public interface Observer {
    void update(QuackObservable duck);
}

• Explanation:

1. public interface Observer {
   • This declares a public interface named Observer.
   • It defines the contract for objects that will observe (update) a QuackObservable subject.
2. void update(QuackObservable duck);
   • This declares an abstract method update, taking a QuackObservable parameter (duck).
   • Classes implementing the Observer interface will provide concrete implementations for updating themselves based on changes in the observed QuackObservable object.
3. }
   • Closes the Observer interface.

• Overall Explanation:
  • The Observer interface is part of the Observer Pattern.
  • Objects that implement this interface can register themselves as observers of a QuackObservable subject.
  • When the state of the observed object changes (e.g., when it quacks), the update method of all registered observers is called.
  • This allows observers to react to changes in the observed object without being tightly coupled to its implementation.

The Observer interface is a fundamental part of the Observer Pattern, promoting loose coupling between the subject (observable) and its observers. Classes that need to react to changes in a QuackObservable object will implement this interface and provide their specific behavior in the update method.