java interface

Interfaces

Another way to achieve abstraction in Java, is with interfaces.

An interface is a completely "abstract class"

that is used to group related methods with empty bodies:

Example

// interface

interface Animal {

  public void animalSound(); // interface method (does not have a body)

  public void run(); // interface method (does not have a body)

}

To access the interface methods, the interface must be "implemented"

(kinda like inherited) by another class with the implements

keyword (instead of extends). The body of the

interface method is provided by the "implement" class:

Example

// Interface

interface Animal {

  public void animalSound(); // interface method (does not have a body)

  public void sleep(); // interface method (does not have a body)

}



// Pig "implements" the Animal interface

class Pig implements Animal {

  public void animalSound() {

    // The body of animalSound() is provided here

    System.out.println("The pig says: wee wee");

  }

  public void sleep() {

    // The body of sleep() is provided here

    System.out.println("Zzz");

  }

}



class Main {

  public static void main(String[] args) {

    Pig myPig = new Pig();  // Create a Pig object

    myPig.animalSound();

    myPig.sleep();

  }

}

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Notes on Interfaces:

Like abstract classes, interfaces cannot be used to create objects (in the example above,

it is not possible to create an "Animal" object in the MyMainClass)

Interface methods do not have a body - the

body is provided by the "implement" class

On implementation of an interface, you must override all of its methods

Interface methods are by default abstract and

public

Interface attributes are by default public,

static and final

An interface cannot contain a constructor (as it cannot be used to create objects)

Why And When To Use Interfaces?

1) To achieve security - hide certain details and only show the important

details of an object (interface).

2) Java does not support "multiple inheritance" (a class can only inherit from one superclass). However, it can be achieved

with interfaces, because the class can implement multiple interfaces.

Note: To implement multiple interfaces, separate them with a comma (see example below).

Multiple Interfaces

To implement multiple interfaces, separate them with a comma:

Example

interface FirstInterface {

  public void myMethod(); // interface method

}



interface SecondInterface {

  public void myOtherMethod(); // interface method

}



class DemoClass implements FirstInterface, SecondInterface {

  public void myMethod() {

    System.out.println("Some text..");

  }

  public void myOtherMethod() {

    System.out.println("Some other text...");

  }

}



class Main {

  public static void main(String[] args) {

    DemoClass myObj = new DemoClass();

    myObj.myMethod();

    myObj.myOtherMethod();

  }

}

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