利用Java提供的Observer接口和Observable类实现观察者模式

import java.util.Observable; 

public class Target extends Observable {

    private double height = 20;

    public double getHeight() {
        return height;
    }

    public void setHeight(double height) { 
        this.height = height;
        //将观察对象标记为已发生改变
         setChanged();
        //通知其所有观察者，对象发生了改变
         notifyObservers();
    }
}

import java.util.Observable;
import java.util.Observer;

public class Customer implements Observer {

    public Customer(Observable o) {
        //将观察者添加到该对象的观察者集合中
        o.addObserver(this);
    }
    
    private double height;
    
    public double getHeight() {
        return height;
    }

    public void setHeight(double height) {
        this.height = height;
    }

        /*
     *实现接口Observer的update方法
     *当观察对象发生改变时，调用该方法
     */
    @Override
    public void update(Observable o, Object arg) {
        Target t = (Target) o;
        double h = t.getHeight();
        setHeight(h);
    }
}

public class Test01 { 
    public static void main(String[] args) {
         Target target=new Target();
        
         Customer customer=new Customer(target);
         
         target.setHeight(30);
         
         System.out.println(customer.getHeight()); 
    } 
}