Java Interface

Data Abstraction in Java is also achieved by Java Interface. An Interface is a completely “abstract class” which is used to group related methods with empty bodies.

Example

//interface
interface AnimalSound {
       public void animalSound();
       public void sleep();
}

To access the interface methods, the interface must be “implemented” (like inherited) by another class using the implements keyword (instead of extends keyword).

Example

In the below example, we implemented the two interface methods animalSound() and sleep() in another classes Dog and Cow.

AnimalSound.java

//interface
interface AnimalSound {
       public void animalSound();
       public void sleep();
}
 
class Dog implements AnimalSound{
       public void animalSound(){
              System.out.println("Dog makes bow bow sound");
       }
       public void sleep(){
              System.out.println("Zzzzz");
       }
}
 
class Cow implements AnimalSound{
       public void animalSound(){
              System.out.println("Cow makes moo moo sound");
       }
       public void sleep(){
              System.out.println("Zzzzz");
       }
}

AnimalSoundClass.java

public class AnimalSoundClass {
       public static void main(String args[]){
              AnimalSound myDog = new Dog();
              AnimalSound myCow = new Cow();
             
              myDog.animalSound();
              myDog.sleep();
              myCow.animalSound();
       }
}

Output

Dog makes bow bow sound
Zzzzz
Cow makes moo moo sound

Notes

• We cannot create a object for Interfaces like abstract classes. If we try to create an object for the interface in the main class, it throws an error.
• Interface methods do not have a body. The body is provided by the “implement” class.
•  Interface methods are abstract by default.
•  Interface methods are also public by default. Hence it can be implemented in other classes.
• Interface attributes are by default public, static and final.
•  An interface class cannot contain a constructor  (as we are not allowed to create objects for interfaces).

Why Interface?

• We could able to achieve security using Interface. We can hide details and only show the important details of the object.
• Java does not support “multiple inheritance”. A class can only inherited from one class. However, we can implement more than one interface in a class. To implement multiple interfaces, we separate them with commas.

Multiple Interfaces

To implement multiple interfaces, we separate them with commas.

Example

We implement two interfaces in the MultipleInterface class:

FirstInterface.java

interface FirstInterface {
       public void firstMethod();
}
 
interface SecondInterface {
       public void secondMethod();
}
 
class MultipleInterfaces implements FirstInterface, SecondInterface {
       public void firstMethod() {
              System.out.println("First Method called");
       }
       public void secondMethod() {
              System.out.println("Second Method called");
       }
}

MyMainJavaClass.java

public class MyMainJavaClass {
       public static void main(String args[]){
              MultipleInterfaces myObj = new MultipleInterfaces();
              myObj.firstMethod();
              myObj.secondMethod();
       }
}

Output

First Method called
Second Method called