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Java – Interface with Syntax and Example

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An Interface in Java is the same as a class, like any other class, an interface can have methods and Variables. But unlike a class, the method in it is an abstract method (only method signature, contain no body), and also by default, the declared variables in an interface are public, static & final.

Uses of Interfaces:

  • It is used to achieve abstraction.
  • As we have learned that Java does not support multiple inheritance but through the interface, we can achieve multiple inheritance.
  • It is also used to achieve loose coupling.

Note: Abstraction cannot be used to create objects. Hence it cannot contain a constructor.

Declaring an Interface in Java:

The keyword ‘interface‘ is used to declare an interface. Syntax of Interface in Java:

interface interface_name 
 {
    
  // declare constant fields, abstract methods

 }

Example of interface:

interface Dog
 {
    public void bark(); // interface method 
    public void run(); // interface method
 }

Implementation of Interface in Java:

To implement an interface in a class, we use the keyword ‘implements’ The following example shows the use of implements. In the example below, the class Puppy implements the interface Dog.

interface Dog 
 {
    public void bark(); // interface method 
    public void eat(); // interface method 
 }

 // implementing Dog
 class Puppy implements Dog 
 {
    public void bark() 
    {
    // Body of a bark() method
    System.out.println("Dog Barks");
    }
    public void eat() 
    {
    // Body of a eat() method
    System.out.println("Dog eat Bones");
    }
 }

 public class MyMainClass 
 {
    public static void main(String[] args) 
    {
    Puppy pup = new Puppy();  //Puppy object
    pup.bark();
    pup.eat();
    }
 }

Output of Interfaces in Java:

Dog Barks
Dog eat Bones

Multiple Interfaces and inheritance in Java:

We know class in java allows only a single extends but with the use of interfaces, we can implement more than one parent interface.

Java Interface

In the diagram above,

  • The class extends a class
  • The interface extends an interface
  • class implements interface

Example of Multiple Interface in Java:

interface Interface1 
 {
    public void firstMethod(); // interface method
 }

 interface Interface2 
 {
    public void secondMethod(); // interface method
 }

 // InClass "implements" the above two interface
 class Inclass implements Interface1, Interface2 
 {
    public void firstMethod() 
    {
    System.out.println("Displaying first method");
    }
    public void secondMethod() 
    {
    System.out.println("Displaying second method");
    }
 }

 public class MyMainClass 
 {
    public static void main(String[] args) 
    {
    Inclass Obj = new Inclass();
    Obj.firstMethod();
    Obj.secondMethod();
    }
 }

Output:

Displaying first method
Displaying second method

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