Java中对List数据进行分组的例子

例：在实体类people中，按照人员的地址分组，使用map<key,List<people>>接收。


package com.felix;

import com.cars.ict.rbpsems.RbpsemsWebApplication;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

@RunWith(SpringRunner.class)
@SpringBootTest(classes = RbpsemsWebApplication.class)
public class CollectorsGroupingBy {

    @Test
    public void groupingTest() {
        People people1 = new People(1, "only-qi1", "11", 11);
        People people2 = new People(2, "only-qi2", "12", 12);
        People people3 = new People(3, "only-qi3", "14", 13);
        People people4 = new People(1, "only-qi4", "14", 14);
        People people5 = new People(2, "only-qi5", "13", 15);
        People people6 = new People(1, "only-qi6", "11", 16);

        ArrayList<People> arrayList = new ArrayList<>();
        arrayList.add(people1);
        arrayList.add(people2);
        arrayList.add(people3);
        arrayList.add(people4);
        arrayList.add(people5);
        arrayList.add(people6);

        Map<String, List<People>> map = arrayList.stream().collect(Collectors.groupingBy(People::getAddress));
        for(String s : map.keySet()) {
            System.out.println("key的值是：" + s + "=============" + "value的值是：" + map.get(s));
        }
    }

}

class People {
    private Integer id;
    private String name;
    private String address;
    private Integer age;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

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

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public People() {
    }

    public People(Integer id, String name, String address, Integer age) {
        this.id = id;
        this.name = name;
        this.address = address;
        this.age = age;
    }

    @Override
    public String toString() {
        return "People{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", address='" + address + '\'' +
                ", age=" + age +
                '}';
    }
}

结果如下：

 

也可以按照多个字段进行分组：

 

Map<String, List<People>> collect = arrayList.stream().collect(Collectors.groupingBy(p -> p.getAddress() + "_" + p.getId()));
for(String s : collect.keySet()) {
    System.out.println("key的值是：" + s + "=============" + "value的值是：" + collect.get(s));
}

 

 

结果如下：