The following code examples demonstrate where and how to use nested classes in your interfaces. It may look a bit strange at first, an interface with a nested class but after following the examples below it should make sense and you'll see how it fits together. The main thing to come away with is that the interface strictly types the classes tha…

Input.java

import java.util.*;

public interface Input {
    
    public class KeyEvent {
         public static final int KEY_DOWN = 0;
         public static final int KEY_UP = 1;
         public int type;
         public int keyCode;
         public char keyChar;
    }
 
    public void keyPressed(KeyEvent keyCode);
    public List<KeyEvent> getKeyEvents();
}

KeyboardInput.java

import java.util.*;

class KeyboardInput implements Input {

    private ArrayList<KeyEvent> keyEvents = new ArrayList<>();

    public void keyPressed(KeyEvent keyEvent) {
    	keyEvents.add(keyEvent);
    }

    public List<KeyEvent> getKeyEvents() {
    	return keyEvents;
    }
}

KeyboardTest.java

class KeyboardTest {

	public static void main(String... args) {

		Input input = new KeyboardInput();

		// This works, our KeyEvent is of type Input
		Input.KeyEvent keyEvent = new Input.KeyEvent();
		keyEvent.keyCode = 65;
		input.keyPressed(keyEvent);

		// or this...still of type Input
		KeyboardInput.KeyEvent keyEvent2 = new KeyboardInput.KeyEvent();
		keyEvent2.keyCode = 96;
		input.keyPressed(keyEvent2);

		// output the events...
		for (KeyboardInput.KeyEvent theKeyEvent : input.getKeyEvents()) {
			System.out.println("KeyEvent.keyCode : " + theKeyEvent.keyCode);
		}

		// This following fails to compile as this KeyEvent is not of type Input
		// Comment out to compile this class.
		KeyEvent myKeyEvent = new KeyEvent();
		myKeyEvent.keyCode = 96;
		input.keyPressed(myKeyEvent);

	}
}

KeyEvent.java

public class KeyEvent {
     
     public static final int KEY_DOWN = 0;
     public static final int KEY_UP = 1;
     public int type;
     public int keyCode;
     public char keyChar;
}