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1.What is the most important feature of Java?

Java is a platform independent language.

2.What do you mean by platform independence?

Platform independence means that we can write and compile the java code in one platform (eg Windows) and can execute the class in any other supported platform eg (Linux,Solaris,etc).

3.Are JVM's platform independent?

JVM's are not platform independent. JVM's are platform specific run time implementation provided by the vendor.

4.What is a JVM?

JVM is Java Virtual Machine which is a run time environment for the compiled java class files.

5.What is the difference between a JDK and a JVM?

JDK is Java Development Kit which is for development purpose and it includes execution environment also. But JVM is purely a run time environment and hence you will not be able to compile your source files using a JVM.

6.What is a pointer and does Java support pointers?

Pointer is a reference handle to a memory location. Improper handling of pointers leads to memory leaks and reliability issues hence Java doesn't support the usage of pointers.

7.What is the base class of all classes?


8.Does Java support multiple inheritance?

Java doesn't support multiple inheritance.

9.Is Java a pure object oriented language?

Java uses primitive data types and hence is not a pure object oriented language.

10.Are arrays primitive data types?

In Java, Arrays are objects.

11.What is difference between Path and Classpath?

Path and Classpath are operating system level environment variales. Path is used define where the system can find the executables(.exe) files and classpath is used to specify the location .class files.

12.What are local variables?

Local varaiables are those which are declared within a block of code like methods. Local variables should be initialised before accessing them.

13.What are instance variables?

Instance variables are those which are defined at the class level. Instance variables need not be initialized before using them as they are automatically initialized to their default values.

14.How to define a constant variable in Java?

The variable should be declared as static and final. So only one copy of the variable exists for all instances of the class and the value can't be changed also.

static final int PI = 2.14; is an example for constant.

15.Should a main method be compulsorily declared in all java classes?

No not required. main method should be defined only if the source class is a java application.

16.What is the return type of the main method?

Main method doesn't return anything hence declared void.

17.Why is the main method declared static?

main method is called by the JVM even before the instantiation of the class hence it is declared as static.

18.What is the arguement of main method?

main method accepts an array of String object as arguement.

19.Can a main method be overloaded?

Yes. You can have any number of main methods with different method signature and implementation in the class.

20.Can a main method be declared final?

Yes. Any inheriting class will not be able to have it's own default main method.

21.Does the order of public and static declaration matter in main method?

No it doesn't matter but void should always come before main().

22.Can a source file contain more than one Class declaration?

Yes a single source file can contain any number of Class declarations but only one of the class can be declared as public.

23.What is a package?

Package is a collection of related classes and interfaces. package declaration should be first statement in a java class.

24.Which package is imported by default?

java.lang package is imported by default even without a package declaration.

25.Can a class declared as private be accessed outside it's package?

Not possible.

26.Can a class be declared as protected?

A class can't be declared as protected. only methods can be declared as protected.

27.What is the access scope of a protected method?

A protected method can be accessed by the classes within the same package or by the subclasses of the class in any package.

28.What is the purpose of declaring a variable as final?

A final variable's value can't be changed. final variables should be initialized before using them.

29.What is the impact of declaring a method as final?

A method declared as final can't be overridden. A sub-class can't have the same method signature with a different implementation.

30.I don't want my class to be inherited by any other class. What should i do?

You should declared your class as final. But you can't define your class as final, if it is an abstract class. A class declared as final can't be extended by any other class.

31.Can you give few examples of final classes defined in Java API?

java.lang.String,java.lang.Math are final classes.

32.How is final different from finally and finalize?

final is a modifier which can be applied to a class or a method or a variable. final class can't be inherited, final method can't be overridden and final variable can't be changed.

.finally is an exception handling code section which gets executed whether an exception is raised or not by the try block code segment.

finalize() is a method of Object class which will be executed by the JVM just before garbage collecting object to give a final chance for resource releasing activity.

33.Can a class be declared as static?

No a class cannot be defined as static. Only a method,a variable or a block of code can be declared as static.

34.When will you define a method as static?

When a method needs to be accessed even before the creation of the object of the class then we should declare the method as static.

35.What are the restriction imposed on a static method or a static block of code?

A static method should not refer to instance variables without creating an instance and cannot use "this" operator to refer the instance.

36.I want to print "Hello" even before main is executed. How will you acheive that?

Print the statement inside a static block of code. Static blocks get executed when the class gets loaded into the memory and even before the creation of an object. Hence it will be executed before the main method. And it will be executed only once.

37.What is the importance of static variable?

static variables are class level variables where all objects of the class refer to the same variable. If one object changes the value then the change gets reflected in all the objects.

38.Can we declare a static variable inside a method?

Static varaibles are class level variables and they can't be declared inside a method. If declared, the class will not compile.

39.What is an Abstract Class and what is it's purpose?

