1.观察者模式

1. 定义：定义了对象之间的一对多依赖，这样一来，当一个对象改变状态时，它的多有依赖者都会收到通知并自动更新。
2. 理解：观察者模式就是主题类状态发生改变的时，用一个公共的接口更新观察者状态。在使用观察者模式应该注重，多使用组合，少用继承；针对接口编程，不针对实现编程；降低对象之间的松耦合。
3. 举例说明：

/**
 * 观察者显示接口
 * Created by 15692 on 2019/4/9.
 */
public interface DisplayElement {
    void display();
}
/**
 * 观察者接口
 * Created by 15692 on 2019/4/9.
 */
public interface Observer {

    void update(Map<String,Object> map);
}
/**
 * 观察者类
 * Created by 15692 on 2019/4/9.
 */
public class CurrentConditionalDisplay implements Observer, DisplayElement {

    private Subject weatherData;

    private float temperature;

    private float humidity;

    public CurrentConditionalDisplay(Subject weatherData) {
        this.weatherData = weatherData;
        this.weatherData.registerObjserver(this);
    }

    @Override
    public void update(Map<String, Object> map) {
        this.temperature = (float) map.get("temperature");
        this.humidity = (float) map.get("humidity");
        display();
    }

    @Override
    public void display() {
        System.out.println("Current Conditional:" + this.temperature + "F degrees and " + this.humidity + "% humidity");
    }
}
/**
 * 主题接口
 * Created by 15692 on 2019/4/9.
 */
public interface Subject {

    void registerObjserver(Observer o);
    void removeObjserver(Observer o);
    void notifyObservers(Map<String, Object> map);
}
/**
 * 观察者类
 * Created by 15692 on 2019/4/9.
 */
public class CurrentConditionalDisplay implements Observer, DisplayElement {

    private Subject weatherData;

    private float temperature;

    private float humidity;

    public CurrentConditionalDisplay(Subject weatherData) {
        this.weatherData = weatherData;
        this.weatherData.registerObjserver(this);
    }

    @Override
    public void update(Map<String, Object> map) {
        this.temperature = (float) map.get("temperature");
        this.humidity = (float) map.get("humidity");
        display();
    }

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

4.时序图：

5. java内置的观察者模式，在java的util包内含有Observer接口和Observerable类，它和前面示例的subject接口和Observer接口类似，只是功能更加强大，观察者的注册和删除在主题中不用多余的代码注册。