设计模式——观察者模式

2.

OO原则：为对象之间的送耦合设计而努力

OO模式：观察者模式——在对象之间定义一对多的依赖，这样一来，当一个对象改变状态，依赖他的对象都会收到通知，并自动更新。

例子：天气报告站的各版面更新

import java.util.Observable;public class WeatherData extends Observable {private float temperature;private float humidity;private float pressure;public WeatherData() { }public void measurementsChanged() {setChanged();notifyObservers();}public void setMeasurements(float temperature, float humidity, float pressure) {this.temperature = temperature;this.humidity = humidity;this.pressure = pressure;measurementsChanged();}public float getTemperature() {return temperature;}public float getHumidity() {return humidity;}public float getPressure() {return pressure;}}

import java.util.Observable;import java.util.Observer;public class CurrentConditionsDisplay implements Observer, DisplayElement {Observable observable;private float temperature;private float humidity;public CurrentConditionsDisplay(Observable observable) {this.observable = observable;observable.addObserver(this);}public void update(Observable obs, Object arg) {if (obs instanceof WeatherData) {WeatherData weatherData = (WeatherData)obs;this.temperature = weatherData.getTemperature();this.humidity = weatherData.getHumidity();display();}}public void display() {System.out.println("Current conditions: " + temperature + "F degrees and " + humidity + "% humidity");}}

public class WeatherStation {public static void main(String[] args) {WeatherData weatherData = new WeatherData();CurrentConditionsDisplay currentConditions = 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);}}

以下是上述类的关系图：