Question 1

What will be the output of the program?

import java.util.*; 
class H 
{
    public static void main (String[] args) 
    { 
        Object x = new Vector().elements(); 
        System.out.print((x instanceof Enumeration)+","); 
        System.out.print((x instanceof Iterator)+","); 
        System.out.print(x instanceof ListIterator); 
    } 
}

Answer

Answer: D

The Vector.elements method returns an Enumeration over the elements of the vector. Vector implements the List interface and extends AbstractList so it is also possible to get an Iterator over a Vector by invoking the iterator or listIterator method.

Question 2

What will be the output of the program?

public class Test 
{ 
    private static float[] f = new float[2]; 
    public static void main (String[] args) 
    {
        System.out.println("f[0] = " + f[0]); 
    } 
}

Answer

Answer: B

The choices are between Option A and B, what this question is really testing is your knowledge of default values of an initialized array. This is an array type float i.e. it is a type that uses decimal point numbers therefore its initial value will be 0.0 and not 0

Question 3

What will be the output of the program?

import java.util.*; 
class I 
{
    public static void main (String[] args) 
    {
        Object i = new ArrayList().iterator(); 
        System.out.print((i instanceof List)+","); 
        System.out.print((i instanceof Iterator)+","); 
        System.out.print(i instanceof ListIterator); 
    } 
}

Answer

Answer: C

The iterator() method returns an iterator over the elements in the list in proper sequence, it doesn't return a List or a ListIterator object.

A ListIterator can be obtained by invoking the listIterator method.

Question 4

What will be the output of the program?

public class Test 
{ 
    private static int[] x; 
    public static void main(String[] args) 
    { 
        System.out.println(x[0]); 
    } 
}

Answer

Answer: D

In the above code the array reference variable x has been declared but it has not been instantiated i.e. the new statement is missing, for example:

private static int[]x = new int[5];

private static int[x] declares a static i.e. class level array.

the "new" keyword is the word that actually creates said array.

int[5] in association with the new sets the size of the array. so since the above code contains no new or size decalarations when you try and access x[0] you are trying to access a member of an array that has been declared but not intialized hence you get a NullPointerException at runtime.

Question 5

What will be the output of the program?

package foo; 
import java.util.Vector; /* Line 2 */
private class MyVector extends Vector 
{
    int i = 1; /* Line 5 */
    public MyVector() 
    { 
        i = 2; 
    } 
} 
public class MyNewVector extends MyVector 
{
    public MyNewVector () 
    { 
        i = 4; /* Line 15 */
    } 
    public static void main (String args []) 
    { 
        MyVector v = new MyNewVector(); /* Line 19 */
    } 
}

Answer

Answer: B

Option B is correct. The compiler complains with the error "modifier private not allowed here". The class is created private and is being used by another class on line 19.

Questions for: Objects And Collections