类层次结构和对象之间的转换 [英] Classes Hierarchy and Casting between Objects

问题描述

据我所知，无法将超类的Object转换为子类的Object 。

更具体地说，给定下面的类和接口层次结构：

p。 Alpha是Beta和Gamma的超类

。 Gamma是Delta和Omega的超类

。界面In由Beta和Delta实现

在这种情况下，我定义了以下代码：

  Delta r; 
 Gamma q; 
  

这是否正确？

  r =（Delta）q; 
  

我可以将q投射到类型Delta，即使Delta是Gamma的子类吗？
我认为这是不可能的，我的教科书说不然。我已经搜索了很多，根据这我是对的，这是教科书的错误。

我没有任何东西？

解决方案

p>这是合法的：

  Gamma q = new Delta 
 Delta d =（Delta）q; 
  

这将编译，但会给你一个运行时错误：

  Gamma q = new Gamma（）; 
 Delta d =（Δ）q; 
  

在第一种情况下， q Delta ，因此您可以将其投放到 Delta 。在第二种情况下， q 是一个 Gamma ，因此您不能将其转换为 Delta 。

A Gamma Gamma 对象或 Gamma 的子类的对象，例如a Delta 对象;当你把一个 Gamma 转换为 Delta 然后你告诉编译器 Gamma 变量是指 Delta 对象或 Delta 如果你错了，你会得到一个 ClassCastException ）。对象本身的类型是不可变的 - 您不能在运行时将 Gamma 对象的类型更改为 Delta 对象，所以如果一个 Gamma 变量实际上是指一个 Gamma 对象，但你试图把它转换为 Delta 对象，那么你会得到一个运行时异常。

as far as i know it's not possible to cast an Object of a superclass into an Object of a subclass. This will compile but during runtime it will return an error.

More specifically, given the following Hierarchy of Classes and Interfaces:

. Alpha is a superclass for Beta and Gamma
. Gamma is a superclass for Delta and Omega
. Interface "In" is implemented by Beta and Delta

In this scenario i define the following code:

Delta r;
Gamma q;

Is this correct?

r = (Delta) q;

Can i cast q to type Delta even if Delta is a subclass of Gamma? I think this isn't possible, my text book says otherwise. I already searched a lot and according to this i'm right and this is an error from the textbook.

Am i missing anything?

解决方案

This is legal:

Gamma q = new Delta();
Delta d = (Delta)q;

This will compile but will give you a runtime error:

Gamma q = new Gamma();
Delta d = (Delta)q;

In the first case, q is a Delta, so you can cast it to a Delta. In the second case, q is a Gamma, so you cannot case it to a Delta.

A Gamma variable can refer to a Gamma object or to an object that is a subclass of Gamma, e.g. a Delta object; when you cast a Gamma to a Delta then you are telling the compiler that the Gamma variable refers to a Delta object or to an object that is a subclass of Delta (and if you're wrong then you'll get a ClassCastException). The types of the objects themselves are immutable - you cannot change the type of a Gamma object to a Delta object at runtime, so if a Gamma variable actually refers to a Gamma object but you then try to cast it to a Delta object then you'll get a runtime exception.

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