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被观察者类继承 java.util.Observable，观察者类实现接口 java.util.Observer。被观察者在需要的方法中调用父类Observable的 setChanged() 方法和 notifyObservers(Object obj) 通知观察者。观察者类实现接口 Observer 的方法 update(Observable o, Object arg).

例：

public class Product extends Observable {  // 被观察者
       private String name;
       private float price;

       public String getName() {
              return name;
       }

       public void setName(String name) {
              this.name = name;
              // 设置变化点
              setChanged();
              notifyObservers(name);
       }

       public float getPrice() {
              return price;
       }

       public void setPrice(float price) {
              this.price = price;
              // 设置变化点
              setChanged();
              notifyObservers(new Float(price));
       }

       // 以下可以是数据库更新 插入命令.
       public void saveToDb() {

       }
}

public class NameObserver implements Observer { // 观察者1

       private String name = null;

       public void update(Observable o, Object arg) {
              // TODO Auto-generated method stub
              if (arg instanceof String) {
                     name = (String) arg;
                     // 产品名称改变值在name中
                     System.out.println("NameObserver :name changet to " + name);
              }
       }
}

public class PriceObserver implements Observer { // 观察者2 
       private float price = 0;

       public void update(Observable o, Object arg) {
              // TODO Auto-generated method stub
              if (arg instanceof Float) {
                     price = ((Float) arg).floatValue();
                     System.out.println("PriceObserver :price changet to " + price);
              }
       }
}

下面是测试：

public class Test {
       public static void main(String[] args) {
            // TODO Auto-generated method stub
            Product product = new Product();
            product.addObserver(new NameObserver());
            product.addObserver(new PriceObserver());
  
            product.setName("book");
            product.setPrice(66f);
      }
}

将会打印出

NameObserver :name changet to book
PriceObserver :price changet to 66.0