Question

Java Programming Questions (Multiple Choice)

** Please only answer if you know it, not just copy and paste from another user. Please explain the answer.

1) Given code:

public abstract class A{ }

public class B extends A

{

}

The following code sequence would correctly create an object reference of class A holding an object reference for an object of class B:

A c;

c = new B();

	TRUE
	FALSE

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2) After the following code sequence is executed, what are the contents at ArrayList index 1 in a?

ArrayList a = new ArrayList();

a.add(7);

a.add(10);

a.add(21);

a.set(1,4);

a.remove(0);

a	10
b	7
c	4
d	21

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3) For this question, consider the following two classes:

public abstract class C

{

    public void foo1()

   {

     System.out.println("Hello foo1");

    }

    public abstract void foo2();

    public abstract void foo3();

    public void foo1Call()

     {

        foo1();

     }

   }

public class D extends C

{

    public void foo2()

    {

        System.out.println("Hello foo2");

     }

   public void foo4()

{

       System.out.println("Hello D foo4()");

}

}

Which of the following code sequences, if any, will successfully access private method foo1 in class C?

a	C c2 = new C(); c2.foo1();
b	C c2 ; c2 = new D(); c2.foo1();
c	D d1 = new D(); d1.foo1();
d	C c1 = new D(); c1.foo1Call();
e	None of the above

Answer 1

1)True

Explanation: In java we cannot create object directly to a abstract class. But here what we are doing is first we ahve declared reference of class A nd then

we have created an object of class B and assigned to reference variable of class A. Class reference varaiable can can hold object of class B since A is parent of B.

So the following code correctly assigs the object reference of class B to reference of class A.

2) Answer is 21

a.add(7);

a.add(10);

a.add(21);

with these insertions arraylist will have elements in following order [7,10,21]

a.set(1,4);

now with the above statement it overwrites the value at position 1 with 4 so now list will be as follows

[7,4,21]

a.remove(0); will remove 7 from list and now list will be

[4,21]

so now at index 1 21 will be there.

3) none of the above is the right answer.

Explanation: first of all foo1() in class C is not private. And also since D is extending class C it should implement all unimplemented methods of class C. But that

doesnot happen or else class D should be abstract that was not declared. If sumed all are correct then option B and C are right answers.