A Class which doesn't provide complete implementation is defined as an abstract class. Abstract classes enforce abstraction.

40.Can a abstract class be declared final?

Not possible. An abstract class without being inherited is of no use and hence will result in compile time error.

41.What is use of a abstract variable?

Variables can't be declared as abstract. only classes and methods can be declared as abstract.

42.Can you create an object of an abstract class?

Not possible. Abstract classes can't be instantiated.

43.Can a abstract class be defined without any abstract methods?

Yes it's possible. This is basically to avoid instance creation of the class.

44.Class C implements Interface I containing method m1 and m2 declarations. Class C has provided implementation for method m2. Can i create an object of Class C?

No not possible. Class C should provide implementation for all the methods in the Interface I. Since Class C didn't provide implementation for m1 method, it has to be declared as abstract. Abstract classes can't be instantiated.

45.Can a method inside a Interface be declared as final?

No not possible. Doing so will result in compilation error. public and abstract are the only applicable modifiers for method declaration in an interface.

46.Can an Interface implement another Interface?

Intefaces doesn't provide implementation hence a interface cannot implement another interface.

47.Can an Interface extend another Interface?

Yes an Interface can inherit another Interface, for that matter an Interface can extend more than one Interface.

48.Can a Class extend more than one Class?

Not possible. A Class can extend only one class but can implement any number of Interfaces.

49.Why is an Interface be able to extend more than one Interface but a Class can't extend more than one Class?

Basically Java doesn't allow multiple inheritance, so a Class is restricted to extend only one Class. But an Interface is a pure abstraction model and doesn't have inheritance hierarchy like classes(do remember that the base class of all classes is Object). So an Interface is allowed to extend more than one Interface.

50.Can an Interface be final?

Not possible. Doing so so will result in compilation error.

51.Can a class be defined inside an Interface?

Yes it's possible.

52.Can an Interface be defined inside a class?

Yes it's possible.

53.What is a Marker Interface?

An Interface which doesn't have any declaration inside but still enforces a mechanism.

54.Which OO Concept is achieved by using overloading and overriding?


55.If i only change the return type, does the method become overloaded?

No it doesn't. There should be a change in method arguements for a method to be overloaded.

56.Why does Java not support operator overloading?

Operator overloading makes the code very difficult to read and maintain. To maintain code simplicity, Java doesn't support operator overloading.

57.Can we define private and protected modifiers for variables in interfaces?


58.What is Externalizable?

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)

59.What modifiers are allowed for methods in an Interface?

Only public and abstract modifiers are allowed for methods in interfaces.

60.What is a local, member and a class variable?

Variables declared within a method are "local" variables. Variables declared within the class i.e not within any methods are "member" variables (global variables). Variables declared within the class i.e not within any methods and are defined as "static" are class variables

61.What is an abstract method?

An abstract method is a method whose implementation is deferred to a subclass.

62.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.

63.Can a Byte object be cast to a double value?

No, an object cannot be cast to a primitive value.

64.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.

65.What is an object's lock and which object's 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.

66.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.

67.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.

68.Which class is extended by all other classes?

The Object class is extended by all other classes.

69.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

70.What restrictions are placed on method overloading?

Two methods may not have the same name and argument list but different return types.

71.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.

72.What is the return type of a program's main() method?


73.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.

74.What do you understand by private, protected and public?

These are accessibility modifiers. Private is the most restrictive, while public is the least restrictive. There is no real difference between protected and the default type (also known as package protected) within the context of the same package, however the protected keyword allows visibility to a derived class in a different package.

75.What is Downcasting ?

Downcasting is the casting from a general to a more specific type, i.e. casting down the hierarchy

76.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.

77.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.

78.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).

79.What is a native method?

A native method is a method that is implemented in a language other than Java.

80.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

81.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.

82.What is the range of the char type?

The range of the char type is 0 to 2^16 - 1.

83.What is the range of the short type?

The range of the short type is -(2^15) to 2^15 - 1.

84.Why isn't there operator overloading?

Because C++ has proven by example that operator overloading makes code almost impossible to maintain.

85.What does it mean that a method or field is "static"?

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.

86.Is null a keyword?

The null value is not a keyword.

87.Which characters may be used as the second character of an identifier,but not as the first character of an identifier?

88.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.

89.Is the ternary operator written x : y ? z or x ? y : z ?

It is written x ? y : z.

90.How is rounding performed under integer division?

The fractional part of the result is truncated. This is known as rounding toward zero.

91.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.

92.Does a class inherit the constructors of its superclass?

A class does not inherit constructors from any of its superclasses.

93.Name the eight primitive Java types.

The eight primitive types are byte, char, short, int, long, float, double, and boolean.

94.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.

95.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.

96.What modifiers can be used with a local inner class?

A local inner class may be final or abstract.

97.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.

98.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.

99.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.

