简单来说，观察者模式=发布者+订阅者。
下面是一个有关猎头的典型的例子。在下面这张图当中有两个角色：猎头和寻找工作的人。找工作的人向猎头订阅，告知自己希望得到一份工作，当有新的工作机会的时候，猎头就会把这个信息通知给曾经向他订阅过的人。
Java代码
Subject接口：
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public interface Subject { |
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public void registerObserver(Observer observer); |
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public void removeObserver(Observer observer); |
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public void notifyAllObservers(); |
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} |
Observer接口：
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public interface Observer { |
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public void update(Subject subject); |
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} |
HeadCounter实现了Subject接口，实际上就是发布者：
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public class HeadCounter implements Subject { |
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private List<Observer> observerList; |
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private List<String> jobList; |
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public HeadCounter(){ |
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observerList = new ArrayList<Observer>(); |
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jobList = new ArrayList<String>(); |
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} |
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@Override |
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public void registerObserver(final Observer observer) { |
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if(! observerList.contains(observer)){ |
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observerList.add(observer); |
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} |
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} |
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@Override |
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public void removeObserver(final Observer observer) { |
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if(observerList.contains(observer)){ |
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observerList.remove(observer); |
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} |
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} |
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@Override |
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public void notifyAllObservers() { |
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for(Observer observer : observerList){ |
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observer.update(this); |
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} |
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} |
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public void addJob(final String job){ |
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this.jobList.add(job); |
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notifyAllObservers(); |
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} |
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public List<String> getJobs(){ |
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return jobList; |
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} |
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public String toString(){ |
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return jobList.toString(); |
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} |
JobSeeker实现了Observer接口，实际上就是订阅者：
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public class JobSeeker implements Observer { |
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private String name; |
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public JobSeeker(String name){ |
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this.name = name; |
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} |
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@Override |
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public void update(Subject subject) { |
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System.out.println(this.name + " got notified!"); |
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System.out.println(subject); |
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} |
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} |
程序实例入口：
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public class Main { |
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public static void main(String[] args) { |
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HeadCounter headCounter = new HeadCounter(); |
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//注册订阅者 |
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headCounter.registerObserver(new JobSeeker("Mike")); |
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headCounter.registerObserver(new JobSeeker("Chris")); |
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headCounter.registerObserver(new JobSeeker("Jeff")); |
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//通知所有订阅者有新的工作机会 |
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headCounter.addJob("Google Job"); |
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headCounter.addJob("Yahoo Job"); |
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} |
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} |
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