观察者模式(jdk实现)

1.定义

在对象中定义一对多的依赖,当一个对象改变状态,依赖它的对象会收到通知并更新.

2.实现   (主要通过jdk自己定义的观察者实现)

以气象站通知展示板为例子,当气象站收到的各种参数改变的时候,就通知展示板并显示收到的参数.

定义一个气象站,继承jdk的Observable类

public class WeatherData extends Observable{
    private float temperature;
    private float humidity;
    private float pressure;
    public WeatherData() {

    }

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

    public void setMeasurements(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        this.pressure = pressure;
        measurementChanged();//改变状态并通知
    }

    public float getTemperature() {
        return temperature;
    }

    public float getHumidity() {
        return humidity;
    }

    public float getPressure() {
        return pressure;
    }

}

定义观察者------展示板,并且实现了jdk的Observer接口,代码如下

public class CurrentConditionsDisplay implements Observer, DisplayElement{
    Observable observable;
    private float temperature;
    private float humidity;

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

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

    @Override
    public void display() {
        System.out.println(toString());
    }

    @Override
    public String toString() {
        return "CurrentConditionsDisplay [temperature=" + temperature + ", humidity=" + humidity + "]";
    }

}

测试类如下

public class ObserverTest {
    public static void main(String[] args) {
        WeatherData weatherData = new WeatherData();
        CurrentConditionsDisplay display = new CurrentConditionsDisplay(weatherData);

        weatherData.setMeasurements(1.2F, 1.2F, 1.3F);

    }
}

3.总结

jdk实现观察者模式有个缺点就是Observable是类,不是接口,java只支持单继承,类会限制被观察者的拓展性.

而且jdk 的setChanged 和 clearChanged 方法都是protected 修饰,不能通过组合来实现这个方法,灵活性较差.

建议根据具体的需求来决定自己实现还是使用jdk的观察者模式.