Java Lambda 方式将List按照对象属性值分组成Map

Java Lambda 方式将List按照对象属性值分组成Map

---

 

有时候，需要对一个List结果集进行分组处理（按照对象中的某一个属性值进行分组）

例如：使用三国英雄的所属国家，进行分组英雄。

1、英雄实体类（Hero）

public class Hero {
    private String name;
    private String country;

    public Hero(String name, String country) {
        this.name = name;
        this.country = country;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCountry() {
        return country;
    }

    public void setCountry(String country) {
        this.country = country;
    }

    @Override
    public String toString() {
        return "Hero{" +
                "name='" + name + '\'' +
                ", country='" + country + '\'' +
                '}';
    }
}

 

2、代码实现（Lambda方式）进行分组

package com.miracle.luna.lambda;

import com.alibaba.fastjson.JSON;

import java.util.*;
import java.util.stream.Collectors;

/**
 * Created by Miracle Luna on 2020/5/17
 */
public class ThreeCountryLambda {
    public static void main(String[] args) {
        List<Hero> heroList =  new ArrayList<>();
        Map<String, List<Hero>> heroListMap = new HashMap<String, List<Hero>>();

        Hero liubei = new Hero("刘备", "蜀国");
        Hero zhugeliang = new Hero("诸葛亮", "蜀国");

        Hero sunce = new Hero("孙策", "吴国");
        Hero zhouyu = new Hero("周瑜", "吴国");

        Hero caocao = new Hero("曹操", "魏国");
        Hero guojia = new Hero("郭嘉", "魏国");

        heroList.add(liubei);
        heroList.add(zhugeliang);

        heroList.add(sunce);
        heroList.add(zhouyu);

        heroList.add(caocao);
        heroList.add(guojia);

        // 按照所属国家分组
        heroListMap = heroList.stream().collect(Collectors.groupingBy(hero -> hero.getCountry()));
        System.out.println(JSON.toJSONString(heroListMap));

    }
}

 

3、运行结果

{"吴国":[{"country":"吴国","name":"孙策"},{"country":"吴国","name":"周瑜"}],"魏国":[{"country":"魏国","name":"曹操"},{"country":"魏国","name":"郭嘉"}],"蜀国":[{"country":"蜀国","name":"刘备"},{"country":"蜀国","name":"诸葛亮"}]}

 

使用JSON在线解析工具（https://www.json.cn/），查看结果如下：

{
    "吴国":[
        {
            "country":"吴国",
            "name":"孙策"
        },
        {
            "country":"吴国",
            "name":"周瑜"
        }
    ],
    "魏国":[
        {
            "country":"魏国",
            "name":"曹操"
        },
        {
            "country":"魏国",
            "name":"郭嘉"
        }
    ],
    "蜀国":[
        {
            "country":"蜀国",
            "name":"刘备"
        },
        {
            "country":"蜀国",
            "name":"诸葛亮"
        }
    ]
}

 

 

PS：

上述场景，使用传统方式分组的话，需要遍历List，用对象中的所属国家，依次匹配Map中的key。

匹配上，则归类，加到子集中，作为Map的value。

1、代码实现（传统方式）进行分组：

package com.miracle.luna.lambda;

import com.alibaba.fastjson.JSON;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * Created by Miracle Luna on 2020/5/17
 */
public class ThreeCountryTradition {
    public static void main(String[] args) {
        List<Hero> heroList =  new ArrayList<>();
        Map<String, List<Hero>> heroListMap = new HashMap<String, List<Hero>>();

        Hero liubei = new Hero("刘备", "蜀国");
        Hero zhugeliang = new Hero("诸葛亮", "蜀国");

        Hero sunce = new Hero("孙策", "吴国");
        Hero zhouyu = new Hero("周瑜", "吴国");

        Hero caocao = new Hero("曹操", "魏国");
        Hero guojia = new Hero("郭嘉", "魏国");

        heroList.add(liubei);
        heroList.add(zhugeliang);

        heroList.add(sunce);
        heroList.add(zhouyu);

        heroList.add(caocao);
        heroList.add(guojia);

        // 按照所属国家分组
        for (Hero hero : heroList) {
            if (!heroListMap.containsKey(hero.getCountry())) {
                List<Hero> valueList = new ArrayList<>();
                valueList.add(hero);
                heroListMap.put(hero.getCountry(), valueList);
            } else {
                heroListMap.get(hero.getCountry()).add(hero);
            }
        }
        System.out.println(JSON.toJSONString(heroListMap));
    }
}

 

2、运行结果：

{"吴国":[{"country":"吴国","name":"孙策"},{"country":"吴国","name":"周瑜"}],"蜀国":[{"country":"蜀国","name":"刘备"},{"country":"蜀国","name":"诸葛亮"}],"魏国":[{"country":"魏国","name":"曹操"},{"country":"魏国","name":"郭嘉"}]}

 

{
    "吴国":[
        {
            "country":"吴国",
            "name":"孙策"
        },
        {
            "country":"吴国",
            "name":"周瑜"
        }
    ],
    "蜀国":[
        {
            "country":"蜀国",
            "name":"刘备"
        },
        {
            "country":"蜀国",
            "name":"诸葛亮"
        }
    ],
    "魏国":[
        {
            "country":"魏国",
            "name":"曹操"
        },
        {
            "country":"魏国",
            "name":"郭嘉"
        }
    ]
}

 

 

总结：

通过两种实现方式的代码对比，可以看出：Lambda方式的逻辑清晰，代码更加简洁。

强烈推荐大家使用！！！

希望能帮到需要的小伙伴，谢谢~