16. What is an Interface and how to implements & extend the interfaces?

An interface is a reference type in Java, it is similar to class, it is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.

Along with abstract methods an interface may also contain constants, default methods, static methods, and nested types

Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.

An interface is similar to a class in the following ways:

1. An interface can contain any number of methods.
2. An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.
3. The byte code of an interface appears in a .class file.
4. Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

However, an interface is different from a class in several ways, including:

1. You cannot instantiate an interface.
2. An interface does not contain any constructors.
3. All of the methods in an interface are abstract.
4. An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.
5. An interface is not extended by a class; it is implemented by a class.
6. An interface can extend multiple interfaces.

Declaring Interfaces:

The interface keyword is used to declare an interface.

Example:

Below given is an example of an interface:

/* File name : NameOfInterface.java */

import java.lang.*;

public interface NameOfInterface

{

   //Any number of final, static fields

   //Any number of abstract method declarations\

}

Interfaces have the following properties:

1. An interface is implicitly abstract. You do not need to use the abstract keyword while declaring an interface.
2. Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.
3. Methods in an interface are implicitly public.

Example:

/* File name : Animal.java */

interface Animal {

   public void eating ();

   public void traveled ();

} 

Implementing Interfaces:

When a class implements an interface, you can think of the class as signing a contract, agreeing to perform the specific behaviors of the interface. If a class does not perform all the behaviors of the interface, the class must declare itself as abstract.

A class uses the implements keyword to implement an interface. 

/* File name : MammalInt.java */

public class MammalInt implements Animal{

   public void eating (){

      System.out.println("Mammal eats");

   }

   public void traveled (){

      System.out.println("Mammal travels");

   }

   public int noOfLegs(){

      return 0;

   }

   public static void main(String args[]){

      MammalInt m = new MammalInt();

      m. eating ();

      m. traveled ();

   }

}

Output:

Mammal eats

Mammal travels

When implementation interfaces there are several rules:

1. A class can implement more than one interface at a time.
2. A class can extend only one class, but implement many interfaces.
3. An interface can extend another interface, similarly to the way that a class can extend another class. 

Extending Interfaces:

An interface can extend another interface, similarly to the way that a class can extend another class. The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface.

The following Sports interface is extended by Hockey and Football interfaces.

//Filename: Sports.java

public interface Sports

{

   public void setHomeTeam(String name);

   public void setVisitingTeam(String name);

}

//Filename: Football.java

public interface Football extends Sports

{

   public void homeTeamScored(int points);

   public void visitingTeamScored(int points);

   public void endOfQuarter(int quarter);

}

//Filename: Hockey.java

public interface Hockey extends Sports

{

   public void homeGoalScored();

   public void visitingGoalScored();

   public void endOfPeriod(int period);

   public void overtimePeriod(int ot);

}

The Hockey interface has four methods, but it inherits two from Sports; thus, a class that implements Hockey needs to implement all six methods. Similarly, a class that implements Football needs to define the three methods from Football and the two methods from Sports. 

Extending Multiple Interfaces:

A Java class can only extend one parent class. Multiple inheritance is not allowed. Interfaces are not classes, however, and an interface can extend more than one parent interface.

The extends keyword is used once, and the parent interfaces are declared in a comma-separated list.

For example, if the Hockey interface extended both Sports and Event, it would be declared as:

public interface Hockey extends Sports, Event