设计模式————观察者模式

观察者模式

定义了对象之间的一对多依赖，这样一来，当一个对象改变状态时，它的所有依赖者都会收到通知并自动更新。

 

一个气象站项目

主题(可观察者)

public class WeatherData implements Subject {
  private ArrayList observers;
  private float temperature;
  private float humidity;
  private float pressure;

  public WeatherData() {
      observers = new ArrayList();
  }

  @Override
  public void registerObserver(Observer o) {
      observers.add(o);
  }

  @Override
  public void removeObserver(Observer o) {
      int i = observers.indexOf(o);
      if (i >= 0) {
          observers.remove(i);
      }
  }

  @Override
  public void notifyObserver() {
      for(int i = 0; i < observers.size(); i++) {
          Observer observer = (Observer)observers.get(i);
          observer.update(temperature,humidity,pressure);
      }
  }

  public void measurementChanged() {
      notifyObserver();
  }

  public void setMeasurements(float temperature,float humidity, float pressure) {
      this.temperature = temperature;
      this.humidity = humidity;
      this.pressure = pressure;
      measurementChanged();
  }
}

Subject.java

public interface Subject {
    void registerObserver(Observer o);
    void removeObserver(Observer o);
    void notifyObserver();
}

其中一个观察者,实现Observer接口

public class CurrentConditionsDisplay implements Observer, DisplayElement {
   private float temperature;
   private float humidity;
   private Subject weatherData;


   public CurrentConditionsDisplay(Subject weatherData) {
       this.weatherData = weatherData;
       weatherData.registerObserver(this);
   }

    @Override
    public void display() {
       System.out.println("Current conditions:" + temperature + "F degrees and " + humidity + "% humidity");
    }

    @Override
    public void update(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        display();
    }
}

Observer.java

public interface Observer {
    void update(float temp, float humidity, float pressure);
}

测试

public class WeatherStation {
    public static void main(String[] args) {
        WeatherData weatherData = new WeatherData();
        CurrentConditionsDisplay currentConditionsDisplay = new CurrentConditionsDisplay(weatherData);
        weatherData.setMeasurements(80, 65, 30.4f);
    }
}

 

同样，也可以使用java.util包下的内置观察者模式。

区别:java.util.Observable是一个类，必须设计一个类继承它。