Interfaces in Java

An interface is a template for another class to follow. It defines strictly all of the abstract methods that any class that wants to implement must include. Interfaces are similar to abstract classes except that every method in an interface must be abstract, it does not contain a constructor, and it does not contain instance fields.

Declaring Instances

To declare a new instance, use the instance keyword:

public interface Phone {
    public void call();
    public void text();
}

Let's say we want to represent phones in our code. Phones can both call and text, but each phone does it slightly differently. This interface defines the methods that all phones need to do, so any class that implements this interface, will need to include their version of it.

Implementing Interfaces

Now let's create classes that implement our Phone interface using the implements keyword:

public class iPhone implements Phone {

   public void call() {
      System.out.println("iPhone call");
   }

   public void text() {
      System.out.println("iPhone Text");
   }
}

public class Pixel implements Phone {

   public void call() {
      System.out.println("Pixel call");
   }

   public void text() {
      System.out.println("Pixel Text");
   }
}

Now we can create actual Phone objects and invoke their interface methods:

Phone iPhone = new iPhone();
iPhone.call();
iPhone.text();

Phone pixel = new Pixel();
pixel.call();
pixel.text();

iPhone call
iPhone Text
Pixel call
Pixel Text

And just like that, we can define multiple instances of Phone and be guaranteed that each one will have the methods outlined in the interface.