JAVA Interfaces and Abstract Classes
Determine output of the following code.interface A { }class C { }class D extends C { }class B extends D implements A { }public class Test extends Thread{ public static void main(String[] args){ B b = new B(); if (b instanceof A) System.out.println("b is an instance of A"); if (b instanceof C) System.out.println("b is an instance of C"); }}

b is an instance of
b is an instance of
Nothing.
b is an instance of A followed by b is an instance of

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JAVA Interfaces and Abstract Classes
Which of the following statements regarding abstract classes are true?

A subclass can override a concrete method in a superclass to declare it abstract.
An abstract class can be used as a data type.
A subclass of a non-abstract superclass can be abstract.
An abstract class can be extended.
All of these

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JAVA Interfaces and Abstract Classes
interface Base{ boolean m1 (); byte m2(short s);}which two code fragments will compile?1. interface Base2 implements Base {}2. abstract class Class2 extends Base { public boolean m1(){ return true; }}3. abstract class Class2 implements Base {}4. abstract class Class2 implements Base { public boolean m1(){ return (7 > 4); }}5. abstract class Class2 implements Base { protected boolean m1(){ return (5 > 7) }}

4 and 5
1 and 2
1 and 3
2 and 3
3 and 4

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JAVA Interfaces and Abstract Classes
Given the following piece of code:public class School{ public abstract double numberOfStudent();}which of the following statements is true?

You must add a return statement in method numberOfStudent().
Class School must be defined abstract.
The keywords public and abstract cannot be used together.
The method numberOfStudent() in class School must have a body.

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JAVA Interfaces and Abstract Classes
In Java, declaring a class abstract is useful

To force developers to extend the class not to use its capabilities.
When it makes sense to have objects of that class.
To prevent developers from further extending the class.
When it doesn't make sense to have objects of that class.
When default implementations of some methods are not desirable.

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