克隆已在Object中保护访问权限

Question

我不知道为什么以下程序会产生此错误 - ＆gt; clone has protected access in Object。派生类不能访问Object类的受保护方法吗？如果我跳过此语句cloned.id=(String)id.clone();，程序将成功编译！

class Example implements Cloneable{
String id;
void display(){
    System.out.println("ID="+id);
    }
public Example clone() throws CloneNotSupportedException{
    Example cloned;
    cloned=(Example)super.clone();
    cloned.id=(String)id.clone();               //troublemaker
    return cloned;
    }
}
class CloneInterface{
    public static void main(String args[]){
        try
        {
        Example e1=new Example();
        e1.id="a";
        Example e2=e1.clone();
        e2.id="b";
        e1.display();
        e2.display();
        }
        catch(CloneNotSupportedException e)
        {
        e.printStackTrace();
        }
    }
}