关于Java8中map()和flatMap()的一些事

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 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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