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Top 10 Java interview questions and programs on this keyword

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instanceof java

Posts: 576
Nickname: instanceof
Registered: Jan, 2015

instanceof java is a java related one.
Top 10 Java interview questions and programs on this keyword Posted: Mar 9, 2016 11:41 AM
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1.What is this key word in java?

  • "this"is a predefined instance variable to hold current object reference

2.What are the uses of this keyword in constructor?



1.It must used explicitly if non-static variable and local variable name is same.

  1. package com.instanceofjava;
  2.  
  3. public class ThisDemo {
  4.     int a, b;
  5.  
  6.  ThisDemo(int a, int b){
  7.  
  8.         a=a;
  9.         b=b;
  10.  }
  11.  
  12.     public static void main(String[] args){
  13.  
  14.         ThisDemo obj= new ThisDemo(1,2);
  15.         
  16.         System.out.println(obj.a);
  17.         System.out.println(obj.b);
  18. }
  19. }

Output:

  1. 0
  2. 0

  • We can use this keyword useful in constructor overloading i.e to call other constructor from a constructor in a class

2.Used to invoke current class constructor:

 

  1. package com.instanceofjava;
  2.  
  3. public class ThisDemo {
  4.     int a, b;
  5.  
  6. ThisDemo(){
  7. System.out.println("Default constructor called");
  8. }

  10.  ThisDemo(int a, int b){
  11.        this();
  12.         this.a=a;
  13.        this.b=b;
  14.  }
  15.  
  16.     public static void main(String[] args){
  17.  
  18.         ThisDemo obj= new ThisDemo(1,2);
  19.         
  20.         System.out.println(obj.a);
  21.         System.out.println(obj.b);
  22. }
  23. }

Output:

 

  1. Default constructor called
  2. 1
  3. 2


3. Can we call methods using this keyword?

  • Yes we can use this keyword to call current class non static methods .

  1. package com.instanceofjava;
  2.  
  3. public class Test{
  4.     int a, b;
  5.  

  7.  Test(int a, int b){
  8.       
  9.        this.a=a;
  10.        this.b=b;
  11.  }
  12.  
  13. void show(){
  14.  
  15. System.out.println("Show() method called");
  16.    
  17. }
  18.  
  19. void print(){
  20.  
  21.     this.show();
  22.     System.out.println(obj.a);
  23.     System.out.println(obj.b);
  24.  
  25.  }
  26.     public static void main(String[] args){
  27.  
  28.         Test obj= new Test(1,2);
  29.         
  30.       
  31.         obj.print()
  32. }
  33. }

Output:

 

  1. Show() method called
  2. 1
  3. 2

3. Can we call method on this keyword on this keyword?

  • Yes we can call non static methods from constructor using this keyword.

this keyword in java interview questions for freshers




4.Is it possible to assign reference to this ?

  • No we can not assign any value to "this" because its always points to current object and it is a final reference in java.
  • If we try to change or assign value to this compile time error will come.
  • The left-hand side of an assignment must be a variable
this keyword in java with example program
5.Can we return this from a method?

  • Yes We can return this as current class object. 

    1. public class B{

    3.    int a;
    4.     
    5.  public int getA() {
    6.         return a;
    7.  }
    8.  
    9. public void setA(int a) {
    10.         this.a = a;
    11. }
    12.  
    13. B show(){
    14.     return this;
    15. }
    16.  
    17. public static void main(String[] args) {
    18.        
    19.  B obj = new B();
    20.  
    21.   obj.setA(10);
    22.  
    23.  System.out.println(obj.getA());
    24.  B obj2= obj.show();
    25.  System.out.println(obj2.getA());
    26.  
    27. }

    29. }

    Output:

    1. 10
    2. 10

    6.Can we pass this as parameter of method?

    • Yes we can pass this as parameter in a method

    7. Can we use this to refer static members?

    •  Yes its possible to access static variable of a class using this but its discouraged and as per best practices this should be used on non static reference.

    8.Is it possible to use this in static blocks?

    •  No its not possible to use this keyword in static block.

    use this keyword in static block


    9.Can we use this in static methods? 

    • No we can not use this in static methods. if we try to use compile time error will come:Cannot use this in a static context

    10.What are all the differences between this and super keyword?

    • This refers to current class object where as super refers to super class object
    • Using this we can access all non static methods and variables. Using super we can access super class variable and methods from sub class.
    • Using this(); call we can call other constructor in same class. Using super we can call super class constructor from sub class constructor.

    Read: Top 10 Java interview questions and programs on this keyword

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