java 观察者模式

java 观察者模式是什么？实例如下：

java 观察者模式是什么？

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。

实例如下：

// 简单实例 import java.util.*; interface Observer { // 观察者接口 public void update( int value ); // 更新数值 } interface Subject { // 主题接口 public void registerObserver( Observer o ); // 注册观察者 public void removeObserver( Observer o ); // 去除观察者 public void notifyObservers(); // 通知更新主题的观察者们 } class SimpleObserver implements Observer { // 单纯观察者 private int value; private Subject subject; // 主题引用变量 public SimpleObserver( Subject subject ) { // 创建单纯观察者并注册主题 this.subject = subject; subject.registerObserver( this ); } public void update( int value ) { //更新数值 this.value = value; display(); } public void display() { // 显示内容 System.out.println( "Value: " + value ); } } class SimpleSubject implements Subject { // 简单主题 private ArrayList< Observer > observers; // 观察者数组引用变量,记录注册的观察者们 private int value = 0; // 数值变量，用来改变数值 public SimpleSubject() { // 创建主题，并初始化数组 observers = new ArrayList< Observer >(); } public void registerObserver( Observer o ) { // 注册主题观察者 observers.add( o ); } public void removeObserver( Observer o ) { // 去除主题观察者 int i = observers.indexOf(o); // 查找观察者位置 if ( i >= 0 ) { observers.remove( i ); // 去除指定观察者 } } public void notifyObservers() { // 通知并更新主题的观察者们 for ( Observer observer : observers ) { observer.update( value ); // 更新主题的观察者们内容 } } public void setValue( int value ) { // 更新内容 this.value = value; notifyObservers(); } } class Example { // 例子 public static void main( String[] args ) { SimpleSubject simpleSubject = new SimpleSubject(); // 创建主题 SimpleObserver simpleObserver = new SimpleObserver( simpleSubject ); //创建观察者并注册主题 simpleSubject.setValue( 80 ); // 主题更新内容 } } public class Test0525{ // 测试 public static void main( String[] args ){ Example.main( args ); } } ---------------------------------------------------------------------------------------- E:\java>java Test0525 Value: 80 // 气象站实例 import java.util.*; interface Subject { public void registerObserver( Observer o ); public void removeObserver( Observer o ); public void notifyObservers(); } interface Observer { public void update( float temp, float humidity, float pressure ); } interface DisplayElement { // 显示元素 public void display(); } class WeatherData implements Subject { // 气象资料主题 private ArrayList< Observer > observers; private float temperature; // 温度 private float humidity; // 湿度 private float pressure; // 压力 public WeatherData() { observers = new ArrayList< Observer >(); } public void registerObserver( Observer o ) { observers.add( o ); } public void removeObserver( Observer o ) { int i = observers.indexOf( o ); if ( i >= 0 ) { observers.remove( i ); } } public void notifyObservers() { for ( Observer observer : observers ) { observer.update( temperature, humidity, pressure ); } } public void measurementsChanged() { // 测量已更改 notifyObservers(); } public void setMeasurements( float temperature, float humidity, float pressure ) { // 设置测量值 this.temperature = temperature; this.humidity = humidity; this.pressure = pressure; measurementsChanged(); } public float getTemperature() { // 获得温度 return temperature; } public float getHumidity() { // 获得湿度 return humidity; } public float getPressure() { // 获得压力 return pressure; } } class CurrentConditionsDisplay implements Observer, DisplayElement { // 当前条件显示 private float temperature; private float humidity; private Subject weatherData; public CurrentConditionsDisplay( Subject weatherData ) { this.weatherData = weatherData; weatherData.registerObserver( this ); } public void update( float temperature, float humidity, float pressure ) { this.temperature = temperature; this.humidity = humidity; display(); } public void display() { System.out.println( "Current conditions: " + temperature + "F degrees and " + humidity + "% humidity" ); } } class ForecastDisplay implements Observer, DisplayElement { // 预测显示 private float currentPressure = 29.92f; // 当前压力 private float lastPressure; // 持续压力 private WeatherData weatherData; public ForecastDisplay( WeatherData weatherData ) { this.weatherData = weatherData; weatherData.registerObserver( this ); } public void update( float temp, float humidity, float pressure ) { lastPressure = currentPressure; currentPressure = pressure; display(); } public void display() { System.out.print( "Forecast: " ); // 预报 if ( currentPressure > lastPressure ) { System.out.println( "Improving weather on the way!" ); // 天气转好 } else if ( currentPressure == lastPressure ) { System.out.println( "More of the same" ); // 天气保持不变 } else if (currentPressure < lastPressure) { System.out.println( "Watch out for cooler, rainy weather" ); // 当心凉爽多雨的天气 } } } class HeatIndexDisplay implements Observer, DisplayElement { // 热指数显示 float heatIndex = 0.0f; // 热指数 private WeatherData weatherData; public HeatIndexDisplay( WeatherData weatherData ) { this.weatherData = weatherData; weatherData.registerObserver( this ); } public void update( float t, float rh, float pressure ) { heatIndex = computeHeatIndex( t, rh ); display(); } private float computeHeatIndex( float t, float rh ) { // 计算热指数 float index = ( float )(( 16.923 + ( 0.185212 * t ) + ( 5.37941 * rh ) - ( 0.100254 * t * rh ) + ( 0.00941695 * ( t * t ) ) + ( 0.00728898 * ( rh * rh ) ) + ( 0.000345372 * ( t * t * rh ) ) - ( 0.000814971 * ( t * rh * rh ) ) + ( 0.0000102102 * ( t * t * rh * rh ) ) - ( 0.000038646 * ( t * t * t ) ) + ( 0.0000291583 * ( rh * rh * rh ) ) + ( 0.00000142721 * ( t * t * t * rh ) ) + ( 0.000000197483 * ( t * rh * rh * rh ) ) - ( 0.0000000218429 * ( t * t * t * rh * rh ) ) + 0.000000000843296 * ( t * t * rh * rh * rh ) ) - ( 0.0000000000481975 * ( t * t * t * rh * rh * rh ) ) ); return index; } public void display() { System.out.println( "Heat index is " + heatIndex ); } } class StatisticsDisplay implements Observer, DisplayElement { // 统计显示 private float maxTemp = 0.0f; // 最大温度 private float minTemp = 200; // 最小温度 private float tempSum= 0.0f; // 温度总和 private int numReadings; // 编号 private WeatherData weatherData; public StatisticsDisplay( WeatherData weatherData ) { this.weatherData = weatherData; weatherData.registerObserver( this ); } public void update( float temp, float humidity, float pressure ) { tempSum += temp; numReadings ++; if ( temp > maxTemp ) { maxTemp = temp; } if ( temp < minTemp ) { minTemp = temp; } display(); } public void display() { System.out.println( "Avg/Max/Min temperature = " + ( tempSum / numReadings ) + "/" + maxTemp + "/" + minTemp ); } } class WeatherStation { // 气象站 public static void main( String[] args ) { WeatherData weatherData = new WeatherData(); CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData); StatisticsDisplay statisticsDisplay = new StatisticsDisplay( weatherData ); ForecastDisplay forecastDisplay = new ForecastDisplay( weatherData ); weatherData.setMeasurements( 80, 65, 30.4f ); weatherData.setMeasurements( 82, 70, 29.2f ); weatherData.setMeasurements( 78, 90, 29.2f ); } } class WeatherStationHeatIndex { // 气象站热指数 public static void main( String[] args ) { WeatherData weatherData = new WeatherData(); CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay( weatherData ); StatisticsDisplay statisticsDisplay = new StatisticsDisplay( weatherData ); ForecastDisplay forecastDisplay = new ForecastDisplay( weatherData ); HeatIndexDisplay heatIndexDisplay = new HeatIndexDisplay( weatherData ); weatherData.setMeasurements( 80, 65, 30.4f ); weatherData.setMeasurements( 82, 70, 29.2f ); weatherData.setMeasurements( 78, 90, 29.2f ); } } public class Test0525{ public static void main( String[] args ){ WeatherStation.main( args ); System.out.println(); WeatherStationHeatIndex.main( args ); } } ----------------------------------------------------------------- E:\java>java Test0525 Current conditions: 80.0F degrees and 65.0% humidity Avg/Max/Min temperature = 80.0/80.0/80.0 Forecast: Improving weather on the way! Current conditions: 82.0F degrees and 70.0% humidity Avg/Max/Min temperature = 81.0/82.0/80.0 Forecast: Watch out for cooler, rainy weather Current conditions: 78.0F degrees and 90.0% humidity Avg/Max/Min temperature = 80.0/82.0/78.0 Forecast: More of the same Current conditions: 80.0F degrees and 65.0% humidity Avg/Max/Min temperature = 80.0/80.0/80.0 Forecast: Improving weather on the way! Heat index is 82.95535 Current conditions: 82.0F degrees and 70.0% humidity Avg/Max/Min temperature = 81.0/82.0/80.0 Forecast: Watch out for cooler, rainy weather Heat index is 86.90124 Current conditions: 78.0F degrees and 90.0% humidity Avg/Max/Min temperature = 80.0/82.0/78.0 Forecast: More of the same Heat index is 83.64967