java8 stream flatMap 笔记

package org.test;

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

import lombok.Data;

public class FlatMap {
	private static List computerClub = Arrays.asList(
        new Student("2015134001", "小明", 15, "1501"), 
        new Student("2015134003", "小王", 14, "1503"),
		new Student("2015134006", "小张", 15, "1501"), 
        new Student("2015134008", "小梁", 17, "1505")
    );
	
    private static List basketballClub = Arrays.asList(
        new Student("2015134012", "小c", 13, "1503"), 
        new Student("2015134013", "小s", 14, "1503"),
		new Student("2015134015", "小d", 15, "1504"), 
        new Student("2015134018", "小y", 16, "1505")
    );
	
     private static List pingpongClub = Arrays.asList(
        new Student("2015134022", "小u", 16, "1502"), 
        new Student("2015134021", "小i", 14, "1502"),
		new Student("2015134026", "小m", 17, "1504"), 
        new Student("2015134027", "小n", 16, "1504")
    );

	public static void main(String[] args) {
		List> allClubStu = new ArrayList<>();
		allClubStu.add(computerClub);
		allClubStu.add(basketballClub);
		allClubStu.add(pingpongClub);

        // 心得和经验分享下：你得看flatmap的方法签名，搞明白输入什么，输出什么，这样子
           比较好理解。flatMap的输出，也得是一个stream类型，faltMap做的事情，就是把流里的每一个元素，都放到一个一维数组（或集合）中。

		// flatMap 一对多映射处理，深入到多个stream内部去处理子元素，统一输出
		List studentList = allClubStu.stream().flatMap(e -> e.stream().filter(student -> student.getAge() > 15)).collect(Collectors.toList());
		studentList.forEach(System.out::println);
		
		System.out.println("我是漂亮的分割线");

		// flatMap 一对多映射处理，深入到多个stream内部去处理子元素，统一输出
		List studentList2 = allClubStu.stream().flatMap(e -> e.stream()).collect(Collectors.toList());
		studentList2.forEach(System.out::println);

	}

	@Data
	static class Student {
		private String id;
		private String name;
		private Integer age;
		private String code;

		public Student() {
			super();
		}

		public Student(String id, String name, Integer age, String code) {
			super();
			this.id = id;
			this.name = name;
			this.age = age;
			this.code = code;
		}

	}
}

输出的内容 

FlatMap.Student(id=2015134008, name=小梁, age=17, code=1505)
FlatMap.Student(id=2015134018, name=小y, age=16, code=1505)
FlatMap.Student(id=2015134022, name=小u, age=16, code=1502)
FlatMap.Student(id=2015134026, name=小m, age=17, code=1504)
FlatMap.Student(id=2015134027, name=小n, age=16, code=1504)
我是漂亮的分割线
FlatMap.Student(id=2015134001, name=小明, age=15, code=1501)
FlatMap.Student(id=2015134003, name=小王, age=14, code=1503)
FlatMap.Student(id=2015134006, name=小张, age=15, code=1501)
FlatMap.Student(id=2015134008, name=小梁, age=17, code=1505)
FlatMap.Student(id=2015134012, name=小c, age=13, code=1503)
FlatMap.Student(id=2015134013, name=小s, age=14, code=1503)
FlatMap.Student(id=2015134015, name=小d, age=15, code=1504)
FlatMap.Student(id=2015134018, name=小y, age=16, code=1505)
FlatMap.Student(id=2015134022, name=小u, age=16, code=1502)
FlatMap.Student(id=2015134021, name=小i, age=14, code=1502)
FlatMap.Student(id=2015134026, name=小m, age=17, code=1504)
FlatMap.Student(id=2015134027, name=小n, age=16, code=1504)

代码段2

package org.test;

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

import net.sf.json.JSONArray;



public class MapAndFloatMap {
	public static void main(String[] args) {
		List strs = Arrays.asList("好,好,学", "习,天,天", "向,上");
		 
		List strArray = strs.stream().map(str -> str.split(",")).collect(Collectors.toList());
 
		JSONArray jsonArray = JSONArray.fromObject(strArray);
 
		///strArray => [["好","好","学"],["习","天","天"],["向","上"]]
		System.out.println("strArray => " + jsonArray.toString());  
		///
 
		/ strList => [好, 好, 学, 习, 天, 天, 向, 上]
		// flatMap与map的区别在于 flatMap是将一个流中的每个值都转成一个个流，然后再将这些流扁平化成为一个流 。
		List strList = strs.stream().map(str -> str.split(",")).flatMap(Arrays::stream)
				.collect(Collectors.toList());
 
		System.out.println("strList => " + strList);
	}
}

strArray => [["好","好","学"],["习","天","天"],["向","上"]]
strList => [好, 好, 学, 习, 天, 天, 向, 上]

flatMap说实话有点绕，心得分享下：你得看flatmap的方法签名，搞明白输入什么，输出什么，这样子才比较好理解。flatMap的输出，也得是一个stream类型，faltMap做的事情，就是把流里的每一个元素，都放到一个一维数组（或集合）中。

再次重申一下，看了下别人写的文章，加了一下自己的理解。原创不易，支持原创。

参考文章：

stream中的map和flatMap_爱琴孩的博客-CSDN博客_stream中map和flatmap

Stream 中的map 与flatMap 用法_java编程-IT的博客-CSDN博客