关于Java8中map()和flatMap()的一些事

package java.util.stream;

/**
* @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);

/**
* @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);

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 with simple list
a
e
i
o
u
Output with nested List of List<String>
java.util.stream.ReferencePipeline$3@3b9a45b3
java.util.stream.ReferencePipeline$3@7699a589

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 with nested List of List<String>
HELLO
HAI
HEHE
HI
YOU GOT IT
DON'T MENTION IT
NO WORRIES.
NOT A PROBLEM

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