This and Super Keywords

• this keyword used to store current object reference.

• super keyword used to store super class reference in subclass object.

this keyword:

• Predefined instance variable to hold current object reference.

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

 Top 10 Java interview questions and programs on this keyword

 

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

• Without this keyword if variable names are same we can not assign instance variables so this problem will solve by using this keyword

1. package com.instanceofjava;
2.  
3. public class ThisDemo {
4.     int a, b;
5.  
6.  ThisDemo(int a, int b){
7.  
8.         this.a=a;
9.        this.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. 1
2. 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. }
9. 
   
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

Used to call current class method:

 

1. package com.instanceofjava;
2.  
3. public class Sample{
4.     int a, b;
5.  
6. 
   
7.  Sample(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.         Sample obj= new Sample(1,2);
29.         
30.       
31.         obj.print()
32. }
33. }

Output:

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

super keyword:

• Predefined instance variable used to hold super class object reference through sub class object.

 Super keyword interview questions java

Used to call super class constructor:

1. package com.instanceofjava;
2.  
3. public class A{
4.     int a, b;
5.  
6.  A(int a, int b){
7.  
8.         a=a;
9.         b=b;
10. System.out.println("super class constructor called");
11.  
12.  }
13.  
14. }

1. package com.instanceofjava;
2.  
3. public class B extends A{
4.     int a, b;
5.  
6.  A(int a, int b){
7.     super(a,b);
8.         a=a;
9.         b=b;
10. }
11. public static void main(String[] args){
12.  
13.         B obj= new B(1,2);
14.         
15.     
16. }
17.  
18.  }
19.  
20. }

Output:

1. super class constructor called
2. 1
3. 2

Used to call super class methods from subclass:

1. package com.instanceofjava;
2.  
3. public class A{
4.     int a, b;
5.  
6.  A(int a, int b){
7.  
8.         a=a;
9.         b=b;
10. System.out.println("super class constructor called");
11.  
12.  }
13.  
14. void add(){
15. System.out.println("super class method called");
16. }
17.  
18. }

1. package com.instanceofjava;
2.  
3. public class B extends A{
4.     int a, b;
5.  
6.  A(int a, int b){
7.     super(a,b);
8.         a=a;
9.         b=b;
10. }
11. void print(){ 
12. super.add();
13. System.out.println(this.a);
14. System.out.println(this.b);
15.  
16. 
    
17. 
    
18. }
19. public static void main(String[] args){
20.  
21.         B obj= new B(1,2);
22.         
23.        obj.print();
24. }
25.  
26.  }
27.  
28. }

Output:

1. super class method called
2. 1
3. 2