为什么不能将Object []转换为String [] [英] why can't cast Object[] to String[]
问题描述
-
没有错误
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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