ToList()的实现清楚地返回了一个ArrayList，它保证了可变性:
public static <T>
Collector<T, ?, List<T>> toList() {
return new CollectorImpl<>(ArrayList::new, List::add,
(left, right) -> { left.addAll(right); return left; },
CH_ID);
}
但toList的JavaDoc读作:
/**
* Returns a {@code Collector} that accumulates the input elements into a
* new {@code List}. There are no guarantees on the type, mutability,
* serializability, or thread-safety of the {@code List} returned; if more
* control over the returned {@code List} is required, use {@link #toCollection(Supplier)}.
*
* @param <T> the type of the input elements
* @return a {@code Collector} which collects all the input elements into a
* {@code List}, in encounter order
*/
为什么不能保证可变性？我是否遗漏了返回的集合可能是不可变的情况？当返回的集合需要可变性时，使用Collectors.toList()是否安全？