觀察者模式定義了對象之間的一對多依賴，這樣一來，當一個對象改變狀態時，他的所有依賴者都會收到通知并自動更新。關于觀察者，主題只需要知道觀察者實現了某個接口。主題不需要知道觀察者的具體類是誰，做了什么或者其他的什么細節，任何時候都可以添加新的觀察者或者刪除觀察者。下面就以一個例子來實現觀察者模式。首先新建一個主題、觀察者以及顯示的接口 public interface Subject {public void registerObserver(Observe observe);public void removeObserver(Observe observe);public void notifyObserver(); } public interface Observe {public void update(float temp,float humidity,float pressure); } public interface DisplayElement {public void display(); }

然后通過實現上述的接口實現一個數據源

public class WeatherData implements Subject {private ArrayList<Observe> observes;private float temperature;private float humidity;private float pressure;public WeatherData() {observes = new ArrayList<Observe>();}@Overridepublic void registerObserver(Observe observe) {observes.add(observe);}@Overridepublic void removeObserver(Observe observe) {if(observes.contains(observe)){observes.remove(observe);}}@Overridepublic void notifyObserver() {for (int i = 0; i < observes.size(); i++) {Observe observe = observes.get(i);observe.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();} }

數據源完成時候就是要展示數據，也就是實現觀察者

public class CurrentConditionsDisplay implements Observe, DisplayElement {private float temperature;private float humidity;private Subject weatherData;public CurrentConditionsDisplay(Subject weatherData) {this.weatherData = weatherData;weatherData.registerObserver(this);}@Overridepublic void display() {System.out.println("當前氣溫：："+temperature+"攝氏度，相對濕度為："+humidity+"%");}@Overridepublic void update(float temp, float humidity, float pressure) {this.humidity = humidity;this.temperature = temp;display();}} public class ForecastDisplay implements Observe,DisplayElement{private float temperature;private float humidity;private float pressure;private Subject weatherData;public ForecastDisplay(Subject weatherData) {this.weatherData = weatherData;weatherData.registerObserver(this);}@Overridepublic void display() {System.out.println("天氣不錯，可以出去郊游！");}@Overridepublic void update(float temp, float humidity, float pressure) {this.temperature = temp;this.humidity = humidity;this.pressure = pressure;display();}} public class StatisticsDisplay implements Observe,DisplayElement{private float temperature;private WeatherData weatherData;private List<Float> list;public StatisticsDisplay(WeatherData weatherData) {list = new ArrayList<Float>();this.weatherData = weatherData;weatherData.registerObserver(this);}@Overridepublic void display() {// 排序Collections.sort(list);float temp = 0;for (int i = 0; i < list.size(); i++) {temp+=list.get(i);}int vagtemp = (int) (temp/list.size());System.out.println("氣溫最大值/平均值/最小值："+list.get(list.size()-1)+"/"+vagtemp+"/"+list.get(0)+" 攝氏度");}@Overridepublic void update(float temp, float humidity, float pressure) {this.temperature = temp;list.add(temp);display();}}

上述三個觀察者都是跟著weatherData獲取數據
測試程序：

“`
public class WeatherTest {

public static void main(String[] args) {WeatherData weatherData = new WeatherData();CurrentConditionsDisplay conditionsDisplay = new CurrentConditionsDisplay(weatherData);StatisticsDisplay display = new StatisticsDisplay(weatherData);ForecastDisplay forecastDisplay = new ForecastDisplay(weatherData);weatherData.setMeasurements(20, 75, 30.4f);weatherData.setMeasurements(32, 70, 29.4f);weatherData.setMeasurements(18, 90, 31.4f); }

}

“`

總結

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