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();
    }

    @Override
    public String toString() {
        return "Mallard Duck";
    }
}

• Explanation:

1. public class MallardDuck implements Quackable {
   • This declares a public class named MallardDuck.
   • It implements the Quackable interface, indicating that instances of this class can quack.
2. Observable observable;
   • This declares a member variable named observable of type Observable.
   • It is used to manage observers interested in the quacking events of this mallard duck.
3. public MallardDuck() { observable = new Observable(this); }
   • This is the constructor of the MallardDuck class.
   • It initializes the observable variable with a new instance of Observable, passing itself (this) as the QuackObservable object.
4. public void quack() { System.out.println("Quack"); notifyObservers(); }
   • This method is part of the Quackable interface implementation.
   • It outputs "Quack" to the console, simulating the quacking sound of a mallard duck, and then notifies observers.
5. public void registerObserver(Observer observer) { observable.registerObserver(observer); }
   • This method is part of the Quackable interface implementation.
   • It delegates the responsibility of registering observers to the Observable object.
6. public void notifyObservers() { observable.notifyObservers(); }
   • This method is part of the Quackable interface implementation.
   • It delegates the responsibility of notifying observers to the Observable object.
7. @Override public String toString() { return "Mallard Duck"; }
   • This overrides the toString method to provide a custom string representation of the object.
8. }
   • Closes the MallardDuck class.

• Overall Explanation:
  • The MallardDuck class represents a mallard duck object that can quack.
  • It implements the Quackable interface, providing methods for quacking, registering observers, and notifying observers.
  • The observable member variable is used to manage observers interested in the quacking events of the mallard duck.
  • The toString method provides a descriptive string representation of the object, returning "Mallard Duck".
  • This class is part of the larger design that allows different types of ducks and quackable entities to be treated uniformly in the code.