFrame with no title.
JInternalFrame(String title)
Creates a non-resizable, non-closable, non-maximizable, non-iconifiable JInternalFrame with the specified title.

JInternalFrame(String title, boolean resizable)
Creates a non-closable, non-maximizable, non-iconifiable JInternalFrame with the specified title and resizability.

JInternalFrame(String title, boolean resizable, boolean closable)
Creates a non-maximizable, non-iconifiable JInternalFrame with the specified title, resizability, and closability.

JInternalFrame(String title, boolean resizable, boolean closable, boolean maximizable)
Creates a non-iconifiable JInternalFrame with the specified title, resizability, closability, and maximizability.

JInternalFrame(String title, boolean resizable, boolean closable, boolean maximizable, boolean iconifiable)
Creates a JInternalFrame with the specified title, resizability, closability, maximizability

East way to study Java :: JFrame

Java Swing Tutorial Explaining the JFrame class. The components added to the frame are referred to as its contents; these are managed by the contentPane. To add a component to a JFrame, we must use its contentPane instead.JFrame is a Window with border, title and buttons. When JFrame is set visible, an event dispatching thread is started. JFrame objects store several objects including a Container object known as the content pane. To add a component to a JFrame, add it to the content pane.

JFrame Features

It’s a window with title, border, (optional) menu bar and user-specified components.
It can be moved, resized, iconified.
It is not a subclass of JComponent.
Delegates responsibility of managing user-specified components to a content pane, an instance of JPanel.

Centering JFrame’s

By default, a Jframe is displayed in the upper-left corner of the screen. To display a frame
at a specified location, you can use the setLocation(x, y) method in the JFrame class. This
method places the upper-left corner of a frame at location (x, y).

The Swing API keeps improving with abstractions such as the setDefaultCloseOperation method
for the JFrame

Crating a JFrame Window

Step 1: Construct an object of the JFrame class.

Step 2: Set the size of the Jframe.

Step 3: Set the title of the Jframe to appear in the title bar (title bar will be blank if no title is set).

Step 4: Set the default close operation. When the user clicks the close button, the program stops running.

Step 5: Make the Jframe visible.

How to position JFrame on Screen?

frame.setLocationRelativeTo( null );

JFrame Source Code

import java.awt.*; import java.awt.event.*; import javax.swing.*; public class JFrameDemo { public static void main(String s[]) { JFrame frame = new JFrame("JFrame Source Demo"); // Add a window listner for close button frame.addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); // This is an empty content area in the frame JLabel jlbempty = new JLabel(""); jlbempty.setPreferredSize(new Dimension(175, 100)); frame.getContentPane().add(jlbempty, BorderLayout.CENTER); frame.pack(); frame.setVisible(true); } }

Output

Java JFrame Hierarchy

javax.swing
Class JFrame
java.lang.Object
java.awt.Component
java.awt.Container
java.awt.Window
java.awt.Frame
javax.swing.JFrame
All Implemented Interfaces:
Accessible, ImageObserver, MenuContainer, RootPaneContainer, Serializable, WindowConstants

JFrame Constructor

JFrame()
Constructs a new frame that is initially invisible.

JFrame(GraphicsConfiguration gc)
Creates a Frame in the specified GraphicsConfiguration of a screen device and a blank title.

JFrame(String title)
Creates a new, initially invisible Frame with the specified title.

JFrame(String title, GraphicsConfiguration gc)
Creates a JFrame with the specified title and the specified GraphicsConfiguration of a screen device.

Easy way to study JAVA :: Swing

What is Swings in java ?

• A part of The JFC
• Swing Java consists of
  Look and feel
  Accessibility
  Java 2D
  Drag and Drop, etc
• Compiling & running programs
• if you do not explicitly add a GUI component to a container, the GUI component will not be displayed when the container appears on the screen.

Swing Model/view design: The “view part” of the MV design is implemented with a component object and the UI object. The “model part” of the MV design is implemented by a model object and a change listener object

Swing is built on top of AWT and is entirely written in Java, using AWT’s lightweight component support. In particular, unlike AWT, t he architecture of Swing components makes it easy to customize both their appearance and behavior. Components from AWT and Swing can be mixed, allowing you to add Swing support to existing AWT-based programs. For example, swing components such as JSlider, JButton and JCheckbox could be used in the same program with standard AWT labels, textfields and scrollbars. You could subclass the existing Swing UI, model, or change listener classes without having to reinvent the entire implementation. Swing also has the ability to replace these objects on-the-fly.

