java写个观察者模式

观察者模式是一种行为设计模式，它允许一个对象（称为主题）在其状态改变时通知一组依赖于它的对象（称为观察者）。以下是一个简单的Java代码示例：

主题接口：

public interface Subject {
    public void registerObserver(Observer o);
    public void removeObserver(Observer o);
    public void notifyObservers();
}

观察者接口：

public interface Observer {
    public void update(float temperature, float humidity, float pressure);
}

主题实现：

import java.util.ArrayList;

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

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

    public void registerObserver(Observer o) {
        observers.add(o);
    }

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

    public void notifyObservers() {
        for (Observer observer : observers) {
            observer.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();
    }
}

观察者实现：

public class CurrentConditionsDisplay implements Observer {
    private float temperature;
    private float humidity;
    private Subject weatherData;

    public CurrentConditionsDisplay(Subject weatherData) {
        this.weatherData = weatherData;
        weatherData.registerObserver(this);
    }

    public void update(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        display();
    }

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

示例程序：

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

        CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData);

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

        weatherData.removeObserver(currentDisplay);

        weatherData.setMeasurements(75, 80, 28.5f);
    }
}

在上面的示例中，WeatherData是主题，它维护观察者列表并通知它们。CurrentConditionsDisplay是一个观察者，它实现了Observer接口并在实例化时注册到WeatherData主题中。当WeatherData的状态改变时，它会调用measurementsChanged()方法并通知所有观察者。观察者可以通过update()方法获取主题的状态并更新自己。在示例程序中，可以看到CurrentConditionsDisplay收到了WeatherData状态的更新并显示它们。