1.我认为用简单朴素的理解和使用，是对工具最好的诠释。java jdk8开始提供了stream流，方便我更高效的操作集合和编写代码。其中flatmap流中间操作api，我认为简单来说是对“集合中的集合的操作和展开”。比如说，一个对象集合里面的每个对象还有个集合对象。这时，我们如果需要对这个集合的所有对象的集合对象进行操作，那么flatmap就是一个不错的选择。

2.接下来举个例子，比如有一群人的每个人都有多套房子，我想把这群人的所有房子的地址都去重的统计出来。

首先person类定义开整

class Person{
    private Long id;
    private Integer age;
    private String name;
    private String idCard;
    private List listHouse;
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getIdCard() {
        return idCard;
    }
    public void setIdCard(String idCard) {
        this.idCard = idCard;
    }
    public List getListHouse() {
        return listHouse;
    }
    public void setListHouse(List listHouse) {
        this.listHouse = listHouse;
    }
}

再定义一个房子bigHouse类

class bigHouse{
    private String address;
    private BigDecimal price;
    private Integer useAge;
    public bigHouse(String address, BigDecimal price, Integer useAge) {
        this.address = address;
        this.price = price;
        this.useAge = useAge;
    }
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;
    }
    public BigDecimal getPrice() {
        return price;
    }
    public void setPrice(BigDecimal price) {
        this.price = price;
    }
    public Integer getUseAge() {
        return useAge;
    }
    public void setUseAge(Integer useAge) {
        this.useAge = useAge;
    }
}

定义个main方法进行测试

public static void main(String[] args) {
        //数据填充
        Person p1 = new Person();
        p1.setId(1L);
        p1.setAge(22);
        p1.setName("p1");
        p1.setIdCard("1234");
        ArrayList p1hList = new ArrayList<>();
        bigHouse p1h1 = new bigHouse("重庆",new BigDecimal(23),5);
        bigHouse p1h2 = new bigHouse("开州",new BigDecimal(62),3);
        bigHouse p1h3 = new bigHouse("南雅",new BigDecimal(6),3);
        p1hList.add(p1h1);
        p1hList.add(p1h2);
        p1hList.add(p1h3);
        p1.setListHouse(p1hList);
        Person p2 = new Person();
        p2.setId(2L);
        p2.setAge(23);
        p2.setName("p2");
        p2.setIdCard("1235");
        ArrayList p2hList = new ArrayList<>();
        bigHouse p2h1 = new bigHouse("四川",new BigDecimal(223),7);
        bigHouse p2h2 = new bigHouse("重庆",new BigDecimal(123),9);
        bigHouse p2h3 = new bigHouse("南雅",new BigDecimal(6),3);
        p2hList.add(p2h1);
        p2hList.add(p2h2);
        p2hList.add(p2h3);
        p2.setListHouse(p2hList);
        Person p3 = new Person();
        p3.setId(3L);
        p3.setAge(24);
        p3.setName("p3");
        p3.setIdCard("1236");
        ArrayList p3hList = new ArrayList<>();
        bigHouse p3h1 = new bigHouse("江苏",new BigDecimal(231),17);
        bigHouse p3h2 = new bigHouse("无锡",new BigDecimal(163),8);
        bigHouse p3h3 = new bigHouse("达州",new BigDecimal(8),3);
        p3hList.add(p3h1);
        p3hList.add(p3h2);
        p3hList.add(p3h3);
        p3.setListHouse(p2hList);
        //flatMap handle collection zhong collection
        //需求：提取所有的人房子的地址，并且去除
        List pList = new ArrayList<>();
        pList.add(p1);
        pList.add(p2);
        pList.add(p3);
        List strings = pList.stream().flatMap(p -> {
            Stream stream = p.getListHouse().stream().map(bigHouse::getAddress).distinct();
            return stream;
        }).distinct().collect(Collectors.toList());
        strings.forEach(System.out::print);
        ArrayList list = CollUtil.newArrayList("ABC", "DEF", "GHI");
        List collect = list.stream().flatMap(ele -> Stream.of(ele.split(""))).collect(Collectors.toList());
        collect.forEach(System.out::println);
//        List list = Arrays.asList("l,y,w", "8,6,8");
//        List collect = list.stream().flatMap(s -> {
//            String[] split = s.split(",");
//            new HashMap<>();
//            return Arrays.stream(split);
//        }).collect(Collectors.toList());
//        collect.forEach(System.out::println);
    }

