DesignPattern学习-----Observer
Observer直译为中文为观察者。所以此模式又称为观察者模式（废话。）不过一直很疑惑，我感觉这个名字起得不好，给人误解很大，特别是初学者。我想大部分人的想法应该和我一样，我第一次看到观察者模式就会想到，观察者观察者，那么肯定观察者是主体了。其实不然。被观察者才是主体。
就小孩睡觉这个例子，如果按观察者的想法，那就观察嘛！！！怎么观察，小孩在睡觉，我就在那观察，他眼一睁，啊，要喝奶了。赶紧去泡奶粉。。。。这个显然就不正常。傻子才这么干。。。。
实际上是你该干嘛就干嘛去，小孩醒了，一哭要喝奶了，你就去泡奶吧。
所以我认为观察者模式叫Call模式或者Call-Response模式更恰当。
下面看程序，模拟找工作。。。。雇主通知他要招聘的雇员，雇员得到通知后做出响应。
先是两个接口，Employer和Employee。

package pig.pattern.Observer;/** * Created by IntelliJ IDEA. * User: Administrator * Date: 2008-8-28 * Time: 14:31:33 * To change this template use File | Settings | File Templates. */public interface Employer {    void agree_call();               void agree(Employee employee);  //相当于swing里的add***Listener()}

package pig.pattern.Observer;/** * Created by IntelliJ IDEA. * User: Administrator * Date: 2008-8-28 * Time: 14:32:14 * To change this template use File | Settings | File Templates. */public interface Employee {    void response(EmployEvent event);//这个相当于ActiongListener里面的actionPerformed(ActionEvent event)}

再来相应的实现类

package pig.pattern.Observer;import java.util.List;import java.util.ArrayList;/** * Created by IntelliJ IDEA. * User: Administrator * Date: 2008-8-28 * Time: 14:32:51 * To change this template use File | Settings | File Templates. */public class Employer_1 implements Employer{    private List<Employee> employees = new ArrayList<Employee>();    private EmployEvent event;    public void agree(Employee employee){        employees.add(employee);    }    public void agree_call() {        event = new EmployEvent();        event.setEmployer_name(this.getClass().getSimpleName());        for(Employee e : employees){            e.response(event);        }    }}

package pig.pattern.Observer;/** * Created by IntelliJ IDEA. * User: Administrator * Date: 2008-8-28 * Time: 14:49:22 * To change this template use File | Settings | File Templates. */public class Employee_1 implements Employee{    public void response(EmployEvent event) {        System.out.println("Oh,I'm lucky.I'm employed by " + event.getEmployer_name());    }}

package pig.pattern.Observer;/** * Created by IntelliJ IDEA. * User: Administrator * Date: 2008-8-28 * Time: 14:49:22 * To change this template use File | Settings | File Templates. */public class Employee_2 implements Employee{    public void response(EmployEvent event) {        System.out.println("Yeah,I'm employed by " + event.getEmployer_name());    }}

再来事件类

package pig.pattern.Observer;/** * Created by IntelliJ IDEA. * User: Administrator * Date: 2008-8-28 * Time: 14:39:48 * To change this template use File | Settings | File Templates. */public class EmployEvent {    private String employer_name;    public String getEmployer_name() {        return employer_name;    }    public void setEmployer_name(String employer_name) {        this.employer_name = employer_name;    }}

最后测试一下。

package pig.pattern.Observer;/** * Created by IntelliJ IDEA. * User: Administrator * Date: 2008-8-28 * Time: 14:51:00 * To change this template use File | Settings | File Templates. */public class Test {    public static void main(String[] args){        Employer e_1 = new Employer_1();        e_1.agree(new Employee_1());        e_1.agree(new Employee_2());        e_1.agree_call();    }}

Employer在众多的简历里面挑选出自己满意的，然后agree到自己的通知列中(list)，全部都确认后，然后一起去call所有的这些应聘者，各个应聘者做出不同的反应。EmployEvent就相当于通知书一样，里面保存了Employer的名称，不然你怎么知道你是被哪个Employer聘了？
这还是说明，观察者(Employee)并不是主体，被观察者(Employer)才是主体。如果Employee是主体，那是不是你投完简历以后就天天跟着那个Employer，他一天不做出决定，你就一天不停了？
至于用到哪儿，根据代码应该就能猜得出来。一个对象的改变会影响其他对象的响应，而且对象个数不确定。
很粗糙的代码，只想说别被名字唬到了。。。。