为什么List< String> .toArray（）返回Object []而不是String []？如何解决这个问题？ [英] why does List<String>.toArray() return Object[] and not String[]? how to work around this?

问题描述

有人知道为什么Java 1.6有这种行为：

Does anybody know why Java 1.6 has this behaviour:

List<String> list = ArrayList<String>();
String[] arr = (String[]) list.toArray();

我得到一个ClassCastException，因为它返回Object []而不是String []。

And I get a ClassCastException, because it returns Object[] and not String[].

我以为
List< T> .toArray（）应该返回T [] - no？有没有人有答案为什么这种不便在语言中存在？
还有如何解决这个问题？如何从 List< String> 得到一个String []而不循环这些项目？

I thought List<T>.toArray() should return T[] - no? Does anyone have an answer why this inconvenience exists in the language? And also how to work around this? How do I get a String[] from List<String> without looping thru the items?

推荐答案

您需要传入一个数组，以便它的运行时类型可以用作 toArray 的提示。尝试 toArray（new String [0]）来代替。你也可以传入一个预先设定大小的数组。

You need to pass in an array so its runtime type can be used as a hint by toArray. Try toArray(new String[0]) instead. You can also pass in a pre-sized array.

为了解，请考虑擦除操作需要做什么类型的操作

To understand, consider what type erasure would have to do to make

new T[4]

工作。如果Java允许的话，它可以在擦除后进行最好的处理是：

work. If Java allowed that, the best it could do post erasure is

new Object[4]

大多数 toArray 实现使用 java.lang.reflect.Array中 来构造一个给定类型提示的正确类型的输出数组，该类型提示作为 Class 传递。

Most toArray implementations use java.lang.reflect.Array to construct an output array of the right type given a type hint passed as a Class.

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