JAVA: 2012

Tuesday, August 7, 2012

Some Intresting questions in java

What is CAFEBABE?
Without using Thread class and Runnable interface can we create Thread?
what is Greey thread?
what is starvation?
What is deadlock?
what is selifish thread?
what is friendly thread?
what is dynamic polymorphishism?
what happens when an object is created?
what is markable interface?

Friday, July 6, 2012

CORE JAVA INTERVIEW QUESTIONS AND ANSWERS

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. What is a JVM?

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

4. Are JVM's platform independent?

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

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?

java.lang.Object

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 object oriented Concept is achieved by using overloading and overriding?

Polymorphism.

55.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.

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

No.

57. 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)

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

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

59. 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.

60. What is an abstract method?

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

61. 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.

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

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

63. 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.

64. 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.

65. 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.

66. 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.

67. Which class is extended by all other classes?

The Object class is extended by all other classes.

68. 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

69. What restrictions are placed on method overloading?

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

70. 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.

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

void.

72. 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.

73. 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.

74. What is Downcasting ?

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

75. 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.

76. 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.

77. 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).

78. What is a native method?

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

79. 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.

80. 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.

81. What is the range of the char type?

The range of the char type is 0 to 2¹⁶ - 1 (i.e. 0 to 65535.)

82. What is the range of the short type?

The range of the short type is -(2¹⁵) to 2¹⁵ - 1. (i.e. -32,768 to 32,767)

83. Why isn't there operator overloading?

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

84. 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.

85. Is null a keyword?

The null value is not a keyword.

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

The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.

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

It is written x ? y : z.

88. How is rounding performed under integer division?

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

89. 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.

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

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

91. Name the eight primitive Java types.

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

92. 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.

93. What is the difference between a while statement and a do while statement?

A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do while statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do whilestatement will always execute the body of a loop at least once.

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

A local inner class may be final or abstract.

95. 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.

96. 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.

97. 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.

98. Are true and false keywords?

The values true and false are not keywords.

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

The result is a String object.

100. 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.

Wednesday, June 20, 2012

Java Object type casting

Java Object type casting

Converting object reference of one type to another type is called Java object typecasting. Casting is a programming term that means, effectively, converting a value or an object from one type to another. The result of a cast is a new value or object; casting does not change the original object or value.

The class of the object you're casting and the class you're casting it to must be related by inheritance; that is, you can cast an object only to an instance of its class's sub- or superclass-not to any random class. Casting downward in the class hierarchy is automatic, but casting upward is not.

There are two types of object typecasting:
1. upcasting
2. downcasting

Upcasting: Upcasting is implicit and safe. Here subclass object is converted to superclass object, because a subclass object is also a superclass object.

Downcasting:Down-casting is potentially unsafe, because you could attempt to use a method that the derived class does not actually implement. With this in mind, down-casting is always explicit, that is, we are always specifying the type we are down-casting to. Converting an instance of a subclass to an instance of a superclass loses the information

Upcasting vs Downcasting

Given Below is a program which easily demonstrates the difference between Upcasting and downcastin in Java.

_____________________________________________________

Vehicle v1 = new Car(); / /Right . upcasting or implicit casting

Vehicle v2= new Vehicle();

Car c0 = v1; // Wrong compile time error "Type Mismatch"

//Explicit or downcasting is required

Car c1 = (Car) v1 // Right. downcasting or explicit casting . v1 has a knowledge //of Car due to line 1

Car c2= (Car) v2; //Wrong Runtime exception ClassCastException because v2 //has no knowledge of Car.

Bus b1 = new BMW(); //Wrong compile time error "type mismatch"

Car c3 = new BMW(); //Right. Upcasting or implicit casting

Car c4 = (BMW) v1; // Wrong Runtime exception ClassCastexception

Object o = v1; //v1 can only be upcast to its parent

Car c5 = (Car) v1; // v1 can be downcast to Car due to line 1 _____________________________________________________

ClassCastException is thrown to indicate that code has attempted to cast an object to a subclass of which it is not an instance. Use exception handling mechanism to catch ClassCastException
Use the instanceof operator to guard against incorrect casting.

what is encapsulation in java programming

Encapsulation is one of the four fundamental OOP concepts. The other three are inheritance, polymorphism, and abstraction. Encapsulation is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. For this reason, encapsulation is also referred to as data hiding. Encapsulation can be described as a protective barrier that prevents the code and data being randomly accessed by other code defined outside the class. Access to the data and code is tightly controlled by an interface. The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this feature Encapsulation gives maintainability, flexibility and extensibility to our code.

