Implementing interfaces in Java

Introduction

Interfaces let you define a set of actions that different classes can do. Implementing interfaces helps make sure classes follow the same rules.

When you want different classes to share the same actions but have different details.

When you want to make sure a class has certain methods before using it.

When you want to write code that works with many types of objects in the same way.

When you want to separate what a class can do from how it does it.

Syntax

Java

interface InterfaceName {
    returnType methodName();
}

class ClassName implements InterfaceName {
    @Override
    public returnType methodName() {
        // method body
    }
}

Use the implements keyword to say a class follows an interface.

All methods in an interface must be implemented in the class.

Examples

This example shows an interface Animal with a method sound. The class Dog implements it and defines the sound.

Java

interface Animal {
    void sound();
}

class Dog implements Animal {
    @Override
    public void sound() {
        System.out.println("Woof");
    }
}

Here, Vehicle interface has two methods. Car class implements both methods.

Java

interface Vehicle {
    void start();
    void stop();
}

class Car implements Vehicle {
    @Override
    public void start() {
        System.out.println("Car started");
    }
    @Override
    public void stop() {
        System.out.println("Car stopped");
    }
}

Sample Program

This program defines an interface Printer with a method print. Two classes implement it differently. The Main class creates objects and calls their print methods.

Java

interface Printer {
    void print();
}

class InkjetPrinter implements Printer {
    @Override
    public void print() {
        System.out.println("Printing with Inkjet Printer");
    }
}

class LaserPrinter implements Printer {
    @Override
    public void print() {
        System.out.println("Printing with Laser Printer");
    }
}

public class Main {
    public static void main(String[] args) {
        Printer inkjet = new InkjetPrinter();
        Printer laser = new LaserPrinter();
        inkjet.print();
        laser.print();
    }
}

Important Notes

Interfaces cannot have method bodies (except default or static methods in newer Java versions).

A class can implement multiple interfaces separated by commas.

Use @Override to help catch mistakes when implementing methods.

Summary

Interfaces define what methods a class must have.

Use implements keyword for a class to follow an interface.

All interface methods must be implemented in the class.