Java Interfaces Tutorial

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.

Why use Java Interface?

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

Syntax

interface <interface_name> {
    // declare constant fields
    // declare methods that abstract 
    // by default.
}

Java Interface Example

interface Drawable {
    void draw();
}

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

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

class TestInterface1 {
    public static void main(String args[]) {
        Drawable d = new Circle();
        d.draw();  
    }
}

Java 8 Interface Improvements

Since Java 8, we can have default and static methods in an interface.

Default Method Example

interface Drawable {
    void draw();
    default void msg() {
        System.out.println("default method");
    }
}