声明一种类型的对象和另一种类型的对象有什么好处? [英] What is the advantage to declaring an object of one type as in instance of another?

问题描述

可能重复:
Base b2 = new Child()是什么?表示?

Possible Duplicate:
What does Base b2 = new Child(); signify?

我是Java初学者.我了解类继承的概念，但是有一件事我不太了解.我正在阅读Java for Dummies，它正在解释多态.它以以下代码为例:

I'm a Java beginner. I understand the concepts of class inheritance, but there's one thing I don't quite understand. I'm reading through Java for Dummies, and it's explaining polymorphism. It gives this code as an example:

class Player {
    public void move() {...

class BetterPlayer extends Player {
    public void move() {...

public class TicTacToeApp {
    public static void main(String[] args) {
        Player p1 = new Player();
        Player p2 = new BetterPlayer();
        playTheGame(p1, p2);
    }

    public static void playTheGame(Player p1, Player p2) {
        p1.move();
        p2.move();
    }
}

为什么将p2创建为Player对象?这是我的理解:

Why was p2 created as a Player object? Here's my understanding:

如果p2是BetterPlayer对象(这样声明:BetterPlayer p2 =新的BetterPlayer ...):
-upcasting是自动的，因此仍然可以使用playTheGame方法
-p2可以用于需要BetterPlayer对象的任何方法

If p2 were a BetterPlayer object (declared like this: BetterPlayer p2 = new BetterPlayer...):
-upcasting is automatic, so it would still work with the playTheGame method
-p2 can be used by any methods requiring a BetterPlayer object

但是因为它是作为Player对象创建的，所以现在只要有需要BetterPlayer对象的方法使用p2，就必须将其显式转换为BetterPlayer对象，对吗?这似乎是更多的工作，没有任何好处，所以我猜想这样做一定会有好处.是什么?

But since it was created as a Player object, now any time p2 is used by a method requiring a BetterPlayer object, it must be explicitly cast to a BetterPlayer object, right? That seems like more work with no benefit, so I'm guessing there has to be some benefit to doing it that way; what is it?

推荐答案

未将变量p2创建为Player对象.它是指向BetterPlayer对象的参考.

The variable p2 was not created as a Player object. It's a reference that points to a BetterPlayer object.

无论将p2声明为Player引用还是BetterPlayer引用，都可以将其用作Player引用.

Regardless whether p2 is declared as a Player or BetterPlayer reference, it can be used as a Player reference.

将其声明为Player的一个优点是，可以防止您耦合仅与BetterPlayer一起使用的任何代码，并在以后保留将代码用于另一个子类选项(例如BestPlayer)的可能性.

An advantage of declaring it as a Player is that you are prevented from coupling any code that works only with BetterPlayer, keeping open the option of using the code for another subclass option in the future -- e.g., BestPlayer.

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