Subject.java

/**
 * Objects implementing this interface allow observers
 * to register with it. When this object changes state,
 * it notifies all registered observers.
 */

public interface Subject {
	/* allows an observer to register with the subject */
	public void addObserver (Observer o);
	
	/* removes an observer */	
	public void removeObserver (Observer o);
	
	/* notify all registered observers when its state changes */	
	public void notifyObservers();
}