Menu
×
   ❮     
HTML CSS JAVASCRIPT SQL PYTHON JAVA PHP HOW TO W3.CSS C C++ C# BOOTSTRAP REACT MYSQL JQUERY EXCEL XML DJANGO NUMPY PANDAS NODEJS R TYPESCRIPT ANGULAR GIT POSTGRESQL MONGODB ASP AI GO KOTLIN SASS VUE DSA GEN AI SCIPY AWS CYBERSECURITY DATA SCIENCE
     ❯   

Java Tutorial

Java HOME Java Intro Java Get Started Java Syntax Java Output Java Comments Java Variables Java Data Types Java Type Casting Java Operators Java Strings Java Math Java Booleans Java If...Else Java Switch Java While Loop Java For Loop Java Break/Continue Java Arrays

Java Methods

Java Methods Java Method Parameters Java Method Overloading Java Scope Java Recursion

Java Classes

Java OOP Java Classes/Objects Java Class Attributes Java Class Methods Java Constructors Java Modifiers Java Encapsulation Java Packages / API Java Inheritance Java Polymorphism Java Inner Classes Java Abstraction Java Interface Java Enums Java User Input Java Date Java ArrayList Java LinkedList Java List Sorting Java HashMap Java HashSet Java Iterator Java Wrapper Classes Java Exceptions Java RegEx Java Threads Java Lambda Java Advanced Sorting

Java File Handling

Java Files Java Create/Write Files Java Read Files Java Delete Files

Java How To's

Add Two Numbers Count Words Reverse a String Sum of Array Elements Convert String to Array Sort an Array Find Array Average Find Smallest Element ArrayList Loop HashMap Loop Loop Through an Enum Area of Rectangle Even or Odd Number Positive or Negative Square Root Random Number

Java Reference

Java Reference Java Keywords Java String Methods Java Math Methods Java Output Methods Java Arrays Methods Java ArrayList Methods Java LinkedList Methods Java HashMap Methods Java Scanner Methods Java Iterator Methods Java Errors & Exceptions

Java Examples

Java Examples Java Compiler Java Exercises Java Quiz Java Server Java Certificate


Java interface Keyword

❮ Java Keywords


Example

An interface is an abstract "class" that is used to group related methods with "empty" bodies:

To access the interface methods, the interface must be "implemented" (kinda like inherited) by another class with the implements keyword (instead of extends). The body of the interface method is provided by the "implement" class:

// interface
interface Animal {
  public void animalSound(); // interface method (does not have a body)
  public void sleep(); // interface method (does not have a body)
}

// Pig "implements" the Animal interface
class Pig implements Animal {
  public void animalSound() {
    // The body of animalSound() is provided here
    System.out.println("The pig says: wee wee");
  }
  public void sleep() {
    // The body of sleep() is provided here
    System.out.println("Zzz");
  }
}

class MyMainClass {
  public static void main(String[] args) {
    Pig myPig = new Pig();  // Create a Pig object
    myPig.animalSound();
    myPig.sleep();
  }
}

Try it Yourself »


Definition and Usage

The interface keyword is used to declare a special type of class that only contains abstract methods.

To access the interface methods, the interface must be "implemented" (kinda like inherited) by another class with the implements keyword (instead of extends). The body of the interface method is provided by the "implement" class.

Notes on Interfaces:

  • It cannot be used to create objects (in the example above, it is not possible to create an "Animal" object in the MyMainClass)
  • Interface methods does not have a body - the body is provided by the "implement" class
  • On implementation of an interface, you must override all of its methods
  • Interface methods are by default abstract and public
  • Interface attributes are by default public, static and final
  • An interface cannot contain a constructor (as it cannot be used to create objects)

Why And When To Use Interfaces?

To achieve security - hide certain details and only show the important details of an object (interface).

Java does not support "multiple inheritance" (a class can only inherit from one superclass). However, it can be achieved with interfaces, because the class can implement multiple interfaces. Note: To implement multiple interfaces, separate them with a comma (see example below).


Multiple Interfaces

To implement multiple interfaces, separate them with a comma:

Example

interface FirstInterface {
  public void myMethod(); // interface method
}

interface SecondInterface {
  public void myOtherMethod(); // interface method
}

// DemoClass "implements" FirstInterface and SecondInterface
class DemoClass implements FirstInterface, SecondInterface {
  public void myMethod() {
    System.out.println("Some text..");
  }
  public void myOtherMethod() {
    System.out.println("Some other text...");
  }
}

class MyMainClass {
  public static void main(String[] args) {
    DemoClass myObj = new DemoClass();
    myObj.myMethod();
    myObj.myOtherMethod();
  }
}

Try it Yourself »


Related Pages

Read more about interfaces in our Java Interface Tutorial.


❮ Java Keywords

×

Contact Sales

If you want to use W3Schools services as an educational institution, team or enterprise, send us an e-mail:
sales@w3schools.com

Report Error

If you want to report an error, or if you want to make a suggestion, send us an e-mail:
help@w3schools.com

W3Schools is optimized for learning and training. Examples might be simplified to improve reading and learning. Tutorials, references, and examples are constantly reviewed to avoid errors, but we cannot warrant full correctness of all content. While using W3Schools, you agree to have read and accepted our terms of use, cookie and privacy policy.

Copyright 1999-2024 by Refsnes Data. All Rights Reserved. W3Schools is Powered by W3.CSS.