[color=red]Purpose[/color]
Lets one or more objects be notified of state changes in other
objects within the system.
[color=red]Use When[/color]
1 State changes in one or more objects should trigger behavior
in other objects
2 Broadcasting capabilities are required.
3 An understanding exists that objects will be blind to the
expense of notification.
[color=red]Example[/color]
This pattern can be found in almost every GUI environment.
When buttons, text, and other fields are placed in applications
the application typically registers as a listener for those controls.
When a user triggers an event, such as clicking a button, the
control iterates through its registered observers and sends a
notification to each.
package javaPattern.observer;
import java.util.ArrayList;
public interface Observer {
public void update();
}
class ConcreteObserver implements Observer{
@Override
public void update() {
System.out.println("update...");
}
}
interface Subject{
public void attach(Observer o);
public void detach(Observer o);
public void notifyObserver();
}
class ConreteSubject implements Subject{
private ArrayList<Observer> observers = new ArrayList<Observer>();
@Override
public void attach(Observer o) {
observers.add(o);
}
@Override
public void detach(Observer o) {
observers.remove(o);
}
@Override
public void notifyObserver() {
for(Observer o : observers){
o.update();
}
}
}
class Client{
public static void main(String[] args) {
Subject subject = new ConreteSubject();
Observer observer1 = new ConcreteObserver();
Observer observer2 = new ConcreteObserver();
Observer observer3 = new ConcreteObserver();
subject.attach(observer1);
subject.attach(observer2);
subject.attach(observer3);
subject.notifyObserver();
}
}