Java Interfaces

Java Interfaces

An interface is a contract of methods that a class promises to implement. Interfaces support multiple inheritance, which classes alone cannot.

Declaring an interface

interface Animal {
  public void animalSound();
  public void sleep();
}

Methods in an interface are abstract by default.

Implementing an interface

Use the implements keyword:

class Pig implements Animal {
  public void animalSound() {
    System.out.println("The pig says: wee wee");
  }

  public void sleep() {
    System.out.println("Zzz");
  }
}

Multiple interfaces

A class can implement many interfaces at once:

class Demo implements FirstInterface, SecondInterface {
  // implement all methods from both interfaces
}

Interface vs abstract class

TL;DR

• Interfaces declare method contracts.
• Classes implement them with the implements keyword.
• A class can implement multiple interfaces.