public class Quackologist implements Observer {
    @Override
    public void update(QuackObservable duck) {
        System.out.println("Quackologist: " + duck + " just quacked.");
    }
}

• Explanation:

1. public class Quackologist implements Observer {
   • This declares a public class named Quackologist.
   • It implements the Observer interface, indicating that instances of this class can observe quackable entities.
2. @Override public void update(QuackObservable duck) { ... }
   • This method is part of the Observer interface implementation.
   • It is called when the observed QuackObservable (duck) notifies its observers about a change.
3. System.out.println("Quackologist: " + duck + " just quacked.");
   • This line prints a message indicating that the Quackologist observed a quack from the provided duck.
   • The duck object is concatenated to the message to provide information about which duck just quacked.
4. }
   • Closes the Quackologist class.

• Overall Explanation:
  • The Quackologist class is a concrete observer that prints a message when it observes a quack from a QuackObservable (duck).
  • The update method is invoked by the observed duck when it quacks, notifying the observer of the event.
  • The message printed by the Quackologist includes information about the specific duck that quacked.
  • This class is part of the Observer pattern, where an object (observer) maintains a dependents list (observers) and is notified of changes by the subject (observable). In this case, Quackologist is an observer of quackable entities.