Observer接口

package java.util;
//用于实现观察者对象
public interface Observer {
    /**
     * This method is called whenever the observed object is changed. An
     * application calls an <tt>Observable</tt> object's
     * <code>notifyObservers</code> method to have all the object's
     * observers notified of the change.
     *
     * @param   o     the observable object.
     * @param   arg   an argument passed to the <code>notifyObservers</code>
     *                 method.
     */
    //当被观察对象o改变时，update方法通知所有arg观察者对象
    void update(Observable o, Object arg);
}