10 Interesting Core Java Interview Coding Questions and Answers

1. What is the output of following program?

1. package com.instanceofjava;
2.  
3. public class B{
4.  
5.   B b= new B();
6.  
7.  public int show(){
8.       return (true ? null : 0);
9.  }
10.  
11.  public static void main(String[] args)  {
12.  
13.         B b= new B();
14.         b.show();
15.     }
16.  
17. }

1. Exception in thread "main" java.lang.StackOverflowError
2. at com.instanceofjava.B.<init>(B.java:3)
3. at com.instanceofjava.B.<init>(B.java:5)
4. at com.instanceofjava.B.<init>(B.java:5)

Exaplanation:

• Whenever we create the object of any class constructor will be called first and memory allocated for all non static variables
• Here  B b= new B(); variable is object and assigned to new object of same class
• B b= new B(); statement leads to recursive execution of constructor will create infinite objects so at run time an exception will be raised
• Exception in thread "main" java.lang.StackOverflowError
• The common cause for a stack overflow exception  is a bad recursive call. Typically this is caused when your recursive functions doesn't have the correct termination condition

2. What is the output of following program?

1. package com.instanceofjava;
2.  
3. public class A{
4. 
   
5.  
6.  public static void show(){
7.  
8.         System.out.println("Static method called");
9.  }
10.  
11.  public static void main(String[] args)  {
12.  
13.         A obj=null;
14.         obj.show();
15.  
16.     }
17.  
18. }

1. Static method called

Exaplanation:

• We can call static methods using reference variable which is pointing to null because static methods are class level so we can either call using class name and reference variable which is pointing to null.

3. What is the output of following program?

1. package com.instanceofjava;
2.  
3. public class A{
4. 
   
5.  
6.  static int a = 1111;
7.  static
8.  {
9.         a = a-- - --a;
10.  }
11.     
12. {
13.         a = a++ + ++a;
14.  }
15.  
16.  public static void main(String[] args)  {
17.  
18.        System.out.println(a);
19.  
20.     }
21.  
22. }

1. 2

Explanation:

Top 10 Increment and decrement  operators interview Questions

4. What is the output of following program?

1. package com.instanceofjava;
2.  
3. public class A{
4. 
   
5.  
6.  int GetValue()
7.  {
8.         return (true ? null : 0);
9.  }
10.  
11.  public static void main(String[] args)  {
12.  
13.    A obj= new A();
14. 
    
15.       obj.GetValue();
16.  
17.     }
18.  
19. }

1. Exception in thread "main" java.lang.NullPointerException

5. What is the output of following program?

1. package com.instanceofjava;
2.  
3. public class A{
4. 
   
5.   
6.  public static void main(String[] args)  {
7.  
8.    Integer i1 = 128;
9.  
10.    Integer i2 = 128;
11.  
12.      System.out.println(i1 == i2);
13.  
14.    Integer i3 = 127;
15.    Integer i4 = 127;
16.  
17.       System.out.println(i3 == i4);
18.  
19.     }
20.  
21. }

1. false
2. true

6. What is the output of following program?

1. package com.instanceofjava;
2. class A
3. {
4.     
5. void method(int i)
6. {
7.  
8.  }
9.  
10.     }
11.  
12.  class B extends A
13.  {
14.  
15. @Override
16. void method(Integer i)
17.  {
18.  
19.  }
20.           
21. 
    
22. 
    
23. }

1. Compile time Error:The method method(Integer) of type B must override or implement a supertype method

7. Which line will throw compile time error? 8 or 9?

1. package com.instanceofjava;
2. class A
3. {
4.     
5. public static void main(String [] args)
6. {
7.  
8.   Integer i = new Integer(null);
9.   String s = new String(null);
10.  
11.  }
12.  
13. }

1. Compile time Error at line number 9:The constructor String(String) is ambiguous

8.What is the output of following program?

1. package com.instanceofjava;
2. class A
3. {
4.     
5. public static void main(String [] args)
6. {
7.  
8.   String s = "ONE"+3+2+"TWO"+"THREE"+5+4+"FOUR"+"FIVE"+5;
9.   System.out.println(s);
10.  
11.  }
12.  
13. }

1. ONE32TWOTHREE54FOURFIVE5

9.What is the output of following program?

1. package com.instanceofjava;
2. class A
3. {
4.  
5. static int method1(int i)
6. {
7. return method2(i *= 11);
8. }
9.  
10.  static int method2(int i)
11. {
12.   return method3(i /= 11);
13. }
14.  
15.  static int method3(int i)
16. {
17.  return method4(i -= 11);
18. }
19.  
20.  static int method4(int i)
21. {
22.   return i += 11;
23. }
        
24. public static void main(String [] args)
25. {
26.  
27.   System.out.println(method1(11));
28.  
29.  }
30.  
31. }

1. 11

10.What is the output of following program?

1. package com.instanceofjava;
2. class A
3. {
4.     
5. public static void main(String [] args)
6. {
7.  
8.   System.out.println(null);
9. 
   
10.  }
11.  
12. }

1. Compilation Error

Explanation:

What happens When System.out.println(null)?

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