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  The element type of the new stream
* @param mapper a non-interfering,
* stateless
*    function to apply to each element
* @return the new stream
*/
  Stream map(Function<? super T, ? extends R> mapper);

flatMap()方法

/**
* @param  The element type of the new stream
* @param mapper a non-interfering,
* stateless
* function to apply to each element which produces a stream
* of new values
* @return the new stream
*/
 Stream 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 vowels = Arrays.asList("A", "E", "I", "O", "U");
  vowels.stream().map(vowel -> vowel.toLowerCase())
    .forEach(value -> System.out.println(value));
  List haiList = new ArrayList<>();
  haiList.add("hello");
  haiList.add("hai");
  haiList.add("hehe");
  haiList.add("hi");
  System.out.println("Output with nested List of List");
  List welcomeList = new ArrayList<>();
  welcomeList.add("You got it");
  welcomeList.add("Don't mention it");
  welcomeList.add("No worries.");
  welcomeList.add("Not a problem");
  List> 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 list
a
e
i
o
u
Output with nested List of List
java.util.stream.ReferencePipeline$3@3b9a45b3
java.util.stream.ReferencePipeline$3@7699a589

flatMap()方法

public static void main(String[] args) {
  List haiList = new ArrayList<>();
  haiList.add("hello");
  haiList.add("hai");
  haiList.add("hehe");
  haiList.add("hi");
  System.out.println("Output with nested List of List");
  List welcomeList = new ArrayList<>();
  welcomeList.add("You got it");
  welcomeList.add("Don't mention it");
  welcomeList.add("No worries.");
  welcomeList.add("Not a problem");
  List> 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
HELLO
HAI
HEHE
HI
YOU GOT IT
DON'T MENTION IT
NO WORRIES.
NOT A PROBLEM

Java 8 map() vs flatMap()

• map（）和flatMap（）方法都可以应用于Stream 和Optional 。 并且都返回Stream 或Optional 。
• 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 together = Stream.of(Arrays.asList(1, 2), Arrays.asList(3, 4)) // Stream of List
    .map(List::stream)
    .collect(Collectors.toList());

  System.out.println("Output with map() -> "+together);


  List togetherFlatMap = Stream.of(Arrays.asList(1, 2), Arrays.asList(3, 4)) // Stream of List
    .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]