Let us look at an example that depicts encapsulation:

public class EncapTest{

private String name;

private String idNum;

private int age;

public int getAge(){

return age;

}

public String getName(){

return name;

}

public String getIdNum(){

return idNum;

}

public void setAge( int newAge){

age = newAge;

}

public void setName(String newName){

name = newName;

}

public void setIdNum( String newId){

idNum = newId;

}

The public methods are the access points to this class.s fields from the outside java world. Normally these methods are referred as getters and setters. Therefore any class that wants to access the variables should access them through these getters and setters.
The variables of the EncapTest class can be access as below::

/* File name : RunEncap.java */

public class RunEncap{

public static void main(String args[]){

EncapTest encap = new EncapTest();

encap.setName("chittampally kirran");

encap.setAge(22);

encap.setIdNum("12343ms");

System.out.print("Name : " + encap.getName()+

" Age : "+ encap.getAge());

}

This would produce following result:

Name : chittampally kirran :22

Benefits of Encapsulation:

1. The fields of a class can be made read-only or write-only.

2. A class can have total control over what is stored in its fields.

3. The users of a class do not know how the class stores its data. A class can change the data type of a field, and users of the class do not need to change any of their code.

Wednesday, June 13, 2012

Overriding and Hiding Methods

Java-Method-Overriding-Overloading

Java Implements the Polymorphisim in one of the two ways that is Method Overriding and Method Overloading

Method Overriding :

Method overriding occurs when sub class declares a method that has the same signature (name, plus the number and the type of its parameters) and return type as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior thatfs specific to a particular subclass type. An overriding method can also return a subtype of the type returned by the overridden method. This is called a covariant return type. Which overridden version of the method to call is decided at runtime based on the object type.

The rules for overriding a method are as follows:

The argument list must exactly match that of the overridden method. If doesn't match we will get a overloaded method not a overriding method.
The return type must be same as, or a subtype of, the return type declared in the original overridden method in the superclass.
We can not override method marked final. Static methods can't be overridden but they can be redefined.
If a method can't be inherited , you cannot override it. Overriding is nothing but we are reimplementing a method that we are inherited
Overriding method must not throw checked exceptions that are new and broader than the those declared by the overridden method
The access level can't be more restrictive than the overridden method. It can be less restrictive than that of overridden method
The overriding method can throw any unchecked or runtime exception, regardless of weather overridden method declares the exception or not.
The overbidding method doesn't have to declare any exceptions that it will never throw, regardless of what the overridden method declares.
A subclass within the same package as the instance's superclass can override any superclass method that is not declared private or final. A subclass in a different package can only override the non-final methods declared public or protected.
Constructors cannot be overridden.

A program to explain the above rules of Java Method Overridding

public class SuperClass{

public void method1() {

System.out.println("SuperClass.method1()");

}

private void method2() {

System.out.println("SuperClass.method2()");

}

public final void method3(){

System.out.println("SuperClass.method3()");

}

private final void method4(){

System.out.println("SuperClass.method4()");

}

public static void method5() {

System.out.println("SuperClass.method5()");

}

public void method6() throws Exception {

System.out.println("SuperClass.method6()");

}

private void method7(){

System.out.println("SuperClass.method7()");

}

private void method8(int x){

System.out.println("SuperClass.method8()");

}

public static void method9() {

System.out.println("SuperClass.method9()");

}

public class SubClass extends SuperClass {

public void method1() {

System.out.println("OverridingClass.method1()");

}

private void method2(){

System.out.println("OverridingClass.method2()");

}

//We can't override a public final method

/* public final void method3(){

System.out.println("OverridingClass.method3()");

}*/

private final void method4(){

System.out.println("OverridingClass.method4()");

}

public static void method5() {

System.out.println("OverridingClass.method5()");

}

public void method6() throws IOException{

System.out.println("OverridingClass.method6()");

}

public void method7(){

System.out.println("OverridingClass.method7()");

}

public void method8(final int x){

System.out.println("OverridingClass.method8()");

}

//A static method cannot be overridden to be non-static instance method

/*public void method9() {

System.out.println("OverridingClass.method9()");

