Definition

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

UML class diagram

Participants

• Subject
  • knows its observers. Any number of Observer objects may observe a subject
  • provides an interface for attaching and detaching Observer objects.
• ConcreteSubject
  • stores state of interest to ConcreteObserver
  • sends a notification to its observers when its state changes
• Observer
  • defines an updating interface for objects that should be notified of changes in a subject.
• ConcreteObserver
  • maintains a reference to a ConcreteSubject object
  • stores state that should stay consistent with the subject's
  • implements the Observer updating interface to keep its state consistent with the subject's

Sample code in Java

package com.hong.observer;

import java.util.LinkedList;
import java.util.List;

interface Observer { 
    public void update();
}

class ConcreteObserver implements Observer {

    private String name;
    private int state; 
    private ConcreteSubject subject; 
    
    public ConcreteObserver(ConcreteSubject subject, String name)
    {
        this.name = name; 
        this.subject = subject;
    }

    @Override
    public void update() {
        this.state = subject.getSubjectState();
        System.out.printf("%s state is %d
", this.name, this.state);
    }
    
}
class Subject {
    
    private List<Observer>  observers = new LinkedList<Observer>();
    
    public void Attach(Observer o){
        observers.add(o);
    }
    
    public void Detach(Observer o){
        observers.remove(o);
    }
    public void notifyObservers(){
        
        for(Observer observer : observers){
            observer.update();
        }
    }
}

class ConcreteSubject extends Subject{
    
    private int subjectState;
    public ConcreteSubject(){
        subjectState = 0;
    }
    
    public int getSubjectState() {
        return subjectState;
    }
    
    public void setSubjectState(int subjectState) {
        this.subjectState = subjectState;
    }
}

public class Observer_structure {
    
    public static void main(String[] args) {
        
        ConcreteSubject subject = new ConcreteSubject(); 
        
        subject.Attach(new ConcreteObserver(subject, "observer1"));
        subject.Attach(new ConcreteObserver(subject, "observer2"));
        subject.Attach(new ConcreteObserver(subject, "observer3"));
        
        subject.notifyObservers();
        subject.setSubjectState(3);
        subject.notifyObservers();
        subject.setSubjectState(51);
        subject.notifyObservers();
        
    }

}