观察者模式

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);
}

@Data
public class WeatherData implements Subject {

    private ArrayList<Observer> observers;
    private float temperature;
    private float humidity;
    private float pressure;

    public WeatherData() {
        observers = new ArrayList<Observer>();
    }

    @Override
    public void registerObserver(Observer o) {

        observers.add(o);
    }

    @Override
    public void removeObserver(Observer o) {

        observers.remove(o);
    }

    @Override
    public void notifyObservers() {

        for (Observer o : observers) {
            o.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 class ForecastDisplay implements Observer, DisplayElement {
    private float currentPressure = 29.92f;
    private float lastPressure;

    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;

	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);
	}
}

@GetMapping("/observer")
public RestResponse<Object> observer() {

    // 主题
    WeatherData weatherData=new WeatherData();
    
    // 对象
    Observer forecastDisplay=new ForecastDisplay();
    Observer statisticsDisplay=new StatisticsDisplay();

    // 注册观察者
    weatherData.registerObserver(forecastDisplay);
    weatherData.registerObserver(statisticsDisplay);

    weatherData.setMeasurements(1, 2, 3);
    weatherData.setMeasurements(4, 5, 6);
    
    return RestResponse.success();
}