Head First 设计模式（2）：观察者模式

观察者模式：定义了对象之间的一对多依赖，当一个对象状态改变时，他的所有依赖者都会收到通知并自动更新

例子：

代码：

package headfirst.designpatterns.observer.weather;public interface Subject {public void registerObserver(Observer o);public void removeObserver(Observer o);public void notifyObservers();}public interface Observer {    public void update(float temp, float humidity, float pressure);}public interface DisplayElement {    public void display();}import java.util.*;public class WeatherData implements Subject {    private ArrayList<Observer> observers;    private float temperature;    private float humidity;    private float pressure;        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() {        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;    }}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);    }        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");    }}public class ForecastDisplay implements Observer, DisplayElement {    private float currentPressure = 29.92f;      private float lastPressure;    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: ");        if (currentPressure > lastPressure) {            System.out.println("Improving weather on the way!");        } else if (currentPressure == lastPressure) {            System.out.println("More of the same");        } else if (currentPressure < lastPressure) {            System.out.println("Watch out for cooler, rainy weather");        }    }}public class StatisticsDisplay implements Observer, DisplayElement {    private float maxTemp = 0.0f;    private float minTemp = 200;    private float tempSum= 0.0f;    private int numReadings;    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);    }}public class WeatherStation {    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);    }}