Declaring Interface Program in Java | Interface in Java

Declaring Interface:

In Java interfaces are declaring only the contract and no implementation, like a 100% abstract superclass. All methods declared in an interface are public and abstract (do not need to actually type the public and abstract modifiers in the method declaration, but the method is still always public and abstract). Interface methods must not be static. Because interface methods are abstract, they cannot be marked final, strictfp, or native. All variables in an interface are public, static, and final in interfaces. An interface can extend one or more other interfaces. An interface cannot implement another interface or class.

Declaring Interface Example:-

 interface Bounceable{
    public abstract void bounce();
    void setBounce(int b);
 }

 public class Tire implements Bounceable{
    public void bounce(){
      System.out.println(”Love”);
    }

    public void setBounce(int b){
      int a = b;
      System.out.println(a);
    }

    public static void main(String[]arge){
       Tire t = new Tire();
       t.bounce();
       t.setBounce(15);
   }
 }


Definition of Declaring Interfaces:

The interface keyword is used to declare an interface. Here is a simple example to declare an interface:

Example:-

Let us look at an example that depicts encapsulation:

/* File name : NameOfInterface.java */
import java.lang.*;
//Any number of import statements

public interface NameOfInterface
{
   //Any number of final, static fields
   //Any number of abstract method declarations\
}

Interfaces have the following properties:-

    # An interface is implicitly abstract. You do not need to use the abstract keyword when declaring an interface.

    # Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.

    # Methods in an interface are implicitly public.

Example:

/* File name : Animal.java */
interface Animal {

    public void eat();
    public void travel();
}


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.

Aclass uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.

/* File name : MammalInt.java */
public class MammalInt implements Animal{

   public void eat(){
      System.out.println("Mammal eats");
   }

   public void travel(){
      System.out.println("Mammal travels");
   }

   public int noOfLegs(){
      return 0;
   }

   public static void main(String args[]){
      MammalInt m = new MammalInt();
      m.eat();
      m.travel();
   }
}

This would produce following result:

Mammal eats
Mammal travels

When overriding methods defined in interfaces there are several rules to be followed:

Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.

The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.

An implementation class itself can be abstract and if so interface methods need not be implemented.

When implementation interfaces there are several rules:

    # A class can implement more than one interface at a time.

    # A class can extend only one class, but implement many interface.

    # An interface itself can extend another interface. An interface cannot extend another interface.

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

Tagging Interfaces:

The most common use of extending interfaces occurs when the parent interface does not contain any methods. For example, the MouseListener interface in the java.awt.event package extended java.util.EventListener, which is defined as:

package java.util;
public interface EventListener
{}

An interface with no methods in it is referred to as a tagging interface. There are two basic design purposes of tagging interfaces:

Creates a common parent: As with the EventListener interface, which is extended by dozens of other interfaces in the Java API, you can use a tagging interface to create a common parent among a group of interfaces. For example, when an interface extends EventListener, the JVM knows that this particular interface is going to be used in an event delegation scenario.

Adds a data type to a class: This situation is where the term tagging comes from. A class that implements a tagging interface does not need to define any methods (since the interface does not have any), but the class becomes an interface type through polymorphism.

Java Interface Example:-

    /*
    Java Interface example.
    This Java Interface example describes how interface is defined and
    being used in Java language.
    
    Syntax of defining java interface is,
    <modifier> interface <interface-name>{
      //members and methods()
    }
    */
    
    //declare an interface
    interface IntExample{
    
      /*
      Syntax to declare method in java interface is,
      <modifier> <return-type> methodName(<optional-parameters>);
      IMPORTANT : Methods declared in the interface are implicitly public and abstract.
      */
    
      public void sayHello();
      }
    }

    /*
    Classes are extended while interfaces are implemented.
    To implement an interface use implements keyword.
    IMPORTANT : A class can extend only one other class, while it
    can implement n number of interfaces.
    */
    
    public class JavaInterfaceExample implements IntExample{
      /*
      We have to define the method declared in implemented interface,
      or else we have to declare the implementing class as abstract class.
      */
    
      public void sayHello(){
        System.out.println("Hello Visitor !");
      }
    
      public static void main(String args[]){
        //create object of the class
        JavaInterfaceExample javaInterfaceExample = new JavaInterfaceExample();
        //invoke sayHello(), declared in IntExample interface.
        javaInterfaceExample.sayHello();
      }
    }
    
    /*
    OUTPUT of the above given Java Interface example would be :
    Hello Visitor !
    */