100% Java implementation of components Pluggable Look & Feel Lightweight components Uses MVC Architecture Model represents the data View as a visual representation of the data Controller takes input and translates it to changes in data Three parts Component set (subclasses of JComponent) Support classes Interfaces

In Swing, classes that represent GUI components have names beginning with the letter J. Some examples are JButton, JLabel, and JSlider. Altogether there are more than 250 new classes and 75 interfaces in Swing — twice as many as in AWT.

Java Swing class hierarchy

The class JComponent, descended directly from Container, is the root class for most of Swing’s user interface components.

Swing contains components that you’ll use to build a GUI. I am listing you some of the commonly used Swing components. To learn and understand these swing programs, AWT Programming knowledge is not required.

Java Swing Examples

Below is a java swing code for the traditional Hello World program.

Basically, the idea behind this Hello World program is to learn how to create a java program, compile and run it. To create your java source code you can use any editor( Text pad/Edit plus are my favorites) or you can use an IDE like Eclipse.

import javax.swing.JFrame; import javax.swing.JLabel; //import statements //Check if window closes automatically. Otherwise add suitable code public class HelloWorldFrame extends JFrame { public static void main(String args[]) { new HelloWorldFrame(); } HelloWorldFrame() { JLabel jlbHelloWorld = new JLabel("Hello World"); add(jlbHelloWorld); this.setSize(100, 100); // pack(); setVisible(true); } }

Output

Note: Below are some links to java swing tutorials that forms a helping hand to get started with java programming swing.

• JPanel is Swing’s version of the AWT class Panel and uses the same default layout, FlowLayout. JPanel is descended directly from JComponent.
• JFrame is Swing’s version of Frame and is descended directly from that class. The components added to the frame are referred to as its contents; these are managed by the contentPane. To add a component to a JFrame, we must use its contentPane instead.
• JInternalFrame is confined to a visible area of a container it is placed in. It can be iconified , maximized and layered.
• JWindow is Swing’s version of Window and is descended directly from that class. Like Window, it uses BorderLayout by default.
• JDialog is Swing’s version of Dialog and is descended directly from that class. Like Dialog, it uses BorderLayout by default. Like JFrame and JWindow,
  JDialog contains a rootPane hierarchy including a contentPane, and it allows layered and glass panes. All dialogs are modal, which means the current
  thread is blocked until user interaction with it has been completed. JDialog class is intended as the basis for creating custom dialogs; however, some
  of the most common dialogs are provided through static methods in the class JOptionPane.
• JLabel, descended from JComponent, is used to create text labels.
• The abstract class AbstractButton extends class JComponent and provides a foundation for a family of button classes, including
  JButton.
• JTextField allows editing of a single line of text. New features include the ability to justify the text left, right, or center, and to set the text’s font.
• JPasswordField (a direct subclass of JTextField) you can suppress the display of input. Each character entered can be replaced by an echo character.
  This allows confidential input for passwords, for example. By default, the echo character is the asterisk, *.
• JTextArea allows editing of multiple lines of text. JTextArea can be used in conjunction with class JScrollPane to achieve scrolling. The underlying JScrollPane can be forced to always or never have either the vertical or horizontal scrollbar;
  JButton is a component the user clicks to trigger a specific action.
• JRadioButton is similar to JCheckbox, except for the default icon for each class. A set of radio buttons can be associated as a group in which only
  one button at a time can be selected.
• JCheckBox is not a member of a checkbox group. A checkbox can be selected and deselected, and it also displays its current state.
• JComboBox is like a drop down box. You can click a drop-down arrow and select an option from a list. For example, when the component has focus,
  pressing a key that corresponds to the first character in some entry’s name selects that entry. A vertical scrollbar is used for longer lists.
• JList provides a scrollable set of items from which one or more may be selected. JList can be populated from an Array or Vector. JList does not
  
  support scrolling directly, instead, the list must be associated with a scrollpane. The view port used by the scroll pane can also have a user-defined
  border. JList actions are handled using ListSelectionListener.
• JTabbedPane contains a tab that can have a tool tip and a mnemonic, and it can display both text and an image.
• JToolbar contains a number of components whose type is usually some kind of button which can also include separators to group related components
  within the toolbar.
• FlowLayout when used arranges swing components from left to right until there’s no more space available. Then it begins a new row below it and moves
  from left to right again. Each component in a FlowLayout gets as much space as it needs and no more.
• BorderLayout places swing components in the North, South, East, West and center of a container. You can add horizontal and vertical gaps between
  the areas.
• GridLayout is a layout manager that lays out a container’s components in a rectangular grid. The container is divided into equal-sized rectangles,
  and one component is placed in each rectangle.
• GridBagLayout is a layout manager that lays out a container’s components in a grid of cells with each component occupying one or more cells,
  called its display area. The display area aligns components vertically and horizontally, without requiring that the components be of the same size.
• JMenubar can contain several JMenu’s. Each of the JMenu’s can contain a series of JMenuItem ’s that you can select. Swing provides support for
  pull-down and popup menus.
• Scrollable JPopupMenu is a scrollable popup menu that can be used whenever we have so many items in a popup menu that exceeds the screen visible height.
  

