Thursday, 20 January 2005 21:26

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Source code

/**
 * Knows its observers. Any number of Observer objects
 * may observe a subject. Provides an interface for attaching
 * and detaching Observer objects.
 * @role __Subject
 */
public class Subject {
        private ArrayList observers = new ArrayList();        public void attach(Observer observer) {
                observers.add(observer);        }

        public void detach(Observer observer) {
                int idx = observers.indexOf(observer);                if (idx != -1) {
                        observers.remove(idx);                }
        }

        protected void notifyObservers() {
                Iterator it = observers.iterator();                while (it.hasNext()) {
                        ((Observer) it.next()).update(this);                }
        }

}

/**
 * Defines an updating interface for objects that
 * should be notified of changes in a subject.
 * @role __Observer
 */
public interface Observer {
        /**
  * Update method.
  */
        public void update(Subject subject);}

/**
 * Stores state of interest to ConcreteObserver objects.
 * Sends a notification to its observers when its state changes.
 */
public class ConcreteSubject extends Subject {

        // add your subject's specific stuff here
}

/**
 * Implements the Observer updating interface to keep 
 * its state consistent with the subject's.
 */
public class ConcreteObserver implements Observer {
        public void update(Subject subject) {
                // put your code here
        }

}

Comments

Add New

RSS