观察者模式

出版者 + 订阅者 = 观察者模式
观察者模式定义了一系列对象之间的一对多关系。
当一个对象改变状态，其他依赖者都会收到通知。
设计原则：

1. 为了交互对象之间的松耦合设计而努力。松耦合的设计之所以能让我们建立弹性的OO系统，能够应对变化，是因为对象之间的互相依赖降到了最低。

接口

public interface DisplayElement {
    
    public void display();
}

实现类

import java.util.Observable;
import java.util.Observer;
import pattern.observer.mapper.DisplayElement;

/**
 * @author NNroc
 * @date 2020/8/3 19:28
 */
public class CurrentConditionsDisplay implements Observer, DisplayElement {
    
    Observable observable;
    private double temperature;
    private double humidity;

    public CurrentConditionsDisplay(Observable observable) {
    
        this.observable = observable;
        observable.addObserver(this);
    }

    public void display() {
    
        System.out.println("Current conditions: " + temperature + "F degrees and " + humidity + "% humidity");
    }

    public void update(Observable obs, Object arg) {
    
        if (obs instanceof WeatherData) {
    
            WeatherData weatherData = (WeatherData) obs;
            this.temperature = weatherData.getTemperature();
            this.humidity = weatherData.getHumidity();
            display();
        }
    }
}

import java.util.ArrayList;
import java.util.Observable;

/**
 * @author NNroc
 * @date 2020/8/3 11:04
 */
public class WeatherData extends Observable {
    
    private double temperature;
    private double humidity;
    private double pressure;

    public WeatherData() {
    
    }

    public void measurementsChanged() {
    
        setChanged();
        notifyObservers();
    }

    public void setMeasurements(double temperature, double humidity, double pressure) {
    
        this.temperature = temperature;
        this.humidity = humidity;
        this.pressure = pressure;
        measurementsChanged();
    }

    public double getTemperature() {
    
        return temperature;
    }

    public double getHumidity() {
    
        return humidity;
    }

    public double getPressure() {
    
        return pressure;
    }
}

测试

/**
 * @author NNroc
 * @date 2020/8/3 19:32
 */
public class WeatherStation {
    
    public static void main(String[] args) {
    
        WeatherData weatherData = new WeatherData();
        CurrentConditionsDisplay currentConditionsDisplay = new CurrentConditionsDisplay(weatherData);
//        StaticsDisplay staticsDisplay = new StaticsDisplay(weatherData);
//        ForecastDisplay forecastDisplay = new ForecastDisplay(weatherData);

        weatherData.setMeasurements(80, 65, 30.4f);
        weatherData.setMeasurements(82, 70, 29.2f);
        weatherData.setMeasurements(78, 90, 29.2f);

		// Current conditions: 80.0F degrees and 65.0% humidity
		// Current conditions: 82.0F degrees and 70.0% humidity
		// Current conditions: 78.0F degrees and 90.0% humidity
    }
}