观察者模式(Observer Pattern)（二）：HeadFirst中的气象站的实现

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 class WeatherData implements Subject {
    private ArrayList observers;
    //temperature、humidity、pressure就是WeatherData的“状态”
    private float temperature;
    private float humidity;
    private float pressure;
    
    public WeatherData() {
        observers = new ArrayList();
    }
    
    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 (int i = 0; i < observers.size(); i++) {
            Observer observer = (Observer)observers.get(i);
            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 StatisticsDisplay implements Observer, DisplayElement {
    private float maxTemp = 0.0f;
    private float minTemp = 200;
    private float tempSum= 0.0f;
    private int numReadings;//记录观测的次数以便计算平均温度值
    private Subject weatherData;

    public StatisticsDisplay(Subject weatherData) {
        this.weatherData = weatherData;
        weatherData.registerObserver(this);
    }
    //这属于“推”数据，本类只使用到了第一个参数
    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);
    }
}