兩個方法的背景

這兩個方法看起來做著同樣的事情，但實際上又有些不一樣。看原始碼部分是這樣的

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

看原始碼做推測，map是一種中間操作，返回的是Stream

程式碼測試

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 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

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>
HELLO
HAI
HEHE
HI
YOU GOT IT
DON'T MENTION IT
NO WORRIES.
NOT A PROBLEM

Java 8 map() vs flatMap()

• map（）和flatMap（）方法都可以應用於Stream <T>和Optional <T>。 並且都返回Stream <R>或Optional <U>。
• 區別在於，對映操作為每個輸入值生成一個輸出值，而flatMap操作為每個輸入值生成任意數量（零個或多個）的值。 在flatMap（）中，每個輸入始終是一個集合，可以是List或Set或Map。 對映操作採用一個函式，該函式將為輸入流中的每個值呼叫，並生成一個結果值，該結果值將傳送到輸出流。 flatMap操作採用的功能在概念上想消耗一個值併產生任意數量的值。 但是，在Java中，方法返回任意數量的值很麻煩，因為方法只能返回零或一個值。
  

程式碼

 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(1,4)) // 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]

總結

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