我应该返回集合还是流?

Should I return a Collection or a Stream?(我应该返回集合还是流?)

问题描述

假设我有一个方法可以将只读视图返回到成员列表中:

Suppose I have a method that returns a read-only view into a member list:

class Team {
    private List<Player> players = new ArrayList<>();

    // ...

    public List<Player> getPlayers() {
        return Collections.unmodifiableList(players);
    }
}

进一步假设客户端所做的所有事情都是立即对列表进行一次迭代.也许将玩家放入 JList 或其他东西中.客户端确实不存储对列表的引用以供以后检查！

Further suppose that all the client does is iterate over the list once, immediately. Maybe to put the players into a JList or something. The client does not store a reference to the list for later inspection!

鉴于这种常见情况，我应该返回一个流吗?

Given this common scenario, should I return a stream instead?

public Stream<Player> getPlayers() {
    return players.stream();
}

或者在 Java 中返回一个非惯用的流?流是否设计为始终终止"?在创建它们的同一表达式中?

Or is returning a stream non-idiomatic in Java? Were streams designed to always be "terminated" inside the same expression they were created in?

推荐答案

答案一如既往，视情况而定".这取决于返回的集合有多大.这取决于结果是否随时间变化，以及返回结果的一致性有多重要.这在很大程度上取决于用户可能如何使用答案.

The answer is, as always, "it depends". It depends on how big the returned collection will be. It depends on whether the result changes over time, and how important consistency of the returned result is. And it depends very much on how the user is likely to use the answer.

首先，请注意，您始终可以从 Stream 中获取 Collection，反之亦然:

First, note that you can always get a Collection from a Stream, and vice versa:

// If API returns Collection, convert with stream()
getFoo().stream()...

// If API returns Stream, use collect()
Collection<T> c = getFooStream().collect(toList());

所以问题是，哪个对您的来电者更有用.

So the question is, which is more useful to your callers.

如果您的结果可能是无限的，那么只有一个选择:Stream.

If your result might be infinite, there's only one choice: Stream.

如果您的结果可能非常大，您可能更喜欢 Stream，因为一次将其全部实现可能没有任何价值，而且这样做可能会产生巨大的堆压力.

If your result might be very large, you probably prefer Stream, since there may not be any value in materializing it all at once, and doing so could create significant heap pressure.

如果调用者要做的只是遍历它(搜索、过滤、聚合)，你应该更喜欢 Stream，因为 Stream 已经内置了这些并且不需要具体化集合(特别是如果用户可能不会处理整个结果.)这是一种非常常见的情况.

If all the caller is going to do is iterate through it (search, filter, aggregate), you should prefer Stream, since Stream has these built-in already and there's no need to materialize a collection (especially if the user might not process the whole result.) This is a very common case.

即使您知道用户会多次迭代它或以其他方式保留它，您仍然可能希望返回一个 Stream，因为无论 Collection(例如，ArrayList)可能不是他们想要的形式，然后调用者无论如何都必须复制它.如果您返回 Stream，他们可以执行 collect(toCollection(factory)) 并以他们想要的形式得到它.