为什么`Stream.collect`类型安全，而`Stream.toArray（IntFunction< A []>）`不是？ [英] Why is `Stream.collect` type-safe and `Stream.toArray(IntFunction<A[]>)` is not?

问题描述

考虑以下代码片段

String strings[] = {"test"};
final List<String> collect = java.util.Arrays.stream(strings).collect(java.util.stream.Collectors.toList());
final Double[] array = java.util.Arrays.stream(strings).toArray(Double[]::new);

为什么Java可以保证collect-case中的正确类型（将collect的泛型类型更改为例如Double导致编译时错误），但不在数组的情况下（编译很好，尽管 apply（int） Double [] :: new 给出 Double [] ，而不是对象[] ，但抛出 ArrayStoreException 如果上面使用不正确的话？）

Why can Java guarantee the correct type in the collect-case (changing the generic type of collect to e.g. Double leads to a compile time error), but not in the array case (compiles fine, despite apply(int) of Double[]::new gives a Double[], not an Object[], but throws ArrayStoreException if used incorrectly as above)?

如果我改变了，那么产生编译时错误的最佳方法是什么？流的类型没有更改 toArray 调用中的给定 IntFunction ？

What would be the best way to generate a compile time error in case I change the type of the stream without changing the given IntFunction in the toArray call?

推荐答案

方法 Stream :: toArray 的签名如下所示。请注意，类型参数 T 和 A 完全不相关。

The signature of the method Stream::toArray looks as follows. Please note that the type parameters T and A are completely unrelated.

public interface Stream<T> {
    <A> A[] toArray(IntFunction<A[]> generator);
}

在 ReferencePipeline.java ，你可以找到以下评论：

In the source of ReferencePipeline.java, you can find the following comment:

由于 A 与<$ c $无关c> U （无法声明 A 是 U 的上限
将不会进行静态类型检查。
因此使用原始类型并假设 A == U 而不是传播 A 和<的分离整个代码库中code> U
。
永远不会检查 U 的运行时类型是否与运行时类型 A [] 。
当元素存储在 A [] 中时，将执行运行时检查，因此如果 A 不是将抛出
超类型 U ArrayStoreException 。

Since A has no relation to U (not possible to declare that A is an upper bound of U) there will be no static type checking. Therefore use a raw type and assume A == U rather than propagating the separation of A and U throughout the code-base. The runtime type of U is never checked for equality with the component type of the runtime type of A[]. Runtime checking will be performed when an element is stored in A[], thus if A is not a super type of U an ArrayStoreException will be thrown.

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