Java8 Stream的map和flatMap的區別

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的區別:
 * map會將一個元素變成一個新的Stream
 * 但是flatMap會將結果打平，得到一個單個元素
 */
public class Test09StreamMapFlatMap {
    public static void main(String[] args) {
        /**獲取單詞，並且去重**/
        List<String> list = Arrays.asList("hello welcome", "world hello", "hello world",
            "hello world welcome");

        //map和flatmap的區別
        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);

        //實際上返回的類似是不同的
        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("---------- ");

        //也可以這樣
        list.stream().map(item -> item.split(" ")).flatMap(Arrays::stream).distinct().collect(Collectors.toList()).forEach(System.out::println);

        System.out.println("================================================");

        /**相互組合**/
        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);

        //實際上返回的類似是不同的
        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());



    }
}