JAVA INTERFACES AND ABSTRACT CLASSES QUIZ

Which of the following statements regarding abstract classes are true?

All of these

An abstract class can be extended.

An abstract class can be used as a data type.

A subclass can override a concrete method in a superclass to declare it abstract.

A subclass of a non-abstract superclass can be abstract.

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YOUR ANSWER : ?

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().

The method numberOfStudent() in class School must have a body.

The keywords public and abstract cannot be used together.

Class School must be defined abstract.

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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 A followed by b is an instance of

b is an instance of

Nothing.

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Which two of the following are legal declarations for abstract classes and interfaces?1. final abstract class Test {}2. public static interface Test {}3. final public class Test {}4. protected abstract class Test {}5. protected interface Test {}6. abstract public class Test {}

3 and 6

5 and 6

1 and 2

2 and 4

3 and 5

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What will happen after compiling this program code?abstract class MyClass{ //line 1 private int a, b; public void call(int a, int b){ this.a = a; this.b = b; System.out.print(a+b); }}public class Test{ public static void main(String args[]){ MyClass m = new MyClass(); //line 2 m.call(12,25); }}

Compilation error due to line 1

Successful run and print 37

Runtime error

None of these

Compilation error due to line 2

TRUE ANSWER : ?
YOUR ANSWER : ?

Which of the following class definitions defines a legal abstract class?

abstract class A { abstract void unfinished(); }

class A { abstract void unfinished() { } }

class A { abstract void unfinished(); }

public class abstract A { abstract void unfinished(); }

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Suppose A is an abstract class, B is a concrete subclass of A, and both A and B have a default constructor. Which of the following is correct?1. A a = new A();2. A a = new B();3. B b = new A();4. B b = new B();

2 and 3

3 and 4

2 and 4

1 and 2

1 and 3

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YOUR ANSWER : ?

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

3 and 4

1 and 3

1 and 2

2 and 3

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Given the following piece of code:public interface Guard{ void doYourJob();}abstract public class Dog implements Guard{ }which of the following statements is correct?

This code will not compile, because in the declaration of class Dog we must use the keyword extends instead of implements.

This code will not compile, because class Dog must implement method doYourJob() from interface Guard.

This code will compile without any errors.

This code will not compile, because method doYourJob() in interface Guard must be defined abstract.