public class ExampleClass implements KeyListener {
  public ExampleClass() { // constructor
    addKeyListener(this); // add this class as keylistener. if we don't implement keylistener, you can't use this function.
  }

  public void keyTyped(KeyEvent e) { // when a key is typed. This function will get called through the KeyListener interface.
    System.out.println("A key was typed!");
  }

  public void keyPressed(KeyEvent e) {
 /* we leave this function empty. we have to declare it though,
  * because the class has to implement all the functions of
  * KeyListener.
  */
  }
  public void keyReleased(KeyEvent e) { /* same counts for this function */ }
}

interface Animal {
  String noise();
}

class Dog implements Animal {
  String noise() {
    return "Woof";
  }
}

class Cat implements Animal {
  String noise() {
    return "Meow";
  }
}

class Test {
  static void speak(Animal animal) {
    System.out.println(animal.noise() + "!");
  }

  static void main(String[] args) {
    Dog pooch = new Dog();
    Cat percy = new Cat();
    speak(pooch);
    speak(percy);
  }
}

using System;

namespace InterfaceTesting
{
	// Our interface which describes the function Noise()
	// By default this function is public and virtual
	public interface IAnimal
	{
		void Noise();
	}
	
	// The Dog class implements the interface and thereby the method Noise()
	public class Dog : IAnimal
	{
		public void Noise()
		{
			Console.WriteLine("Woof!");
		}
	}
	
	// Same in the Cat class, though it differs slightly to differentiate it from Dog
	public class Cat : IAnimal
	{
		public void Noise()
		{
			Console.WriteLine("Meow!");
		}
	}
	
	// Here we test our two IAnimal classes
	public class TestAnimal
	{
		public TestAnimal()
		{
			// Declare objects
			Dog myDog = new Dog();
			Cat myCat = new Cat();
			
			// Use the Speak() method on both objects
			// These objects both conform to the IAnimal interface
			// and can such be treated as IAnimal objects
			Speak(myDog);
			Speak(myCat);
		}
		
		// This method takes an IAnimal type object for a parameter
		// IAnimal isn't an instance of a specific class, but whatever objects implement that interface
		public void Speak(IAnimal animal)
		{
			// So this doesn't produce an error
			// Because animal implements the IAnimal interface and MUST have a Noise() method
			animal.Noise();
		}
	
		public static void Main()
		{
			new TestAnimal();
		}
	}
}