Java 8 Streams FlatMap 方法示例

Question

我一直在检查即将发布的 Java update ，即:Java 8 or JDK 8 .是的，我很不耐烦，有很多新东西，但是，有一些我不明白的东西，一些简单的代码:

final Stream<Integer>stream = Stream.of(1,2,3,4,5,6,7,8,9,10);
stream.flatMap();

javadocs 是

public <R> Stream<R> flatMap(Function<? super T,? extends Stream<? extends R>> mapper)

Returns a stream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element. Each mapped stream is closed after its contents have been placed into this stream. (If a mapped stream is null an empty stream is used, instead.) This is an intermediate operation.

如果有人创建了一些关于 flatMap 的简单现实示例，我将不胜感激。 ，如何在以前的 Java 版本中对其进行编码 Java[6,7]以及如何使用 Java 8 编写相同的例程.

Answer 1

flatMap 没有意义一个 Stream这已经是平坦的了，就像 Stream<Integer>你已经在你的问题中展示了。

但是，如果您有 Stream<List<Integer>>那么这是有道理的，你可以这样做:

Stream<List<Integer>> integerListStream = Stream.of(
    Arrays.asList(1, 2), 
    Arrays.asList(3, 4), 
    Arrays.asList(5)
);

Stream<Integer> integerStream = integerListStream .flatMap(Collection::stream);
integerStream.forEach(System.out::println);

哪个会打印:

1
2
3
4
5

要在 Java 8 之前执行此操作，您只需要一个循环:

List<List<Integer>> integerLists = Arrays.asList(
    Arrays.asList(1, 2), 
    Arrays.asList(3, 4), 
    Arrays.asList(5)
)

List<Integer> flattened = new ArrayList<>();

for (List<Integer> integerList : integerLists) {
    flattened.addAll(integerList);
}

for (Integer i : flattened) {
    System.out.println(i);
}