在Stream :: flatMap中使用Java 8的Optional [英] Using Java 8's Optional with Stream::flatMap

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问题描述

新的Java 8流框架和朋友们提供了一些非常简洁的Java代码,但是我遇到了一个看似简单的情况,这个问题很简洁。

The new Java 8 stream framework and friends make for some very concise java code, but I have come across a seemingly-simple situation that is tricky to do concisely.

考虑列表< Thing>东西和方法可选<其他>解决(事情)。我想将 Thing 映射到可选< Other> s并获取第一个其他。显而易见的解决方案是使用 things.stream()。flatMap(this :: resolve).findFirst(),但 flatMap 要求您返回一个流,并且可选没有 stream()方法(或者是它是集合或提供一种方法将其转换为或以集合的形式查看。

Consider a List<Thing> things and method Optional<Other> resolve(Thing thing). I want to map the Things to Optional<Other>s and get the first Other. The obvious solution would be to use things.stream().flatMap(this::resolve).findFirst(), but flatMap requires that you return a stream, and Optional doesn't have a stream() method (or is it a Collection or provide a method to convert it to or view it as a Collection).

我能想到的最好的是:

things.stream()
    .map(this::resolve)
    .filter(Optional::isPresent)
    .map(Optional::get)
    .findFirst();

但这似乎是一个非常常见的案例。任何人都有更好的主意吗?

But that seems awfully long-winded for what seems like a very common case. Anyone have a better idea?

推荐答案

Java 9



Optional.stream 已添加到JDK 9.这使您可以执行以下操作,而无需任何帮助方法:

Java 9

Optional.stream has been added to JDK 9. This enables you to do the following, without the need of any helper method:

Optional<Other> result =
    things.stream()
          .map(this::resolve)
          .flatMap(Optional::stream)
          .findFirst();



Java 8



是的,这是API中的一个小洞,因为将Optional变为零或一个长度的流有点不方便。你可以这样做:

Java 8

Yes, this was a small hole in the API, in that it's somewhat inconvenient to turn an Optional into a zero-or-one length Stream. You could do this:

Optional<Other> result =
    things.stream()
          .map(this::resolve)
          .flatMap(o -> o.isPresent() ? Stream.of(o.get()) : Stream.empty())
          .findFirst();

在flatMap中使用三元运算符有点麻烦,所以写起来可能会更好一个小帮手功能:

Having the ternary operator inside the flatMap is a bit cumbersome, though, so it might be better to write a little helper function to do this:

/**
 * Turns an Optional<T> into a Stream<T> of length zero or one depending upon
 * whether a value is present.
 */
static <T> Stream<T> streamopt(Optional<T> opt) {
    if (opt.isPresent())
        return Stream.of(opt.get());
    else
        return Stream.empty();
}

Optional<Other> result =
    things.stream()
          .flatMap(t -> streamopt(resolve(t)))
          .findFirst();

在这里,我已经内联调用resolve()而不是单独的map()操作,但这是一个品味问题。

Here, I've inlined the call to resolve() instead of having a separate map() operation, but this is a matter of taste.

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