Java – Interface

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An interface in java is a blueprint of a class. It has static constants and abstract methods.

The interface in java is a mechanism to achieve abstraction. There can be only abstract methods in the java interface not method body. It is used to achieve abstraction and multiple inheritance in Java.

Java Interface also represents IS-A relationship.

It cannot be instantiated just like abstract class.

Why use Java interface?

There are mainly three reasons to use interface. They are given below.

  • It is used to achieve abstraction.
  • By interface, we can support the functionality of multiple inheritance.
  • It can be used to achieve loose coupling.

Java 8 Interface Improvement

Since Java 8, interface can have default and static methods which is discussed later.

Internal addition by compiler

The java compiler adds public and abstract keywords before the interface method. More, it adds public, static and final keywords before data members.

In other words, Interface fields are public, static and final by default, and methods are public and abstract.

interface in java


Understanding relationship between classes and interfaces

As shown in the figure given below, a class extends another class, an interface extends another interface but a class implements an interface.

relationship between class and interface


Java Interface Example

In this example, Printable interface has only one method, its implementation is provided in the A class.

interface printable {
    void print();
}

public class A6 implements printable {
    public void print() {
        System.out.println("Hello");
    }

    public static void main(String args[]) {
        A6 obj = new A6();
        obj.print();
    }
}

Test it NowOutput:

Hello

Java Interface Example: Drawable

In this example, Drawable interface has only one method. Its implementation is provided by Rectangle and Circle classes. In real scenario, interface is defined by someone but implementation is provided by different implementation providers. And, it is used by someone else. The implementation part is hidden by the user which uses the interface.

File: TestInterface1.java

//Interface declaration: by first user
interface Drawable {
    void draw();
}

//Implementation: by second user
class Rectangle implements Drawable {
    public void draw() {
        System.out.println("drawing rectangle");
    }
}

class Circle implements Drawable {
    public void draw() {
        System.out.println("drawing circle");
    }
}

//Using interface: by third user
public class TestInterface1 {
    public static void main(String args[]) {
        Drawable d = new Circle();//In real scenario, object is provided by method e.g. getDrawable()
        d.draw();
    }
}

Test it NowOutput:

drawing circle

Java Interface Example: Bank

Let’s see another example of java interface which provides the implementation of Bank interface.

File: TestInterface2.java

interface Bank {
    float rateOfInterest();
}

class SBI implements Bank {
    public float rateOfInterest() {
        return 9.15f;
    }
}

class PNB implements Bank {
    public float rateOfInterest() {
        return 9.7f;
    }
}

public class TestInterface2 {
    public static void main(String[] args) {
        Bank b = new SBI();
        System.out.println("ROI: " + b.rateOfInterest());
    }
}

Test it NowOutput:

ROI: 9.15

Multiple inheritance in Java by interface

If a class implements multiple interfaces, or an interface extends multiple interfaces i.e. known as multiple inheritance.

 multiple inheritance in java

interface Printable {
    void print();
}

interface Showable {
    void show();
}

public class A7 implements Printable, Showable {
    public void print() {
        System.out.println("Hello");
    }

    public void show() {
        System.out.println("Welcome");
    }

    public static void main(String args[]) {
        A7 obj = new A7();
        obj.print();
        obj.show();
    }
}

Test it Now

Output:Hello
       Welcome

Q) Multiple inheritance is not supported through class in java but it is possible by interface, why?

As we have explained in the inheritance chapter, multiple inheritance is not supported in case of class because of ambiguity. But it is supported in case of interface because there is no ambiguity as implementation is provided by the implementation class. For example:

interface Printable {
    void print();
}

interface Showable {
    void print();
}

public class TestInterface3 implements Printable, Showable {
    public void print() {
        System.out.println("Hello");
    }

    public static void main(String args[]) {
        TestInterface3 obj = new TestInterface3();
        obj.print();
    }
}

Test it NowOutput:

Hello

As you can see in the above example, Printable and Showable interface have same methods but its implementation is provided by class TestTnterface1, so there is no ambiguity.


Interface inheritance

A class implements interface but one interface extends another interface .

interface Printable {
    void print();
}

interface Showable extends Printable {
    void show();
}

class TestInterface4 implements Showable {
    public void print() {
        System.out.println("Hello");
    }

    public void show() {
        System.out.println("Welcome");
    }

    public static void main(String args[]) {
        TestInterface4 obj = new TestInterface4();
        obj.print();
        obj.show();
    }
}

Test it NowOutput:

Hello
Welcome

Q) What is marker or tagged interface?

An interface that have no member is known as marker or tagged interface. For example: Serializable, Cloneable, Remote etc. They are used to provide some essential information to the JVM so that JVM may perform some useful operation.

//How Serializable interface is written?  
public interface Serializable{
}

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