目的

定义对象之间的一对多依赖，当一个对象状态改变时，它的所有依赖都会收到通知并且自动更新状态。
主题（Subject）是被观察的对象，而其所有依赖者（Observer）称为观察者。
观察者1.jpg

类图

主题（Subject）具有注册和移除观察者、并通知所有观察者的功能，主题是通过维护一张观察者列表来实现这些操作的。
观察者（Observer）的注册功能需要调用主题的 registerObserver()方法。
观察者类图.png

实现

天气数据布告板会在天气信息发生改变时更新其内容，布告板有多个，并且在将来会继续增加。
观察者2.jpg

  1. public interface Subject {
  2.     void registerObserver(Observer o);
  4.     void removeObserver(Observer o);
  6.     void notifyObserver();
  7. }
  10. public class WeatherData implements Subject {
  11.     private List<Observer> observers;
  12.     private float temperature;
  13.     private float humidity;
  14.     private float pressure;
  16.     public WeatherData() {
  17.         observers = new ArrayList<>();
  18.     }
  20.     public void setMeasurements(float temperature, float humidity, float pressure) {
  21.         this.temperature = temperature;
  22.         this.humidity = humidity;
  23.         this.pressure = pressure;
  24.         notifyObserver();
  25.     }
  27.     @Override
  28.     public void registerObserver(Observer o) {
  29.         observers.add(o);
  30.     }
  32.     @Override
  33.     public void removeObserver(Observer o) {
  34.         int i = observers.indexOf(o);
  35.         if (i >= 0) {
  36.             observers.remove(i);
  37.         }
  38.     }
  40.     @Override
  41.     public void notifyObserver() {
  42.         for (Observer o : observers) {
  43.             o.update(temperature, humidity, pressure);
  44.         }
  45.     }
  46. }
  49. public interface Observer {
  50.     void update(float temp, float humidity, float pressure);
  51. }
  54. public class StatisticsDisplay implements Observer {
  56.     public StatisticsDisplay(Subject weatherData) {
  57.         weatherData.reisterObserver(this);
  58.     }
  60.     @Override
  61.     public void update(float temp, float humidity, float pressure) {
  62.         System.out.println("StatisticsDisplay.update: " + temp + " " + humidity + " " + pressure);
  63.     }
  64. }
  67. public class CurrentConditionsDisplay implements Observer {
  69.     public CurrentConditionsDisplay(Subject weatherData) {
  70.         weatherData.registerObserver(this);
  71.     }
  73.     @Override
  74.     public void update(float temp, float humidity, float pressure) {
  75.         System.out.println("CurrentConditionsDisplay.update: " + temp + " " + humidity + " " + pressure);
  76.     }
  77. }
  80. public class WeatherStation {
  81.     public static void main(String[] args) {
  82.         WeatherData weatherData = new WeatherData();
  83.         CurrentConditionsDisplay currentConditionsDisplay = new CurrentConditionsDisplay(weatherData);
  84.         StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
  86.         weatherData.setMeasurements(0, 0, 0);
  87.         weatherData.setMeasurements(1, 1, 1);
  88.     }
  89. }
  92. 运行结果
  93. CurrentConditionsDisplay.update: 0.0 0.0 0.0
  94. StatisticsDisplay.update: 0.0 0.0 0.0
  95. CurrentConditionsDisplay.update: 1.0 1.0 1.0
  96. StatisticsDisplay.update: 1.0 1.0 1.0

JDK中的体现