Core Java Coding Questions

1) What will be the output of this program?
   
public class MainClass
{
    static int i = 1;

    static
    {
        i = i-- + --i;
    }

    {
        i = i++ - ++i;
    }

    int methodOfTest()
    {
        return i + i - i * i / i;
    }

    public static void main(String[] args)
    {
        System.out.println(new MainClass().methodOfTest());
    }
}

Ans : -2

2) What will be the output of the below program?
   
class A
{
    static String s = "AAA";

    static
    {
        s = s + "BBB";
    }

    {
        s = "AAABBB";
    }
}

class B extends A
{
    static
    {
        s = s + "BBBAAA";
    }

    {
        System.out.println(s);
    }
}

public class MainClass
{
    public static void main(String[] args)
    {
        B b = new B();
    }
}

Ans : AAABBB

3) What will be the output of this program?
   
abstract class X
{
    int i = 11;

    int methodOneOfX()
    {
        return i *= i /= i;
    }

    abstract void methodTwoOfX(int i);
}

class Y extends X
{
    @Override
    void methodTwoOfX(int i)
    {
        System.out.println(i);
    }
}

public class Test
{
    public static void main(String[] args)
    {
        new Y().methodTwoOfX(new Y().methodOneOfX());
    }
}

Ans : 11

4) What will be the output of this program?
   
public class Test
{
    int i = 10;

    static Test t;

    public static void main(String[] args)
    {
        System.out.println(Test.t.t.t.t.t.t.i);
    }
}

Ans : It throws NullPointerException at run time. Because, ‘t’ is not initialized.

5) How do you access field ‘i’ of InnerClassFour in the below example?
   
class OuterClass
{
    class InnerClassOne
    {
        class InnerClassTwo
        {
            class InnerClassThree
            {
                class InnerClassFour
                {
                    int i;
                }
            }
        }
    }
}

Ans : new OuterClass().new InnerClassOne().new InnerClassTwo().new InnerClassThree().new InnerClassFour().i

6) Does below program compile successfully?
   
public class Test
{
    public static void main(String[] args)
    {
        try
        {
            System.out.println("This is try block");
        }

        System.out.println("statement after try block");

        catch(Exception ex)
        {
            System.out.println("This is catch block");
        }

         System.out.println("statement after catch block");

        finally
        {
            System.out.println("This is finally block");
        }
    }
}

Ans : No, you can’t put other statements in between try-catch blocks and catch-finally blocks.

7) Will this program run successfully without catch block?
   
public class Test
{
    public static void main(String[] args)
    {
        try
        {
            System.out.println("This is try block");
        }

        finally
        {
            System.out.println("This is finally block");
        }
    }
}

Ans : Yes, output will be
This is try block
This is finally block

8) What is wrong with this program. why it is showing compilation error?
   
public class Test
{
    public static void main(String[] args)
    {
        String s;

        System.out.println(s);
    }
}

Ans : ‘s’ is a local variable. Local variables must be initialized before they are used.

9) Which line of the below program shows compilation error?

public class Test
{
    static
    {
        d = 10;

        System.out.println(d);
    }

    static double d;
}

Ans : Line 7. Because, variables can be initialized before they are declared. But you can’t use the variables before they are declared.

10) Does the below code compile successfully?
   
class A
{
    int method()
    {
        return 0;
    }
}

class B extends A
{
    @Override
    Integer method()
    {
        return 10;
    }
}

Ans : No, return type of the overridden method is incompatible with it’s super class method.

11) What happens when you compile and run this program?
   
public class ArraysInJava
{
    public static void main(String[] args)
    {
        int[] i = new int[0];

        System.out.println(i[0]);
    }
}

Ans : You will get ArrayIndexOutOfBoundsException at run time.

