两个方法的背景

这两个方法看起来做着同样的事情，但实际上又有些不一样。看源码部分是这样的

package java.util.stream;

map()方法

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

flatMap()方法

/*** @param <R> The element type of the new stream* @param mapper a <a href="package-summary.html#NonInterference" rel="external nofollow" rel="external nofollow" >non-interfering</a>,* <a href="package-summary.html#Statelessness" rel="external nofollow" rel="external nofollow" >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

看源码做推测，map是一种中间操作，返回的是Stream

代码测试

map()方法

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()方法

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（）方法都可以应用于Stream <T>和Optional <T>。 并且都返回Stream <R>或Optional <U>。
• 区别在于，映射操作为每个输入值生成一个输出值，而flatMap操作为每个输入值生成任意数量（零个或多个）的值。 在flatMap（）中，每个输入始终是一个集合，可以是List或Set或Map。 映射操作采用一个函数，该函数将为输入流中的每个值调用，并生成一个结果值，该结果值将发送到输出流。 flatMap操作采用的功能在概念上想消耗一个值并产生任意数量的值。 但是，在Java中，方法返回任意数量的值很麻烦，因为方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]

总结

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