Java 8流和简单类型(Java 8 streams and simple types)

Java 8流和简单类型(Java 8 streams and simple types)

任何人都可以向我解释为什么以下不起作用：

long la[] = new long[] {1,2,3}; Arrays.stream(la).map(Long::valueOf).collect(Collectors.toSet());

这样做：

String la[] = new String[] {"1","2","3"}; Arrays.stream(la).map(Long::valueOf).collect(Collectors.toSet());

前者给出了编译错误，而后者没有。 编译错误是如此神秘（Eclipse），我无法理解它。

Can anyone explain to me why the following does not work:

long la[] = new long[] {1,2,3}; Arrays.stream(la).map(Long::valueOf).collect(Collectors.toSet());

When this does:

String la[] = new String[] {"1","2","3"}; Arrays.stream(la).map(Long::valueOf).collect(Collectors.toSet());

The former gives a compilation error while the latter does not. The compilation error is so cryptic (Eclipse) that I cannot make sense of it.

最满意答案

Arrays.stream(la)执行生成public static LongStream stream(long[] array)的方法public static LongStream stream(long[] array) 。 LongStream的map方法返回一个LongStream （即源LongStream每个long元素都映射到目标LongStream的long元素）。 LongStream没有接受单个参数的collect方法，这就是collect(Collectors.toSet())不通过编译的原因。

如果你使用mapToObj它应该工作：

Set<Long> set = Arrays.stream(la).mapToObj(Long::valueOf).collect(Collectors.toSet());

你的第二个代码片段起作用，因为Arrays.stream产生一个引用类型的Stream<String> （ Stream<String> ），其map方法产生另一个引用类型的Stream<Long> （在你的情况下为Stream<Long> ）。 这里， Stream有一个接受单个参数的collect方法 - collect(Collector<? super T, A, R> collector) - 所以collect(Collectors.toSet())可以工作。

Arrays.stream(la) executes the method public static LongStream stream(long[] array) which produces a LongStream. LongStream's map method returns a LongStream (i.e. each long element of the source LongStream is mapped to a long element in the target LongStream). LongStream doesn't have a collect method that accepts a single argument, which is why collect(Collectors.toSet()) doesn't pass compilation.

It should work if you use mapToObj :

Set<Long> set = Arrays.stream(la).mapToObj(Long::valueOf).collect(Collectors.toSet());

Your second snippet works since here Arrays.stream produces a Stream of a reference type (Stream<String>) whose map method produces another Stream of a reference type (Stream<Long> in your case). Here, Stream has a collect method that accepts a single argument - collect(Collector<? super T, A, R> collector) - so collect(Collectors.toSet()) works.