Question

Abstract classes and Interfaces problems

10. Explain one similarity and one difference between abstract classes and interfaces.

11. Consider the following declarations.

public interface Shape{

int someFunction(Shape other);

//other functions not shown

}

public class Square implements Shape

{/*implementation not shown*/}

Which of the following function headings of someFunction must be added to the declaration of the Square class such that it will satisfy the Shape interface?

1. public int someFunction (Shape other)
2. public int someFunction (Square other)
3. public boolean someFunction(Object other)

12. In Java, all functions implement late binding, except when they are labeled with two keywords: which ones?

13. What is the output of the following code?

interface Vessel { }

interface Toy { }

class Boat implements Vessel { }

class Speedboat extends Boat implements Toy { }

public class Test {

     public static void main(String[] args) {

          String s = "0";

          Boat b = new Boat();

          Boat b2 = new Speedboat();

          Speedboat s2 = new Speedboat();

if((b instanceof Vessel) && (b2 instanceof Toy))

s += "1";

          if((s2 instanceof Vessel) && (s2 instanceof Toy))

s += "2";

          System.out.println(s);

     }

}

14. True/false

_____ When implementing an interface, the compiler will ensure that the method implementation has the intended behavior.

_____ An interface can be extended through inheritance.

_____ Variables can be added to an interface only if they are static, public and final.

_____ An interface should have a constructor.

_____ An abstract class can not implement an interface.   

_____ The Serializable interface includes no methods and no defined constants

15. What is the output of the following code?

public abstract class Base {

     public Base()

     { System.out.println(“Building Base class”); };

     public void message()

     { System.out.println(“Greetings from Base class”); };

}

public class Derived extends Base {

     public Derived()

     { System.out.println(“Building Derived class”);   };

     public void message()

     { System.out.println(“Greetings from Derived

class”); };

}

public class Test{

     public static void main(String[] args) {

          Base b = new Base();

          Base d = new Derived();

          b.message();

          d.message();

     }

}

Answer 1

10)

Difference

Main difference is methods of a Java interface are implicitly abstract and cannot have implementations. A Java abstract class can have instance methods that implements a default behavior

Similarity

Interface can not be instantiated. Same way, you can not instantiate abstract class.

That means you can not create object of interface or abstract class.

11)

public int someFunction (Shape other)

because this function is declared in the interface called shape and Square class is implements that interface so Square class have to implements this method.

12)

abstract and interface

13)

012

because Boat class implements vessel and Speed class extends from the Boat and implements the Toy so all the condition is true.

14)

1)True

2)True

A class implements an interface, thereby inheriting the abstract methods of the interface.

3)True

Any implementations can change value of fields if they are not defined as final. Then they would become a part of the implementation.An interface is a pure specification without any implementation.

If they are static, then they belong to the interface, and not the object, nor the run-time type of the object.

An interface provide a way for the client to interact with the object. If variables were not public, the clients would not have

access to them.

4)False

The answer is No, interface cannot have constructors. ... In order to call any method we need an object since there is no need to have object of interface, there is no need of having constructor in interface .

5)false

abstract class implements the interface

6) True

15)

Ouput

Building Base class

Building Derived class

Greetings from Base class

Greetings from Derived class

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