}*/

}

public class MethodOverrdingSample{

public static void main(String[] args) {

SubClass sub = new SubClass();

SuperClass sc = new SubClass();

sub.method1();

sub.method3();

/* Since its private, the below 2 methods are not visible

sub.method2();

sub.method4();*/

sub.method5();

try {

sub.method6();

} catch (IOException e) {

e.printStackTrace();

}

sub.method7();

sub.method8(100);

sc3.method5();

SubClass subClass = new SubClass();

SuperClass supClass = (SuperClass)subClass;

supClass.method5();

supClass.method1();

}

To invoke a superclass version of overridden method we use a super keyword. For example

public class Superclass{

public void method1(){

System.out.println(gSuperclass Versionh);}

}

public class SubClass{

public method1(){

super.method1(); //Invoke first superclass code

//then do the subclass specific code

System.out.println(gSubclass Versionh);

}

Method Overloading :

Method overloading means to have two or more methods with same name in the same class and its subclass with different arguments and optionally different return type. Which overloaded version of the method to be called is based on the reference type of the argument passed at compile time.

The rules for overloading a method are as follows:

ðL Overloaded methods must change the argument list.

ðL Overloaded methods can change the return type.

ðL Overloaded methods can declare new or broader checked exacpetions

ðL Overloaded methods can change the access modifier.

ðL Constructors can be overloaded.

A program to explain the above rules of Java Method Overloading

public class A {

protected A(){

System.out.println("Hi A Constructor");

}

public A(int i,int j){

System.out.println("Hi A Constructor A,B");

}

public void method1(int i){

System.out.println("Hi A in method1(i)");

}

public class B extends A{

public B(){

System.out.println("Hi B Constructor");

}

public B(int i, int j){

super(i,j);

System.out.println("Hi B Constructor with i,j");

}

public void method1(int j){

System.out.println("Hi B in method1(i)");

}

public void method1(int i, int j){

System.out.println("Hi B in method1(i,j)");

}

public class C extends A{

public C(){

System.out.println("Hi C Constructor");

}

public C(int i, int j){

super(i,j);

System.out.println("Hi C Constructor with i,j");

}

public void method1(int j){

System.out.println("Hi C in method1(i)");

}

public void method1(int i, int j){

System.out.println("Hi C in method1(i,j)");

}

public class D {

public static void main(String[] args) {

// TODO Auto-generated method stub

int k=0;

int l=0;

A a=new A();

B b= new B();

a= b;

C c= new C();

A ac=c;

a.method1(k);

b.method1(k,l);

b.method1(k);

c.method1(k);

//Compiler erroe! Compiler looks only at the reference and sees that Class A does not have method1(int,int) method. Compiler doesn't care about the actual object at runtime

}

Output for the above program

Hi A Constructor

Hi B Constructor

Hi A Constructor

Hi C Constructor

Hi B in method1(i)

Hi B in method1(i,j)

Hi B in method1(i)

Hi C in method1(i)

Instance Methods

An instance method in a subclass with the same signature and return type as an instance method in the superclass overrides the superclass's method.The ability of a subclass to override a method allows a class to inherit from a superclass whose behavior is "close enough" and then to modify behavior as needed. The overriding method has the same name, number and type of parameters, and return type as the method it overrides. An overriding method can also return a subtype of the type returned by the overridden method. This is called a covariant return type.

When overriding a method, you might want to use the @Override annotation that instructs the compiler that you intend to override a method in the superclass. If, for some reason, the compiler detects that the method does not exist in one of the superclasses, it will generate an error.

Class Methods

If a subclass defines a class method with the same signature as a class method in the superclass, the method in the subclass hides the one in the superclass.The distinction between hiding and overriding has important implications. The version of the overridden method that gets invoked is the one in the subclass. The version of the hidden method that gets invoked depends on whether it is invoked from the superclass or the subclass.

let us see the following example

public class Person {

void drive()

{

    System.out.println("we are iin person class"); }

}

class Father extends Person {

    void drive()
    {
        System.out.println("father drives smmothly yar");}
    }

class Son extends Father {

        void drive()
        {
            System.out.println("Son drives roughly yar"); }
        }

class Overridedemo extends Son {

public static void main(String[] args) {

    Person P = new Person();
    Father F = new Father();
    Father S = new Son();
    P.drive();
    P = F;
    P.drive();
    P=S;
    P.drive();
}
}

JAVA

kirran