java stream map和 flatmap区别

区别：
map mapper返回R，flatMap mapper返回Stream<R>

官网解释

1，<R>Stream<R> flatMap(Function<? super T,? extends Stream<? extends R>> mapper) 

Returns a stream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.

2，<R>Stream<R> map(Function<? super T,? extends R> mapper)

Returns a stream consisting of the results of applying the given function to the elements of this stream.

map(Function f)

接收一个函数作为参数，该函数会被应用到每个元素上，并将其映射成一个新的元素，返回的数据还是一个流。

flatMap(Function f)

接收一个函数作为参数，将流中的每个值都换成另一个流，然后把所有流连接成一个流。

示例代码

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class MapFlatMap {
    public static void main(String[] args) {
        List<String> list1 = new ArrayList<>();
        list1.add("A");
        list1.add("B");
        List<String> list2 = new ArrayList<>();
        list2.add("C");
        list2.add("D");

        List<List<String>> list = new ArrayList<>();
        list.add(list1);
        list.add(list2);

        System.out.println("----------对比举例------------");

        Stream stream1 = list.stream().map(i-> {
            List<String> listT = new ArrayList<>();
            i.stream().forEach(j->listT.add("map1>" + j));
            return listT;
        });
        stream1.forEach(System.out::println);

        System.out.println();

        Stream stream2 = list.stream().flatMap(i->i.stream().map(j->"flatMap2>" + j));
        stream2.forEach(System.out::println);

        System.out.println("-----------单词合并-----------");
        List<String> words = Arrays.asList("hello c++", "hello java", "hello python");
        List<String> result = words.stream()
                .map(word -> word.split(" ")) // 将单词按照空格切合，返回Stream<String[]>类型的数据
                .flatMap(Arrays::stream) // 将Stream<String[]>转换为Stream<String>
                .distinct() // 去重
                .collect(Collectors.toList());
        System.out.println(result);

    }
}

输出：

----------对比举例------------
[map1>A, map1>B]
[map1>C, map1>D]

flatMap2>A
flatMap2>B
flatMap2>C
flatMap2>D
-----------单词合并-----------
[hello, c++, java, python]