package headfirst.combining.observer;public class MallardDuck implements Quackable {	Observable observable; 	public MallardDuck() {		observable = new Observable(this);	} 	public void quack() {		System.out.println("Quack");		notifyObservers();	} 	public void registerObserver(Observer observer) {		observable.registerObserver(observer);	} 	public void notifyObservers() {		observable.notifyObservers();	} 	public String toString() {		return "Mallard Duck";	}}