完整代码如下:

package com.conpany.project.stream;
import cn.hutool.core.collection.CollUtil;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class FlatMap {
    public static void main(String[] args) {
        //数据填充
        Person p1 = new Person();
        p1.setId(1L);
        p1.setAge(22);
        p1.setName("p1");
        p1.setIdCard("1234");
        ArrayList p1hList = new ArrayList<>();
        bigHouse p1h1 = new bigHouse("重庆",new BigDecimal(23),5);
        bigHouse p1h2 = new bigHouse("开州",new BigDecimal(62),3);
        bigHouse p1h3 = new bigHouse("南雅",new BigDecimal(6),3);
        p1hList.add(p1h1);
        p1hList.add(p1h2);
        p1hList.add(p1h3);
        p1.setListHouse(p1hList);
        Person p2 = new Person();
        p2.setId(2L);
        p2.setAge(23);
        p2.setName("p2");
        p2.setIdCard("1235");
        ArrayList p2hList = new ArrayList<>();
        bigHouse p2h1 = new bigHouse("四川",new BigDecimal(223),7);
        bigHouse p2h2 = new bigHouse("重庆",new BigDecimal(123),9);
        bigHouse p2h3 = new bigHouse("南雅",new BigDecimal(6),3);
        p2hList.add(p2h1);
        p2hList.add(p2h2);
        p2hList.add(p2h3);
        p2.setListHouse(p2hList);
        Person p3 = new Person();
        p3.setId(3L);
        p3.setAge(24);
        p3.setName("p3");
        p3.setIdCard("1236");
        ArrayList p3hList = new ArrayList<>();
        bigHouse p3h1 = new bigHouse("江苏",new BigDecimal(231),17);
        bigHouse p3h2 = new bigHouse("无锡",new BigDecimal(163),8);
        bigHouse p3h3 = new bigHouse("达州",new BigDecimal(8),3);
        p3hList.add(p3h1);
        p3hList.add(p3h2);
        p3hList.add(p3h3);
        p3.setListHouse(p2hList);
        //flatMap handle collection zhong collection
        //需求：提取所有的人房子的地址，并且去重
        List pList = new ArrayList<>();
        pList.add(p1);
        pList.add(p2);
        pList.add(p3);
        List strings = pList.stream().flatMap(p -> {
            Stream stream = p.getListHouse().stream().map(bigHouse::getAddress).distinct();
            return stream;
        }).distinct().collect(Collectors.toList());
        strings.forEach(System.out::print);
        ArrayList list = CollUtil.newArrayList("ABC", "DEF", "GHI");
        List collect = list.stream().flatMap(ele -> Stream.of(ele.split(""))).collect(Collectors.toList());
        collect.forEach(System.out::println);
//        List list = Arrays.asList("l,y,w", "8,6,8");
//        List collect = list.stream().flatMap(s -> {
//            String[] split = s.split(",");
//            new HashMap<>();
//            return Arrays.stream(split);
//        }).collect(Collectors.toList());
//        collect.forEach(System.out::println);
    }
}
class Person{
    private Long id;
    private Integer age;
    private String name;
    private String idCard;
    private List listHouse;
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getIdCard() {
        return idCard;
    }
    public void setIdCard(String idCard) {
        this.idCard = idCard;
    }
    public List getListHouse() {
        return listHouse;
    }
    public void setListHouse(List listHouse) {
        this.listHouse = listHouse;
    }
}
class bigHouse{
    private String address;
    private BigDecimal price;
    private Integer useAge;
    public bigHouse(String address, BigDecimal price, Integer useAge) {
        this.address = address;
        this.price = price;
        this.useAge = useAge;
    }
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;
    }
    public BigDecimal getPrice() {
        return price;
    }
    public void setPrice(BigDecimal price) {
        this.price = price;
    }
    public Integer getUseAge() {
        return useAge;
    }
    public void setUseAge(Integer useAge) {
        this.useAge = useAge;
    }
}

分析：flatmap看源码需要返回一个stream流

 核心实现部分：

List strings = pList.stream().flatMap(p -> {
    Stream stream = p.getListHouse().stream().map(bigHouse::getAddress);
    return stream;
}).distinct().collect(Collectors.toList());

这个p就是有人对象，我们把每个人对象里面的房子的集合通过map返回地址这个字符串流，就是一个展开操作，最后再收集所有人的地址流，统一返回字符串。

看运行效果：