java - 为什么Stream＃toList的默认实现似乎过于复杂/不够理想？

Question

在查看Stream#toList的实现时，我只是注意到它看起来过于复杂和欠佳。
就像在上面的javadoc中提到的那样，大多数default实现都没有使用此Stream实现，但是，我认为可能是其他原因。
资料来源

/**
 * Accumulates the elements of this stream into a {@code List}. The elements in
 * the list will be in this stream's encounter order, if one exists. The returned List
 * is unmodifiable; calls to any mutator method will always cause
 * {@code UnsupportedOperationException} to be thrown. There are no
 * guarantees on the implementation type or serializability of the returned List.
 *
 * <p>The returned instance may be <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>.
 * Callers should make no assumptions about the identity of the returned instances.
 * Identity-sensitive operations on these instances (reference equality ({@code ==}),
 * identity hash code, and synchronization) are unreliable and should be avoided.
 *
 * <p>This is a <a href="package-summary.html#StreamOps">terminal operation</a>.
 *
 * @apiNote If more control over the returned object is required, use
 * {@link Collectors#toCollection(Supplier)}.
 *
 * @implSpec The implementation in this interface returns a List produced as if by the following:
 * <pre>{@code
 * Collections.unmodifiableList(new ArrayList<>(Arrays.asList(this.toArray())))
 * }</pre>
 *
 * @implNote Most instances of Stream will override this method and provide an implementation
 * that is highly optimized compared to the implementation in this interface.
 *
 * @return a List containing the stream elements
 *
 * @since 16
 */
@SuppressWarnings("unchecked")
default List<T> toList() {
    return (List<T>) Collections.unmodifiableList(new ArrayList<>(Arrays.asList(this.toArray())));
}

我对什么会更好的想法

return (List<T>) Collections.unmodifiableList(Arrays.asList(this.toArray()));

甚至

return Arrays.asList(this.toArray()));

IntelliJ的建议

return (List<T>) List.of(this.toArray());

是否有充分的理由在JDK源代码中实现？

Answer 1

可以实现toArray方法来返回一个数组，然后该数组随后发生突变，这将有效地使返回的列表不可变。这就是为什么通过创建新的ArrayList进行显式复制的原因。
本质上是防御性副本。
斯图尔特·马克斯(Stuart Marks)在review of this API期间也对此进行了讨论:

As written it's true that the default implementation does perform apparently redundant copies, but we can't be assured that toArray() actually returns a freshly created array. Thus, we wrap it using Arrays.asList and then copy it using the ArrayList constructor. This is unfortunate but necessary to avoid situations where someone could hold a reference to the internal array of a List, allowing modification of a List that's supposed to be unmodifiable.