100.Are true and false keywords?

The values true and false are not keywords.

101.What happens when you add a double value to a String?

The result is a String object.

102.What is the diffrence between inner class and nested class?

When a class is defined within a scope od another class, then it becomes inner class. If the access modifier of the inner class is static, then it becomes nested class.

103.Can an abstract class be final?

An abstract class may not be declared as final

104.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

105.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.

106.To what value is a variable of the boolean type automatically initialized?

The default value of the boolean type is false

107.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.

108.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.

109.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.

110.What modifiers may be used with a top-level class?

A top-level class may be public, abstract, or final.

111.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.

112.What are the practical benefits, if any, of importing a specific class rather than an entire package (e.g. import java.net.* versus import java.net.Socket)?

It makes no difference in the generated class files since only the classes that are actually used are referenced by the generated class file. There is another practical benefit to importing single classes, and this arises when two (or more) packages have classes with the same name. Take java.util.Timer and javax.swing.Timer, for example. If I import java.util.* and javax.swing.* and then try to use "Timer", I get an error while compiling (the class name is ambiguous between both packages). Let's say what you really wanted was the javax.swing.Timer class, and the only classes you plan on using in java.util are Collection and HashMap. In this case, some people will prefer to import java.util.Collection and import java.util.HashMap instead of importing java.util.*. This will now allow them to use Timer, Collection, HashMap, and other javax.swing classes without using fully qualified class names in.

113.Can a method be overloaded based on different return type but same argument type ?

No, because the methods can be called without using their return type in which case there is ambiquity for the compiler

114.What happens to a static var that is defined within a method of a class ?

Can't do it. You'll get a compilation error

115.How many static init can you have ?

As many as you want, but the static initializers and class variable initializers are executed in textual order and may not refer to class variables declared in the class whose declarations appear textually after the use, even though these class variables are in scope.

116.What is the difference between method overriding and overloading?

Overriding is a method with the same name and arguments as in a parent, whereas overloading is the same method name but different arguments

117.What is constructor chaining and how is it achieved in Java ?

A child object constructor always first needs to construct its parent (which in turn calls its parent constructor.). In Java it is done via an implicit call to the no-args constructor as the first statement.

118.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.

119.Which Java operator is right associative?

The = operator is right associative.

120.Can a double value be cast to a byte?

Yes, a double value can be cast to a byte.

121.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.

122.Can a for statement loop indefinitely?

Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;

123.To what value is a variable of the String type automatically initialized?

The default value of an String type is null.

124.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.

125.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.

126.What does it mean that a class or member is final?

A final class cannot be inherited. A final method cannot be overridden in a subclass. A final field cannot be changed after it's initialized, and it must include an initializer statement where it's declared.

127.What does it mean that a method or class is abstract?

An abstract class cannot be instantiated. Abstract methods may only be included in abstract classes. However, an abstract class is not required to have any abstract methods, though most of them do. Each subclass of an abstract class must override the abstract methods of its superclasses or it also should be declared abstract.

128.What is a transient variable?

transient variable is a variable that may not be serialized.

129.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.

130.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.

131.Is sizeof a keyword?

The sizeof operator is not a keyword.

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How to take a snapshot

How To Create/Take a snap shot of your Error:

Step1: Make Sure that the window displaying the error is on the top of the screen.
Step2: Find the Print Screen key on your keyboard.

Step 3: Clik on print screen button.

Step 4: open the microsoft paint
           Shortcut to open paint :
                                  >> click windows key + r
                                  >> Type mspaint and clik ok

Step 5: click ctrl +v  or select paste option

Step 6: save the file (click ctrl + s and click ok.)

Setting an Icon Image

In this section, you will learn how to set an icon for the frame in Java Swing.

This program helps us to set the icon (image) on the title bar of the frame. When you open frame or window the icon situated on the title bar is seen on the taskbar also. For this purposes, various methods as follows has been used:

Above method sets the icon for the frame or window after getting the image using the Image class method named getImage().

This is the method of the Toolkit class which gets the default toolkit.

Here is the code of program:


Creating a JFrame

This program shows you how to create a frame in Java Swing Application. The frame in java works like the main window where your components (controls) are added to develop an application. In the Java Swing, top-level windows are represented by the JFrame class. Java supports the look and feel and decoration for the frame. 
For creating java standalone application you must provide GUI for a user. The most common way of creating a frame is, using single argument constructor of the JFrame class. The argument of the constructor is the title of the window or frame. Other user interface are added by constructing and adding it to the container one by one. The frame initially are not visible and to make it visible the setVisible(true) function is called passing the boolean value true. The close button of the frame by default performs the hide operation for the JFrame. In this example we have changed this behavior to window close operation by setting the setDefaultCloseOperation() to EXIT_ON_CLOSE value.
setSize (400, 400):
Above method sets the size of the frame or window to width (400) and height (400) pixels.
Above method makes the window visible.
Above code sets the operation of close operation to Exit the application using the System exit method.

