Java8 Stream的map和flatMap的区别

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package com.lgx.jdk8.part02;import java.util.ArrayList;import java.util.Arrays;import java.util.List;import java.util.stream.Collectors;import java.util.stream.Stream;/** * Stream的map和flatMap的区别: * map会将一个元素变成一个新的Stream * 但是flatMap会将结果打平，得到一个单个元素 */public class Test09StreamMapFlatMap {    public static void main(String[] args) {        /**获取单词，并且去重**/        List<String> list = Arrays.asList("hello welcome", "world hello", "hello world",            "hello world welcome");        //map和flatmap的区别        list.stream().map(item -> Arrays.stream(item.split(" "))).distinct().collect(Collectors.toList()).forEach(System.out::println);        System.out.println("---------- ");        list.stream().flatMap(item -> Arrays.stream(item.split(" "))).distinct().collect(Collectors.toList()).forEach(System.out::println);        //实际上返回的类似是不同的        List<Stream<String>> listResult = list.stream().map(item -> Arrays.stream(item.split(" "))).distinct().collect(Collectors.toList());        List<String> listResult2 = list.stream().flatMap(item -> Arrays.stream(item.split(" "))).distinct().collect(Collectors.toList());        System.out.println("---------- ");        //也可以这样        list.stream().map(item -> item.split(" ")).flatMap(Arrays::stream).distinct().collect(Collectors.toList()).forEach(System.out::println);        System.out.println("================================================");        /**相互组合**/        List<String> list2 = Arrays.asList("hello", "hi", "你好");        List<String> list3 = Arrays.asList("zhangsan", "lisi", "wangwu", "zhaoliu");        list2.stream().map(item -> list3.stream().map(item2 -> item + " " + item2)).collect(Collectors.toList()).forEach(System.out::println);        list2.stream().flatMap(item -> list3.stream().map(item2 -> item + " " + item2)).collect(Collectors.toList()).forEach(System.out::println);        //实际上返回的类似是不同的        List<Stream<String>> list2Result = list2.stream().map(item -> list3.stream().map(item2 -> item + " " + item2)).collect(Collectors.toList());        List<String> list2Result2 = list2.stream().flatMap(item -> list3.stream().map(item2 -> item + " " + item2)).collect(Collectors.toList());    }}
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