参考来源：http://www.cnblogs.com/hnrainll/archive/2011/12/29/2305582.html

观察者模式 Observer Pattern

类别：行为型模式
思想：当一个对象（被观察者或目标）的状态发生改变时，所有依赖于它的对象（观察者）都会得到通知并更新自身的状态。
应用：Observer模式的最知名的应用是在MVC结构，Observer模式可以很好的应用在文档和图表程序的制作中。

实现原理：
被观察者或目标（subject）接口：
注册观察者：public void attach(Observer o)；
删除观察者：public void detach(Observer o)；
向所有观察者发布通知：public void notice()；

观察者（observer）接口：
接收到通知后，更新：public void update()；

实例：

Subject代码：
public interface Subject{
    public void attach(Observer o);
    public void detach(Observer o);
    public void notice();
}
Observer代码：
public interface Observer{
    public void update();
}
Teacher代码：
package observerPattern;

import java.util.ArrayList;
import java.util.List;

public class Teacher implements Subject {
    private String phone;
    private List<Observer> students;
    
    public Teacher() {
        phone = "";
        students = new ArrayList<Observer>();
    }
    
    @Override
    public void attach(Observer o) {
        // TODO Auto-generated method stub
        students.add(o);
    }

    @Override
    public void detach(Observer o) {
        // TODO Auto-generated method stub
        students.remove(o);
    }

    @Override
    public void notice() {
        // TODO Auto-generated method stub
        for(Observer student : students) {
            student.update();
        }
    }
    
    public void setPhone(String phone) {
        this.phone = phone;
        notice();         // 关键
    }
    
    public String getPhone() {
        return this.phone;
    }

Student代码：
package observerPattern;

public class Student implements Observer {
    private String name;
    private String phone;
    private Teacher teacher;
    
    public Student(String name, Teacher teacher) {
        this.name = name;
        this.teacher = teacher;
    }
    
    public void show() {
        System.out.println("Name: " + this.name + " Teacher's Phone: " + this.phone);
    }
    
    @Override
    public void update() {
        // TODO Auto-generated method stub
        phone = teacher.getPhone();
    }

}

Client代码：
package observerPattern;

import java.util.ArrayList;
import java.util.List;

public class Client {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<Student>();
        Teacher teacher = new Teacher();
        for(int i = 0; i < 5; i++) {
            Student  student = new Student("s" + i, teacher);
            students.add(student);
            teacher.attach(student);
        }
        teacher.setPhone("123456");
        
        for(Student student : students) {
            student.show();
        }
        teacher.setPhone("654321");
        for(Student student : students) {
            student.show();
        }
    }
}