  Java Swing Projects

• Java Swing Calculator developed using Java Swing. It is a basic four-function calculator java program source code.
• Java Swing Address Book demonstrates how to create a simple free address book program using java swing and jdbc. Also you will learn to use
  the following swing components like Jbuttons, JFrames, JTextFields and Layout Manager (GridBagLayout).

Why Web Services are needed in Industry

Basically Web services mitigate the application integration crisis. It helps integrating applications at a significantly lower price point than any other integration technology.

It’s a new kind of middleware based on XML and the Web. XML and the Web help solve the challenges associated with traditional application-to-application integration like heterogeneity. They are platform and language independent.

Web Services has following advantages:

1. Exposing your API onto a network Connecting Different Applications
2. Low Cost of communication
3. Support for Loosely Coupled Applications
4. Web Services are Self Describing using WSDL
5. Automatic Discovery using UDDI
6. Business Opportunity to grow your business

InterView Java :: Question vs Answer 7

Q: Is Empty .java file a valid source file?
A: Yes, an empty .java file is a perfectly valid source file.

Q: Can a .java file contain more than one java classes?
A: Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.

Q: Is String a primitive data type in Java?
A: No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.

Q: Is main a keyword in Java?
A: No, main is not a keyword in Java.

Q: Is next a keyword in Java?
A: No, next is not a keyword.

Q: Is delete a keyword in Java?
A: No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.

Q: Is exit a keyword in Java?
A: No. To exit a program explicitly you use exit method in System object.

Q: What happens if you dont initialize an instance variable of any of the primitive types in Java?
A: Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false.

Q: What will be the initial value of an object reference which is defined as an instance variable?
A: The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references.

Q: What are the different scopes for Java variables?
A: The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.
2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.

Q: What is the default value of the local variables?
A: The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized..

Q: How many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();
A: Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.

Q: Can a public class MyClass be defined in a source file named YourClass.java?
A: No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.

Q: Can main method be declared final?
A: Yes, the main method can be declared final, in addition to being public static.

Q: What will be the output of the following statement?
System.out.println ("1" + 3);
A: It will print 13.

Q: What will be the default values of all the elements of an array defined as an instance variable?
A: If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.

InterView Java :: Question vs Answer 6

Q: What is daemon thread and which method is used to create the daemon thread?
A: 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.

Q: Can applets communicate with each other?
A: At this point in time applets may communicate with other applets running in the same virtual machine. If the applets are of the same class, they can communicate via shared static variables. If the applets are of different classes, then each will need a reference to the same class with static variables. In any case the basic idea is to pass the information back and forth through a static variable.

An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its public members.

It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial.

Q: What are the steps in the JDBC connection?
A: While making a JDBC connection we go through the following steps :

Step 1 : Register the database driver by using :

Class.forName(\" driver classs for that specific database\" );

Step 2 : Now create a database connection using :

Connection con = DriverManager.getConnection(url,username,password);

Step 3: Now Create a query using :

Statement stmt = Connection.Statement(\"select * from TABLE NAME\");

Step 4 : Exceute the query :

stmt.exceuteUpdate();

Q: How does a try statement determine which catch clause should be used to handle an exception?
A: 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 exceptionis executed. The remaining catch clauses are ignored.

Q: Can an unreachable object become reachable again?
A: 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.

Q: What method must be implemented by all threads?
A: All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

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: What is Externalizable?
A: Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)

Q: What modifiers are allowed for methods in an Interface?
A: Only public and abstract modifiers are allowed for methods in interfaces.

Q: What are some alternatives to inheritance?
A: Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).

Q: What does it mean that a method or field is "static"?
A: Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.

Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.

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: What is the catch or declare rule for method declarations?
A: 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.

InterView Java :: Question vs Answer 5

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: 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: 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: This() 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.

InterView Java :: Question vs Answer 4

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: Give a simplest way to find out the time a method takes for execution without using any profiling tool?
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: 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.

InterView Java :: Question vs Answer 3

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.

QHow 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: When you serialize an object, 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?
A: There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.

InterView Java :: Question vs Answer 2

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.

InterView Java :: Question vs Answer1

Q: What is the difference between an Interface and an Abstract class?
A: 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 and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.
.

