java8 map flatmap

川长思鸟来 2021-06-11 15:14 463阅读 0赞

[https://www.cnblogs.com/diegodu/p/8794857.html][https_www.cnblogs.com_diegodu_p_8794857.html]

map: 对于Stream中包含的元素使用给定的转换函数进行转换操作，新生成的Stream只包含转换生成的元素。这个方法有三个对于原始类型的变种方法，分别是：mapToInt，mapToLong和mapToDouble。这三个方法也比较好理解，比如mapToInt就是把原始Stream转换成一个新的Stream，这个新生成的Stream中的元素都是int类型。之所以会有这样三个变种方法，可以免除自动装箱/拆箱的额外消耗；

map方法示意图：  
![2e301683060e1b68a3aa2fe9cda01262.png][]

flatMap：和map类似，不同的是其每个元素转换得到的是Stream对象，会把子Stream中的元素压缩到父集合中；

flatMap方法示意图：  
![0fe41b2b2afce3d188439048767be1f8.png][]

## 原文地址 ##

[http://www.java67.com/2016/03/how-to-use-flatmap-in-java-8-stream.html][http_www.java67.com_2016_03_how-to-use-flatmap-in-java-8-stream.html]

## 一句话 ##

把几个小的list转换到一个大的list。

## 一张图 ##

![3f74ff613ab866a99c51f9dce4da366d.png][]

flatMap操作

## 一段代码 ##

package test;
    
    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.List;
    import java.util.stream.Collectors;
    
    /**
     * Java Program to demonstrate how to use the flatMap() function in Java 8.
     * The flatMap() function is used to convert a Stream of list of values to
     * just a Stream of values. This is also called flattening of stream.
     *
     * @author Javin Paul
     */
    public class Test {
    
        public static void main(String args[]) {
    
            List<String> teamIndia = Arrays.asList("Virat", "Dhoni", "Jadeja");
            List<String> teamAustralia = Arrays.asList("Warner", "Watson", "Smith");
            List<String> teamEngland = Arrays.asList("Alex", "Bell", "Broad");
            List<String> teamNewZeland = Arrays.asList("Kane", "Nathan", "Vettori");
            List<String> teamSouthAfrica = Arrays.asList("AB", "Amla", "Faf");
            List<String> teamWestIndies = Arrays.asList("Sammy", "Gayle", "Narine");
            List<String> teamSriLanka = Arrays.asList("Mahela", "Sanga", "Dilshan");
            List<String> teamPakistan = Arrays.asList("Misbah", "Afridi", "Shehzad");
            
            List<List<String>> playersInWorldCup2016 = new ArrayList<>();
            playersInWorldCup2016.add(teamIndia);
            playersInWorldCup2016.add(teamAustralia);
            playersInWorldCup2016.add(teamEngland);
            playersInWorldCup2016.add(teamNewZeland);
            playersInWorldCup2016.add(teamSouthAfrica);
            playersInWorldCup2016.add(teamWestIndies);
            playersInWorldCup2016.add(teamSriLanka);
            playersInWorldCup2016.add(teamPakistan);
            
            // Let's print all players before Java 8
            List<String> listOfAllPlayers = new ArrayList<>();
            
            for(List<String> team : playersInWorldCup2016){
                for(String name : team){
                    listOfAllPlayers.add(name);
                }
            }
            
            System.out.println("Players playing in world cup 2016");
            System.out.println(listOfAllPlayers);
            
            
            // Now let's do this in Java 8 using FlatMap
            List<String> flatMapList = playersInWorldCup2016.stream()
                                                            .flatMap(pList -> pList.stream())
                                                            .collect(Collectors.toList());
            
            System.out.println("List of all Players using Java 8");
            System.out.println(flatMapList);
        }
    
    }
    
    Output
    run:
    Players playing in world cup 2016
    [Virat, Dhoni, Jadeja, Warner, Watson, Smith, Alex, Bell, Broad, Kane, Nathan, Vettori, AB, Amla, Faf, Sammy, Gayle, Narine, Mahela, Sanga, Dilshan, Misbah, Afridi, Shehzad]
    List of all Players using Java 8
    [Virat, Dhoni, Jadeja, Warner, Watson, Smith, Alex, Bell, Broad, Kane, Nathan, Vettori, AB, Amla, Faf, Sammy, Gayle, Narine, Mahela, Sanga, Dilshan, Misbah, Afridi, Shehzad]
    BUILD SUCCESSFUL (total time: 0 seconds)

\-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

flatMap的用法和含义住要通过一个案例来讲解，

案例：对给定单词列表 \["Hello","World"\],你想返回列表\["H","e","l","o","W","r","d"\]

第一种方式

1.  `String[] words = new String[]{"Hello","World"};`
2.  `List<String[]> a = Arrays.stream(words)`
3.  `.map(word -> word.split(""))`
4.  `.distinct()`
5.  `.collect(toList());`
6.  `a.forEach(System.out::print);`

代码输出为：\[Ljava.lang.String;@12edcd21\[Ljava.lang.String;@34c45dca

（返回一个包含两个String\[\]的list）

这个实现方式是由问题的，传递给map方法的lambda为每个单词生成了一个String\[\](String列表)。因此，map返回的流实际上是Stream<String\[\]> 类型的。你真正想要的是用Stream<String>来表示一个字符串。

下方图是上方代码stream的运行流程

![70][]

第二种方式：flatMap（对流扁平化处理）

1.  `String[] words = new String[]{"Hello","World"};`
2.  `List<String> a = Arrays.stream(words)`
3.  `.map(word -> word.split(""))`
4.  `.flatMap(Arrays::stream)`
5.  `.distinct()`
6.  `.collect(toList());`
7.  `a.forEach(System.out::print);`

结果输出：HeloWrd

使用flatMap方法的效果是，各个数组并不是分别映射一个流，而是映射成流的内容，所有使用map(Array::stream)时生成的单个流被合并起来，即扁平化为一个流。

下图是运用flatMap的stream运行流程，

![70 1][]

[https_www.cnblogs.com_diegodu_p_8794857.html]: https://www.cnblogs.com/diegodu/p/8794857.html
[2e301683060e1b68a3aa2fe9cda01262.png]: /images/20210527/b79f3308ee2843ac8b585adf4996c652.png
[0fe41b2b2afce3d188439048767be1f8.png]: /images/20210527/53de6d8771f54d59bf0bec79cf4e6f8e.png
[http_www.java67.com_2016_03_how-to-use-flatmap-in-java-8-stream.html]: https://link.jianshu.com/?t=http://www.java67.com/2016/03/how-to-use-flatmap-in-java-8-stream.html
[3f74ff613ab866a99c51f9dce4da366d.png]: /images/20210527/53d8e40b91f04860ac42f6871ea4ed0d.png
[70]: /images/20210527/0b3aa8b6ca5847c58de13186c53aa6c1.png
[70 1]: /images/20210527/26f6aad6fa9f4517b695d5ab2c509274.png