Java: Java8中stream Collectors.groupingBy将List转为分组Map

package com.github.mouday.reggie;
public class User {
    private Integer id;
    private String name;
    private Integer age;
    public User(Integer id, String name, Integer age) {
        this.id = id;
        this.name = name;
        this.age = 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 Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

package com.github.mouday.reggie;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Demo {
    public static void main(String[] args) {
        List<User> users = Arrays.asList(
                new User(1, "Tom", 20),
                new User(2, "Jack", 30),
                new User(3, "Steve", 20)
        );
        // 按照年龄分组
        Map<Integer, List<User>> map = users.stream().collect(Collectors.groupingBy(User::getAge));
        System.out.println(map);
        // {
        //     20=[
        //          User{id=1, name='Tom', age=20},
        //          User{id=3, name='Steve', age=20}
        //        ],
        //     30=[
        //          User{id=2, name='Jack', age=30}
        //          ]
        //  }
    }
}