Java interface Keyword

❮ Java Keywords

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 »

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.
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)
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).
To implement multiple interfaces, separate them with a comma:
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 »

Read more about interfaces in our Java Interface Tutorial.

❮ Java Keywords

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