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 synchonization, 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. Passby 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 doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized 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 componenets. 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 Iterator?
A: Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through 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 or null?
A: It is empty. But not null.

Q: How can one prove that the array is not null but empty using one line of code?
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.

Easy way to study JAVA :: Static

-Static variable :: can be access without creating any object. There is only one copy of static variable in the class and every object of that class access this Static variable

-Static method :: is a method which accepts arguments instead of using instance variables.The opposite of a static method is an instance method.The default for a method is to be an instance method. Methods must be explicitly defined as static methods.A static method is also refer to as a class method.

Example1 ::

Class Demo {

int i=10 //instant variable

static int a=50;

public static void display(){

System.out.println(“display method”);

}

public static void main(String arg[]){

System.out.println(a); //directly call the variable

display(); //directly calles function

Demo d = new Demo();

System.out.println(d.a); //called by object

System.out.println(d.display()); //calles by object.

}

}

Example2 ::

class Ax {

int a;

Static int s;

public static void main(String arg[]) {

Ax a1 = new Ax();

a1.a++;

a1.s++;

System.out.println(a1.a);

Ax a2 = new Ax();

System.out.println(a2.a);

System.out.println(a2.s);

}

}

output

1

1

0

1

Easy step to study JAVA :: Separators

Separators help define the structure of a program. The separators used in HelloWorld are parentheses, ( ), braces, { }, the period, ., and the semicolon, ;. The table lists the six Java separators (nine if you count opening and closing separators as two).

Separator Purpose
( )	Encloses arguments in method definitions and calling; adjusts precedence in arithmetic expressions; surrounds cast types and delimits test expressions in flow control statements
{ }	defines blocks of code and automatically initializes arrays
[ ]	declares array types and dereferences array values
;	terminates statements
,	separates successive identifiers in variable declarations; chains statements in the test, expression of a for loop
.	Selects a field or method from an object; separates package names from sub-package and class names
:	Used after loop labels

Easy step to study JAVA :: Keyword

Keywords are identifiers like public, static and class that have a special meaning inside Java source code and outside of comments and Strings. Four keywords are used in Hello World, public, static, void and class.

Keywords are reserved for their intended use and cannot be used by the programmer for variable or method names.

There are fifty reserved keywords in Java 1.1, 51 in Java 1.2, 52 in Java 1.4, and 54 in Java 5. The forty-eight that are actually used in are listed below. Don't worry if the purposes of the keywords seem a little opaque at this point. They will all be explained in much greater detail later.

Keywords Used in Java 1.1

Keyword	Purpose
boolean	declares a boolean variable or return type
byte	declares a byte variable or return type
char	declares a character variable or return type
double	declares a double variable or return type
float	declares a floating point variable or return type
short	declares a short integer variable or return type
void	declare that a method does not return a value
int	declares an integer variable or return type
long	declares a long integer variable or return type
while	begins a while loop
for	begins a for loop
do	begins a do while loop
switch	tests for the truth of various possible cases
break	prematurely exits a loop
continue	prematurely return to the beginning of a loop
case	one case in a switch statement
default	default action for a switch statement
if	execute statements if the condition is true
else	signals the code to be executed if an if statement is not true
try	attempt an operation that may throw an exception
catch	handle an exception
finally	declares a block of code guaranteed to be executed
class	signals the beginning of a class definition
abstract	declares that a class or method is abstract
extends	specifies the class which this class is a subclass of
final	declares that a class may not be subclassed or that a field or method may not be overridden
implements	declares that this class implements the given interface
import	permit access to a class or group of classes in a package
instanceof	tests whether an object is an instanceof a class
interface	signals the beginning of an interface definition
native	declares that a method is implemented in native code
new	allocates a new object
package	defines the package in which this source code file belongs
private	declares a method or member variable to be private
protected	declares a class, method or member variable to be protected
public	declares a class, method or member variable to be public
return	returns a value from a method
static	declares that a field or a method belongs to a class rather than an object
super	a reference to the parent of the current object
synchronized	Indicates that a section of code is not thread-safe
this	a reference to the current object
throw	throw an exception
throws	declares the exceptions thrown by a method
transient	This field should not be serialized
volatile	Warns the compiler that a variable changes asynchronously

Two other keywords, const and goto, are reserved by Java but are not actually implemented. This allows compilers to produce better error messages if these common C++ keywords are improperly used in a Java program.

Java 1.2 adds the strictfp keyword to declare that a method or class must be run with exact IEEE 754 semantics.

Java 1.4 adds the assert keyword to specify assertions.

Java 5 adds assert and enum.

true and false appear to be missing from this list. In fact, they are not keywords but rather boolean literals. You still can't use them as a variable name though.