Home Java

Interface in Java

An interface is define as an abstract type in the Java that is used to define a class's behaviour. An interface is a template for behaviour in Java. Static constants and abstract methods are both parts of a Java interface.
An interface is a mechanism for achieving 100% abstraction in Java. Only abstract methods, not method bodies, are not allowed in the Java interface it means only method signature are devlared, no body. It is used in Java to provide abstraction and multiple inheritance. Java Interface represents the IS-A connection.

Table Of Content

Interface in Java
Uses of Interface
How to declare an interface
Java multiple inheritance with interface
JDK 8 : Static Method in Interface
JDK 8 : Default Method in Interface
Marker or Tagged Interface
Difference Between Class and Interface

We should describe an entity as an interface when we identify its type based on its behaviour rather than its properties.

An interface is same as class. it include variables and methods, but by default, the methods declared in an interface are abstract it means just method signature, no body.

Important point for Interface

The interface keyword should be used to declare an interface. It means complete abstraction. By default, all fields in an interface are public, static, and final, and all methods are declared with an empty body and are all public.
Any method that is declared in an interface must be implemented by the class that implements it and need to declare the implements keyword to implement/extends an interface.
The IS-A relationship is represented by the Java interface.
There is no constructor for an interface.
Java Interfaces cannot be created in the same way as we cerate abstract classes.
We can include default and static methods in an interface as of Java 8 and Private methods are now possible in interfaces from Java 9.
Interfaces define what a class needs to do it, not how to do it. It is the behavior's design manual.
If any class implements an interface but it does not provide method bodies for all of the interface's method then the class must be declared as abstract.

Uses an Interface

Total abstraction is achieved by using interface.
Java does not support multiple inheritances for classes, so multiple inheritances can be achived by using an interface.
Any class can implement an infinite number of interfaces but can only extend one other class.
To achieve loose coupling, interface is also used.
Abstraction improves the application's modularity.
The enhancement will be very simple in abstraction because we will be able to make changes to our internal system without affecting end users.

The compiler's internal addition to the interface

Before the interface method, the Java compiler adds the keywords public and abstract.

Before data members, the compiler also adds the public, static, and final keywords.

By default, interface fields are public, static, and final and interface methods are public and abstract .

Relationship between class and interface

How to declare an interface?

Syntax

 interface {

   // declare constant fields

   // declare methods that abstract

   // by default.

Java Interface Example

 // Java Interface Example

interface Displayable {
	void display();
}
class Test implements Displayable {
	public void display(){
	System.out.println("Hello");
	}
}
class Main{
	public static void main(String[] args){
		Test test = new Test();
		test.display();
	}
}

Output:

Hello

Java Interface Example

In below example Shape Interface has only one method draw but its implementation is provided by circle and triangle classes.It means interface is defined by someone else, but its implementation is provided by some other people and used by someone else but implementation part is hidden by the user who uses the interface.

interface Shape {
	void Draw();
}
class Triangle implements Shape {
	public void Draw(){
	 System.out.println("Drawing Triangle......");
	}
}
class Circle implements Shape {
	public void Draw(){
	 System.out.println("Drawing Circle......");
	}
}
class Main{
	public static void main(String[] args){
		Shape shape = new Triangle();
		shape.Draw();
		
		Shape shape2 = new Circle();
		shape2.Draw();
		
	}
}

Output:

Drawing Triangle

Drawing Circle

Java multiple inheritance with interface

Due to ambiguity, multiple inheritance is not supported in the case of classes. However, since there is no ambiguity in the case of an interface, it is supported.

Multiple inheritance occurs when a class implements multiple interfaces or when an interface extends multiple interfaces.

Multiple inheritance with interface

interface Shape {
	void Draw();
}
interface Displayable{  
  void Display();  
}  
class Circle implements Shape,Displayable { // multiple inheritance
	@Override
	public void Draw() {
		// TODO Auto-generated method stub
		System.out.println("Drawing Circle......");
	}
	
	@Override
	public void Display() {
		// TODO Auto-generated method stub
		System.out.println("Display Circle.");
	}
}
class Main{
	public static void main(String[] args){
		Circle circle = new Circle();
		circle.Draw();
		circle.Display();		
	}
}

Output:

Drawing Circle......

Display Circle.

Interface Inheritance

Example of Interface inheritance

interface Displayable{  
    void Display();  
} 
interface Shape extends Displayable {
	void Draw();
}
class Circle implements Shape{ 
	@Override
	public void Draw() {
		// TODO Auto-generated method stub
		System.out.println("Drawing Circle......");
	}
	
	@Override
	public void Display() {
		// TODO Auto-generated method stub
		System.out.println("Display Circle");
	}
}
class Main{
	public static void main(String[] args){
		Circle circle = new Circle();
		circle.Draw();
		circle.Display();		
	}
}

Output:

Drawing Circle......

Display Circle

JDK 8 : Static Method in Interface

Since java 8 static methods are defined in interfaces, they can be called without an object. however, these method are not inherited.

static method example

interface  Bike{
	void MaxSpeed();
    static void StaticCheck(){ //  Static method in interface 
        System.out.println("Static method  from interface ");
    }
}
class Hero implements Bike{   
   	 public void MaxSpeed() {
 		System.out.println("Max Speed is : 140 kmph");	
 	 }  
}
class Main{
	public static void main(String[] args) {
		Hero hero=new Hero();
		hero.MaxSpeed();
		Bike.StaticCheck(); // static method called without object creation
	}
}

Output:

Max Speed is : 140 kmph

Static method  from interface

JDK 8 : Default Method in Interface

Since Java 8, method body can be define in interfaces. However, we must make it default method with default keyword.

above concept is describe in below example

interface  Bike{
	void MaxSpeed();
	default void DefaultCheck(){ 
        System.out.println("Default method  from interface");
    }
}
class Hero implements Bike{   
   	 public void MaxSpeed() {
 		System.out.println("Max Speed is : 140 kmph");	
 	 }  
}
class Main{
	public static void main(String[] args) {
		Bike bike=new Hero();
		bike.MaxSpeed();
		bike.DefaultCheck();	
	}
}

Output:

Max Speed is : 140 kmph

Default method  from interface

Marker or Tagged Interface

An interface that contains neither methods nor constants is referred to as a marker interface. The compiler and JVM can learn more about the object because it provides run-time type information.

Marker interface is also called tagging interface.

Syntax

 public interface  Deletable {

Comparison Chart

Difference Between Class and Interface are mention below

S.N	Class	Interface
1.	we can instantiate variables and make objects in a class.	we cannot instantiate variables and make an object in an interface.
2.	Classes may include methods with implementation	The implementation of methods is not permitted in the interface.
3.	Classes can use the private, protected, and public access specifiers.	Public is the only specifier used in Interface.

Abstract class vs Interface Next »

« Perv Next »

Interface in Java

Important point for Interface

Uses an Interface

The compiler's internal addition to the interface

Relationship between class and interface

How to declare an interface?

Java Interface Example

Java Interface Example

Java multiple inheritance with interface

Multiple inheritance with interface

Interface Inheritance

Example of Interface inheritance

JDK 8 : Static Method in Interface

static method example

JDK 8 : Default Method in Interface

above concept is describe in below example

Marker or Tagged Interface

Comparison Chart

Post your comment

Useful Links

Contact Us

About DockerTpoint