java中如何实现观察者模式

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public class Teacher extends Observable {    private String content;    public void setHomework(final String content) {        this.content = content;        System.out.println("布置的作业是：" + content);        /**         * 标记对象已更改，更改后才能通知         */        setChanged();        /**         * 通知观察者，会调用观察者update方法         */        notifyObservers();    }    public String getContent() {        return this.content;    }}

public class Student implements Observer {    private final Observable observable;    private final String name;    public Student(Observable observable, String name) {        this.observable = observable;        this.name = name;        observable.addObserver(this);    }    @Override    public void update(Observable o, Object arg) {        if (o instanceof Teacher) {            Teacher teacher = (Teacher) o;            System.out.println(name + "得到作业信息：" + teacher.getContent());        }    }}

public class Test {    public static void main(String[] args) {        /**         *  instance observable         */        Teacher teacher = new Teacher();        Student s1 = new Student(teacher, "张三");        Student s2 = new Student(teacher, "李四");        Student s3 = new Student(teacher, "王五");        teacher.setHomework("第二页第六题");        teacher.setHomework("第三页第七题");        teacher.setHomework("第五页第八题");    }}

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