为什么不能将Object []转换为String []

问题描述

1. 没有错误

1. No error

Object[] a = new String[]{"12","34","56"};
String[] b = (String[]) a;

没有错误

No error

Object a = new String[]{"12","34","56"};
String[] b = (String[]) a;

运行时错误:ClassCastException

Run time error : ClassCastException

Object[] a = new Object[3];
a[0] = "12";
a[1] = "34";
a[2] = "56";
String[] b = (String[]) a;

运行时错误:ClassCastException

Run time error : ClassCastException

Object[] a = {"12","34","56"};
String[] b = (String[]) a;

当然，如果它是作为 String [] 创建的，我们可以将 Object [] 变量下放回 String [] .

Of course, we can downcast an Object[] variable back to String[] if it was created as an String[].

我的问题是为什么当将 Object [] 创建为 Object [] 时，为什么不能将 Object [] 转换为 String [] ，成员是String吗?是出于安全原因还是实施起来没什么用?

My question is why we can not cast Object[] to String[] when it was created as Object[] but all its members are String? Is it because of security reason or just not that useful to implement this?

推荐答案

这是我能想到的两个原因.

Here's two reasons I can think of.

首先，如果更改原始数组，则强制转换的数组可能变为无效.例如

Firstly, if you change the original array, the casted array can become invalid. e.g.

 Object[] a = {"12","34","56"};
 String[] b = (String[]) a; // pretend this is legal. a and b now point to the same array

 a[0] = new Object(); // clearly ok
 String x = b[0]; // No longer a string! Bad things will happen!

第二，您选择的示例非常简单，但是如果您有一个很大的 Object [] 数组，并且编译器不清楚填充它的内容，那么它将无法验证数组的每个元素都满足转换要求.

Secondly, the example you have chosen is very simple, but if you have a very large Object[] array and it's not clear to the compiler what is filling it, then it has no way of validating that every element of the array satisfies the cast.

Object[] a = new Object[10000];
// lots of weird and whacky code to fill the array with strings

String[] b= (String[]) a; // valid or no? The best-defined answer is to say no.

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