定义：定义了对象之间一对多依赖，这样一来，当一个对象改变状态时，它的所有依赖都会收到通知并自动更新
类图

image.png

案例：布告板实时显示温度。
建立主题接口

public interface Subject {
    void registerObserver(Observer O);
    void removeObserver(Observer O);
    void notifyObserver();

}

建立观察者接口

public interface Observer {
    void update(float temp,float humidity,float pessure);
}

建立显示接口

public interface DisplayElement {
    void display();
}

定义被观察者

public class WeatherData implements Subject {
    private ArrayList<Observer> observers;
    private float temperature;
    private float humidity;
    private float pressure;

    WeatherData(){
        observers = new ArrayList<>();
    }
    @Override
    public void registerObserver(Observer o) {
        observers.add(o);
    }

    @Override
    public void removeObserver(Observer o) {
        int i = observers.indexOf(o);
        if(i >= 0){
            observers.remove(i);
        }

    }

    @Override
    public void notifyObserver() {
        for (Observer observer:observers){
            observer.update(temperature,humidity,pressure);
        }
    }

    public void measurementsChanged(){
        notifyObserver();
    }

    public void setMessurements(float temperature,float humidity,float pressure){
        this.temperature = temperature;
        this.humidity = humidity;
        this.pressure = pressure;
        measurementsChanged();
    };
}

定义观察者

public class CurrentConditionDisplay implements Observer,DisplayElement {

    private float temperature;
    private float humidity;
    private WeatherData weatherData;
    CurrentConditionDisplay(WeatherData weatherData){
        this.weatherData = weatherData;
        weatherData.registerObserver(this);
    }
    public void logOut(){
        weatherData.removeObserver(this);
    }
    @Override
    public void display() {
        System.out.println("Current conditions :" + temperature+"F degrees and " + humidity+"% humdity");
    }

    @Override
    public void update(float temp, float humidity, float pessure) {
        this.temperature = temp;
        this.humidity = humidity;
        display();
    }
}

测试类

public class Test {
    public static void main(String[] args){
        WeatherData weatherData = new WeatherData();
        CurrentConditionDisplay currentConditionDisplay = new CurrentConditionDisplay(weatherData);
        weatherData.setMessurements(80,65,30.4f);
        weatherData.setMessurements(83,60,30.1f);
        currentConditionDisplay.logOut();
        weatherData.setMessurements(85,62,30.3f);
    }
}

java内置的观察者模式

java.util包内包含最基本的Observer接口和Observable类。
修改后的观察者类

public class CurrentConditionDisplay implements Observer, DisplayElement {

    Observable observable;
    private float temperature;
    private float humidity;
    private WeatherData weatherData;
    CurrentConditionDisplay(Observable observable){
       this.observable = observable;
       observable.addObserver(this);
    }

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

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

修改后的被观察者类

public class WeatherData extends Observable {

    private float temperature;
    private float humidity;
    private float pressure;

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

    public void setMessurements(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;
    }
}

java内置观察者的缺点：
1、Observable是一个类，使得被观察者无法继承其他的超类。
2、Observable将关键方法保护起来。