觀察者模式：定義了物件之間的一對多依賴，這樣依賴，當一個物件改變狀態時，它的所有依賴者都會受到通知並自動更新

例子：

package com.zoey.allTest.pattern.headFirst.observerPattern.book;

public interface Subject {
    public void registerObserver(Observer o);
    public void deleteObserver(Observer o);
    public void notifyObservers();//當主題狀態改變時，這個方法被呼叫，通知所有觀察者
 

}

package com.zoey.allTest.pattern.headFirst.observerPattern.book;

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

package com.zoey.allTest.pattern.headFirst.observerPattern.book;

public interface DisplayElement {
    public void display();
}
 

package com.zoey.allTest.pattern.headFirst.observerPattern.book;

public class CurrentConditionsDisplay implements Observer, DisplayElement {
    private float temp;
    private float humidity;
    @SuppressWarnings("unused")
    private Subject weatherData;

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

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

    public void display() {
        System.out.println(temp + " " + humidity);
    }
}

package com.zoey.allTest.pattern.headFirst.observerPattern.book;

import java.util.ArrayList;

public class WeatherData implements Subject {
    @SuppressWarnings("rawtypes")
    private ArrayList observers;
    private float temp;
    private float humidity;
    private float pressure;

    @SuppressWarnings("rawtypes")
    public WeatherData() {
        observers = new ArrayList();
    }

    @SuppressWarnings("unchecked")
    public void registerObserver(Observer o) {
        observers.add(o);
    }

    public void deleteObserver(Observer o) {
        int i = observers.indexOf(o);
        if ( i <= 0) {
            observers.remove(i);
        }
    }

    public void notifyObservers() {
        for (int i = 0; i < observers.size(); i ++) {
            Observer o = (Observer) observers.get(i);
            o.update(temp, humidity, pressure);
        }
    }

    public void measurementsChanged() {
        notifyObservers();
    }

    public void setMeasurements(float temp, float humidity, float pressure) {
        this.temp = temp;
        this.humidity = humidity;
        this.pressure = pressure;
        measurementsChanged();
    }
}

package com.zoey.allTest.pattern.headFirst.observerPattern.book;

public class WeatherDataTest {

    public static void main(String[] args) {
        WeatherData w = new WeatherData();

        new CurrentConditionsDisplay(w);
        w.setMeasurements(45, 52, 33f);
    }
}