12)  What will be the output of this program?
   
public class ArraysInJava
{
    public static void main(String[] args)
    {
        int[] a = new int[3];

        a[1] = 50;

        Object o = a;

        int[] b = (int[])o;

        b[1] = 100;

        System.out.println(a[1]);

        ((int[])o)[1] = 500;

        System.out.println(a[1]);
    }
}

Ans :
100
500

13) What will be the outcome of the following program?
   
public class ArraysInJava
{
    static final int[] a;

    static
    {
        a = new int[] {1, 2, 3};
    }

    public static void main(String[] args)
    {
        a = new int[5];
    }
}

Ans : Compile time error.

14)What will be the outcome of this program?
   
public class ArraysInJava
{
    public static void main(String[] args)
    {
        int[] a = {1, 2, 3, 4, 5, 8};

        System.out.println(a[-1]);
    }
}

Ans : ArrayIndexOutOfBoundsException at run time.

15) What happens when you compile and run the following program?

public class ArraysInJava
{
    public static void main(String[] args)
    {
         int[][] a = {{1,2,}, {3,4}};

         int[] b = (int[]) a[1];

         Object o1 = a;

         int[][] a2 = (int[][]) o1;

         int[] b2 = (int[]) o1;

         System.out.println(b[1]);
    }
}

Ans : Line 13 throws ClassCastException at run time.

16) What will be the output of this program?

public class ArraysInJava
{
    static void methodOne(int[] a)
    {
        int[] b = new int[5];

        a = b;

        System.out.print(a.length);

        System.out.print(b.length);
    }

    public static void main(String[] args)
    {
        int[] a = new int[10];

        methodOne(a);

        System.out.print(a.length);
    }
}

Ans : 5510

17) Will this program compiles and runs successfully?

public class ArraysInJava
{
    public static void main(String[] args)
    {
        int[] a = {1};

        int[] b[] = {{1}};

        int[][] c[] = {{{1}}};

        int[][] d [][] = {{{{1}}}};
    }
}

Ans : Yes.

18) What will be the output of this program?

public class ArraysInJava
{
    public static void main(String[] args)
    {
        String[][][][] colors =
            {
                {
                    {
                        {"RED", "GREEN", "BLUE"},

                        {"GREEN", "RED", "BLUE"}
                    },
                    {
                        {"ORANGE", "GREEN", "WHITE"},

                        {"BLACK", "INDIGO", "BLUE"}
                    }
                },
                {
                    {
                        {"SKY BLUE", "ALMOND", "AQUA"},

                        {"APPLE GREEN", "PINK", "BLUE GREEN"}
                    },
                    {
                        {"VIOLET", "BRASS", "GREY"},

                        {"BROWN", "INDIGO", "CHERRY"}
                    }
                }
            };

        System.out.println(colors[1][0][1][0]);

        System.out.println(colors[0][1][0][1]);

        System.out.println(colors[0][0][0][2]);

        System.out.println(colors[1][1][1][2]);

        System.out.println(colors[0][0][0][0]);

        System.out.println(colors[1][1][1][1]);
    }
}

Ans :
APPLE GREEN
GREEN
BLUE
CHERRY
RED
INDIGO

19) Does below program compile successfully?

class A
{
    int i = 10;
}

class B extends A
{
    int j = 20;
}

class C extends B
{
    int k = 30;
}

class D extends C
{
    int m = 40;
}

public class ArraysInJava
{
    public static void main(String[] args)
    {
        A[] a = {new A(), new B(), new C(), new D()};

        System.out.println(a[3].i);

        System.out.println(a[2].j);

        System.out.println(a[1].k);

        System.out.println(a[0].m);
    }
}

Ans : No, It gives compile time error

20) What will be the output of this program?

public class ArraysInJava
{
    static Double[] methodOne(Double[] D)
    {
        D[1] = 36.25;

        return methodTwo(D);
    }

    static Double[] methodTwo(Double[] D)
    {
        D[1] = 62.36;

        return methodThree(D);
    }

    static Double[] methodThree(Double[] D)
    {
        D[1] = 93.58;

        return D;
    }

    public static void main(String[] args)
    {
        Double[] D = {10.55, 25.36, 58.29, 74.32, 32.21};

        D = methodOne(D);

        System.out.println(D[1]);
    }
}

Ans : 93.58