一、观察者模式概念

　　 观察者模式，顾名思义，就是观察一个对象的变化而做出变化，观察的对象称之为观察者，被观察的对象称之为主体。就比如我们在课堂上上课，当老师在看着我们的时候我们就认真听课，而当老师转过身去的时候，很多同学就会开始开小差。在这当中我们就是观察者，老师就是主体，当老师的行为发生改变时，我们的行为就会做相应的改变，这就是所谓的观察者模式。

二、代码示例             

interface Subject{    void addListener(Observer observer);    void removeListener(Observer observer);    void trunArround();    void faceStudents();}class Teacher implements Subject{         private List
     
       observers = new ArrayList
      
       ();        public void addListener(Observer observer){        observers.add(observer);    }            public void removeListener(Observer observer){        observers.remove(observer);    }        public void trunArround(){        for (Observer observer : observers) {            observer.downAction();        }    }        public void faceStudents(){        for (Observer observer : observers) {            observer.upAction();        }    }}abstract class Observer{        abstract void upAction();    abstract void downAction();}class Student extends Observer{    public  Student(Subject subject) {        subject.addListener(this);    }         @Override    void upAction() {        System.out.println("我在认真学习");    }    @Override    void downAction() {        System.out.println("我在玩手机");    }    }public static void main(String[] args) throws Exception{                Teacher teacher = new Teacher();                Student firstStudent = new Student(teacher);        Student second = new Student(teacher);                        teacher.trunArround();        teacher.faceStudents();}
      
     

运行代码会输出下面的结果

这就是当老师的行动有所改变时，所有的学生都可以感知到，从而做出相应的反应。

 　　上面的实现模式是当老师行动有所改变时，会通知到相关的学生，我做了什么动作，你们快做出相应的反应。这是不合乎情理，不可能老师说我要转身了，你们要玩手机的快玩手机吧。所以在这种场景里，学生作为观察者，需要时刻关注老师的行为，学生主动去获取老师的状态，查看其状态是否已改变，从而做出相应的动作。所以观察者模式还有一种实现，就是观察者能够主动去获取主体的状态。下面是其的实现代码：

interface Subject{    void addListener(Observer observer);    void removeListener(Observer observer);    void trunArround();    void faceStudents();}class Teacher implements Subject{        private Integer status = 1;         private List
     
       observers = new ArrayList
      
       ();        public Integer getStatus() {        return status;    }    public void setStatus(Integer status) {        this.status = status;    }    public void addListener(Observer observer){        observers.add(observer);    }            public void removeListener(Observer observer){        observers.remove(observer);    }        public void trunArround(){        System.out.println("老师转身");        this.status = 2;    }        public void faceStudents(){        System.out.println("老师面对学生");        this.status = 1;    }}abstract class Observer{        abstract void upAction();    abstract void downAction();}class Student extends Observer{    public  Student(Subject subject) {        subject.addListener(this);    }         public void watchTeacher(Subject subject){        if(subject instanceof Teacher){            if(((Teacher) subject).getStatus() == 1){                this.upAction();            }else if(((Teacher) subject).getStatus() == 2){                this.downAction();            }        }    }        @Override    void upAction()     {            System.out.println("我在认真学习");    }    @Override    void downAction() {        System.out.println("我在玩手机");    }    }public static void main(String[] args) throws Exception{                Teacher teacher = new Teacher();                Student student = new Student(teacher);                teacher.trunArround();        student.watchTeacher(teacher);        teacher.faceStudents();        student.watchTeacher(teacher);}