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Java Interfaces

Interface is a concept which is used to achieve abstraction in Java. This is the only way by which we can achieve full abstraction. Interfaces are syntactically similar to classes, but you cannot create instance of an Interface and their methods are declared without any body. It can have When you create an interface it defines what a class can do without saying anything about how the class will do it.

It can have only abstract methods and static fields. However, from Java 8, interface can have default and static methods and from Java 9, it can have private methods as well.

When an interface inherits another interface extends keyword is used whereas class use implements keyword to inherit an interface.

Advantages of Interface

  • It Support multiple inheritance
  • It helps to achieve abstraction
  • It can be used to achieve loose coupling.

Syntax:

interface interface_name {  
// fields
// abstract/private/default methods
}

Interface Key Points

  • Methods inside interface must not be static, final, native or strictfp.
  • All variables declared inside interface are implicitly public, static and final.
  • All methods declared inside interfaces are implicitly public and abstract, even if you don't use public or abstract keyword.
  • Interface can extend one or more other interface.
  • Interface cannot implement a class.
  • Interface can be nested inside another interface.

Time for an Example!

Let's take a simple code example and understand what interfaces are:

interface Moveable
{
 	int AVERAGE-SPEED = 40;
 	void move();
}

Interface in Java

NOTE: Compiler automatically converts methods of Interface as public and abstract, and the data members as public, static and final by default.

Example of Interface implementation

In this example, we created an interface and implemented using a class. lets see how to implement the interface.

interface Moveable
{
 	int AVG-SPEED = 40;
 	void move();
}

class Vehicle implements Moveable
{
 	public void move()
 	{
  		System .out. print in ("Average speed is"+AVG-SPEED");
 	}
 	public static void main (String[] arg)
 	{
  		Vehicle vc = new Vehicle();
  		vc.move();
 	}
}

Average speed is 40.


Interfaces supports Multiple Inheritance

Though classes in Java doesn't support multiple inheritance, but a class can implement more than one interfaces.

In this example, two interfaces are implemented by a class that show implementation of multiple inheritance.

interface Moveable
{
 	boolean isMoveable();
}

interface Rollable
{
 	boolean isRollable
}

class Tyre implements Moveable, Rollable
{
 	int width;

 	boolean isMoveable()
 	{
  		return true;
 	}

 	boolean isRollable()
 	{
  		return true;
 	}
 	
 	public static void main(String args[])
 	{
  		Tyre tr = new Tyre();
  		System.out.println(tr.isMoveable());
  		System.out.println(tr.isRollable());
 	}
}

true true


Interface extends other Interface

Interface can inherit to another interface by using extends keyword. But in this case, interface just inherit, does not provide implementation. Implementation can be provided by a class only.

interface NewsPaper
{
 	news();
}

interface Magazine extends NewsPaper
{
 	colorful();
}

Difference between an interface and an abstract class?

Interface and abstract class both are used to implement abstraction but have some differences as well. Some of differences are listed below.

Abstract classInterface
Abstract class is a class which contain one or more abstract methods, which has to be implemented by its sub classes. Interface is a Java Object containing method declaration but no implementation. The classes which implement the Interfaces must provide the method definition for all the methods.
Abstract class is a Class prefix with an abstract keyword followed by Class definition.Interface is a pure abstract class which starts with interface keyword.
Abstract class can also contain concrete methods.Whereas, Interface contains all abstract methods and final variable declarations.
Abstract classes are useful in a situation that Some general methods should be implemented and specialization behavior should be implemented by child classes.Interfaces are useful in a situation that all properties should be implemented.

Default Methods in Interface – Java 8

In Java 8 version a new feature is added to the interface, which was default method. Default method is a method that can have its body. It means default method is not abstract method, it is used to set some default functionality to the interface.

Java provides default keyword to create default method. Let's see an example:

interface Abc{  
    // Default method   
    default void msg(){  
        System.out.println("This is default method");  
    }  
    // Abstract method  
    void greet(String msg);  
}  
public class Demo implements Abc{  
    public void greet(String msg){        // implementing abstract method   
        System.out.println(msg);  
    }  
    public static void main(String[] args) {  
    	Demo d = new Demo();  
        d.msg();   // calling default method  
        d.greet("Say Hi");  // calling abstract method  
  
    }  
} 

This is default method Say Hi

Static methods in Interface – Java 8

From Java 8, Java allows to declare static methods into interface. The purpose of static method is to add utility methods into the interface. In the below example, we have created an interface Abc that contains a static method and an abstract method as well. See the below example.

interface Abc{  
    // static method   
    static void msg(){  
        System.out.println("This is static method");  
    }  
    // Abstract method  
    void greet(String msg);
    
    
}  
public class Demo implements Abc{  
    public void greet(String msg){        // implementing abstract method   
        System.out.println(msg);  
    }  
    public static void main(String[] args) {  
    	Demo d = new Demo();  
        Abc.msg();   // calling static method  
        d.greet("Say Hi");  // calling abstract method  
  
    }  
} 

This is static method Say Hi

Private methods – Java 9

In Java 9 version, a new feature is added that allows us to declare private methods inside the interface. The purpose of private method is just to share some task between the non-abstract methods of the interface.

In this example, we created an interface Abc that has a default method and a private method as well. Since private methods are not accessible outside to interface. So, we called it from the default method. See the below example.

interface Abc{  
    // Default method   
    default void msg(){  
         greet();
    }  
    // Private method  
    private void greet() {
    	System.out.println("This is private method"); 
    }
    
    
}  
public class Demo implements Abc{    
    public static void main(String[] args) {  
    	Demo d = new Demo();  
        d.msg();   // calling default method  
  
    }  
}  

This is private method