Collectors.toList()返回什么类型的 List < E > ？

我正在阅读 Lambda 状态: 图书馆版，有一句话让我感到惊讶:

在 小溪节中，有以下内容:

List<Shape> blue = shapes.stream()
.filter(s -> s.getColor() == BLUE)
.collect(Collectors.toList());

该文档没有说明 shapes实际上是什么，我甚至不知道它是否重要。

让我感到困惑的是: 这个代码块返回什么样的具体 List？

  • 它将变量赋值给 List<Shape>，这完全没问题。
  • stream()和 filter()决定使用什么类型的列表。
  • Collectors.toList()既不指定 List的具体类型。

那么，这里使用的 List的 混凝土类型(子类)是什么? 有什么保证吗？

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It doesn't matter, but the concrete type is non-generic as indeed all types are non-generic at runtime.

So, what concrete type (subclass) of List is being used here? Are there any guarantees?

I don't think so, but ArrayList or LinkedList seem likely.

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最佳答案

So, what concrete type (subclass) of List is being used here? Are there any guarantees?

If you look at the documentation of Collectors#toList(), it states that - "There are no guarantees on the type, mutability, serializability, or thread-safety of the List returned". If you want a particular implementation to be returned, you can use Collectors#toCollection(Supplier) instead.

Supplier<List<Shape>> supplier = () -> new LinkedList<Shape>();


List<Shape> blue = shapes.stream()
.filter(s -> s.getColor() == BLUE)
.collect(Collectors.toCollection(supplier));

And from the lambda, you can return whatever implementation you want of List<Shape>.

Update:

Or, you can even use method reference:

List<Shape> blue = shapes.stream()
.filter(s -> s.getColor() == BLUE)
.collect(Collectors.toCollection(LinkedList::new));
小开

Navigating through Netbeans (Ctrl + Click), I landed in this code. It seems to be using an ArrayList as Supplier.

public static <T> Collector<T, ?, List<T>> toList() {
return new CollectorImpl<>((Supplier<List<T>>) ArrayList::new, List::add,
(left, right) -> { left.addAll(right); return left; },
CH_ID);
}

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