Java开发网 - 关于derived class 中的被覆写方法的调用

Topic: 关于derived class 中的被覆写方法的调用

1.关于derived class 中的被覆写方法的调用

Posted by: AchillesA
Posted on: 2004-08-06 17:05

先看一段代码：


class  TwoFunction 
{
   int i;
   public void first(){
   i=10;
   System.out.println("before second() => int i = " + i);
   second();
   System.out.println("after second() => int i = " + i);
}
   public void second (){
   i=20;
   first();
}
} 
class first extends TwoFunction
{
    public void second(){
    i=67;
    System.out.println("after second() => int i = " + i);
}
}
class second extends TwoFunction
{
    public void second(){
    i=88;
    System.out.println("after second() => int i = " + i);
}
    public static void main(String args[]){
    TwoFunction TF = new first();//upcasting
    TF.first();
}
}

output如下


---------- output ----------
before second() => int i = 10
after second() => int i = 88
after second() => int i = 88

输出完成 (耗时 0 秒) - 正常终止

改一下：


class  TwoFunction 
{
   int i;
   public void first(){
   i=10;
   System.out.println("before second() => int i = " + i);
   second();
   System.out.println("after second() => int i = " + i);
}
   public void second (){
   i=20;
   first();
}
} 
class first extends TwoFunction
{
    public void second(){
    i=67;
    System.out.println("after second() => int i = " + i);
}
}
class second extends TwoFunction
{
    public void second(){
    i=88;
    System.out.println("after second() => int i = " + i);
}
    /++
    public static void main(String args[]){
    TwoFunction TF = new first();//upcasting
    TF.first();
}*/
}
public class E_12
{
public static void main(String args[]){
    TwoFunction TF = new first();//upcasting
    TF.first();
}
}

output如下：


---------- output ----------
before second() => int i = 10
after second() => int i = 67
after second() => int i = 67

输出完成 (耗时 0 秒) - 正常终止

------------------------------------------------
我的理解是：第一段代码虽然是first的对象向上转型，但是java中总是调用它所能找到的最外层的derived class的对应方法。这里class second在最外层次，所以调用了他的second方法；第二段代码中虽然最外层的derived class还是second，但是由于在class E_12中产生的对象看不见class second，所以对该对象来说它能看见的最外层的derived class就是class first。
我想问我是：我的看法正确吗，java中是不是总是调用对象所能找到的最外层的derived class中的对应方法
(Java will always use the most-derived method it can find for the object type）