java – 为什么不能将Object []转换为String []

>没有错误

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

>没有错误

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

>运行时错误：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

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

当然,如果一个Object []变量被创建为一个String [],我们可以将其转换回String [].

我的问题是为什么当Object []创建为Object []但它的所有成员都是String时,我们不能将Object []转换为String []是因为安全原因还是对实现这个没有帮助？

解决方法

这里有两个原因可以想到.

首先,如果您更改原始数组,则转换的数组可能无效.例如

Object[] a = {"12","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!

其次,您所选择的示例非常简单,但如果您有一个非常大的Object []数组,并且编译器不清楚填充它,那么它无法验证数组的每个元素都满足该转换.

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.