Here is the code of the program : 



 JComponent Class

With the exception of top-level containers, all Swing components whose names begin with "J" descend from the JComponent class. For example, JPanel, JScrollPane, JButton, and JTable all inherit from JComponent. However, JFrame and JDialog don't because they implement top-level containers. The JComponent class extends the Container class, which itself extends Component. The Component class includes everything from providing layout hints to supporting painting and events. The Container class has support for adding components to the container and laying them out. This section's API tables summarize the most often used methods of Component and Container, as well as of JComponent.

JComponent Features

The JComponent class provides the following functionality to its descendants:
Tool tips
By specifying a string with the setToolTipText method, you can provide help to users of a component. When the cursor pauses over the component, the specified string is displayed in a small window that appears near the component. See How to Use Tool Tips for more information.
Painting and borders
The setBorder method allows you to specify the border that a component displays around its edges. To paint the inside of a component, override the paintComponent method. See How to Use Borders and Performing Custom Painting for details.
Application-wide pluggable look and feel
Behind the scenes, each JComponent object has a corresponding ComponentUI object that performs all the drawing, event handling, size determination, and so on for that JComponent. Exactly which ComponentUI object is used depends on the current look and feel, which you can set using the UIManager.setLookAndFeel method. See How to Set the Look and Feel for details.
Custom properties
You can associate one or more properties (name/object pairs) with any JComponent. For example, a layout manager might use properties to associate a constraints object with each JComponent it manages. You put and get properties using the putClientProperty and getClientProperty methods. For general information about properties, see Properties.
Support for layout
Although the Component class provides layout hint methods such as getPreferredSize and getAlignmentX, it doesn't provide any way to set these layout hints, short of creating a subclass and overriding the methods. To give you another way to set layout hints, the JComponent class adds setter methods — setMinimumSize, setMaximumSize, setAlignmentX, and setAlignmentY. See Laying Out Components Within a Container for more information.
Support for accessibility
The JComponent class provides API and basic functionality to help assistive technologies such as screen readers get information from Swing components, For more information about accessibility, see How to Support Assistive Technologies.
Support for drag and drop
The JComponent class provides API to set a component's transfer handler, which is the basis for Swing's drag and drop support. See Introduction to DnD for details.
Double buffering
Double buffering smooths on-screen painting. For details, see Performing Custom Painting.
Key bindings
This feature makes components react when the user presses a key on the keyboard. For example, in many look and feels when a button has the focus, typing the Space key is equivalent to a mouse click on the button. The look and feel automatically sets up the bindings between pressing and releasing the Space key and the resulting effects on the button. For more information about key bindings, see How to Use Key Bindings


    This lesson gives you the background information you need to use the Swing components, and then     describes every Swing component. It assumes that you have successfully compiled and run a program that uses Swing components, and that you are familiar with basic Swing concepts.

Before you get started, you may want to check out these pages (from the Graphical User Interfaces lesson in the Core trail) which have pictures of all the standard Swing components, from top-level containers to scroll panes to buttons. To find the section that discusses a particular component, just click the component's picture.

Using Top-Level Containers

Discusses how to use the features shared by the JFrame, JDialog, and JApplet classes — content panes, menu bars, and root panes. It also discusses the containment hierarchy, which refers to the tree of components contained by a top-level container.

The JComponent Class

Tells you about the features JComponent provides to its subclasses — which include almost all Swing components — and gives tips on how to take advantage of these features. This section ends with API tables describing the commonly used API defined by JComponent and its superclasses, Container and Component.

Using Text Components

Describes the features and API shared by all components that descend from JTextComponent. You probably do not need to read this section if you are just using text fields (formatted or not) or text areas.

How to...

Sections on how to use each Swing component, in alphabetical order. We do not expect you to read these sections in order. Instead, we recommend reading the relevant "How to" sections once you are ready to start using Swing components in your own programs. For example, if your program needs a frame, a label, a button, and a color chooser, you should read How to Make Frames, How to Use Labels, How to Use Buttons, and How to Use Color Choosers.

Using HTML in Swing Components

Describes how to vary the font, color, or other formatting of text displayed by Swing components by using HTML tags.

Using Models

Tells you about the Swing model architecture. This variation on Model-View-Controller (MVC) means that you can, if you wish, specify how the data and state of a Swing component are stored and retrieved. The benefits are the ability to share data and state between components, and to greatly improve the performance of components such as tables that display large amounts of data.

Using Borders

Borders are very handy for drawing lines, titles, and empty space around the edges of components. (You might have noticed that the examples in this trail use a lot of borders.) This section tells you how to add a border to any JComponent.

Using Icons

Many Swing components can display icons. Usually, icons are implemented as instances of the ImageIcon class.

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