两个办法的背景

这两个办法看起来做着同样的事件，但实际上又有些不一样。看源码局部是这样的

package java.util.stream;

map()办法

/*** @param <R> The element type of the new stream* @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,* <a href="package-summary.html#Statelessness">stateless</a>*               function to apply to each element* @return the new stream*/    <R> Stream<R> map(Function<? super T, ? extends R> mapper);

flatMap()办法

/*** @param <R> The element type of the new stream* @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,* <a href="package-summary.html#Statelessness">stateless</a>* function to apply to each element which produces a stream* of new values* @return the new stream*/<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);

Stream map() Method

看源码做揣测，map是一种两头操作，返回的是Stream

代码测试

map()办法

public static void main(String[] args) {        System.out.println("Output with simple list");        List<String> vowels = Arrays.asList("A", "E", "I", "O", "U");        vowels.stream().map(vowel -> vowel.toLowerCase())                .forEach(value -> System.out.println(value));        List<String> haiList = new ArrayList<>();        haiList.add("hello");        haiList.add("hai");        haiList.add("hehe");        haiList.add("hi");        System.out.println("Output with nested List of List<String>");        List<String> welcomeList = new ArrayList<>();        welcomeList.add("You got it");        welcomeList.add("Don't mention it");        welcomeList.add("No worries.");        welcomeList.add("Not a problem");        List<List<String>> nestedList = Arrays.asList(haiList, welcomeList);        nestedList.stream().map(list -> {            return list.stream().map(value -> value.toUpperCase());        }).forEach(value -> System.out.println(value));    }

Output

Output with simple listaeiouOutput with nested List of List<String>java.util.stream.ReferencePipeline$3@3b9a45b3java.util.stream.ReferencePipeline$3@7699a589

flatMap()办法

public static void main(String[] args) {        List<String> haiList = new ArrayList<>();        haiList.add("hello");        haiList.add("hai");        haiList.add("hehe");        haiList.add("hi");        System.out.println("Output with nested List of List<String>");        List<String> welcomeList = new ArrayList<>();        welcomeList.add("You got it");        welcomeList.add("Don't mention it");        welcomeList.add("No worries.");        welcomeList.add("Not a problem");        List<List<String>> nestedList = Arrays.asList(haiList, welcomeList);        nestedList.stream().flatMap(                list -> list.stream())                .map(value -> value.toUpperCase())                .forEach(value -> System.out.println(value));    }

Output

Output with nested List of List<String>HELLOHAIHEHEHIYOU GOT ITDON'T MENTION ITNO WORRIES.NOT A PROBLEM

Java 8 map() vs flatMap()

  • map（）和flatMap（）办法都能够利用于Stream <T>和Optional <T>。 并且都返回Stream <R>或Optional <U>。
  • 区别在于，映射操作为每个输出值生成一个输入值，而flatMap操作为每个输出值生成任意数量（零个或多个）的值。 在flatMap（）中，每个输出始终是一个汇合，能够是List或Set或Map。 映射操作采纳一个函数，该函数将为输出流中的每个值调用，并生成一个后果值，该后果值将发送到输入流。 flatMap操作采纳的性能在概念上想耗费一个值并产生任意数量的值。 然而，在Java中，办法返回任意数量的值很麻烦，因为办法只能返回零或一个值。

代码

 public static void main(String[] args) {        List<Stream> together = Stream.of(Arrays.asList(1, 2), Arrays.asList(3, 4)) // Stream of List<Integer>                .map(List::stream)                .collect(Collectors.toList());        System.out.println("Output with map() -> "+together);        List<Integer> togetherFlatMap = Stream.of(Arrays.asList(1, 2), Arrays.asList(3, 4)) // Stream of List<Integer>                .flatMap(List::stream)                .map(integer -> integer + 1)                .collect(Collectors.toList());        System.out.println("Output with flatMap() -> "+togetherFlatMap);    }

Output

Output with map() -> [java.util.stream.ReferencePipeline$Head@16b98e56, java.util.stream.ReferencePipeline$Head@7ef20235]Output with flatMap() -> [2, 3, 4, 5]