---

一、stream toMap、groupingBy、mapping的综合应用

1、前提准备

①、实体类

package com.cfay.demo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.experimental.Accessors;

import java.io.Serializable;

/**
 * @author lcb
 * @since 2023-07-19
 */
@Data
@NoArgsConstructor
@AllArgsConstructor
@Accessors(chain = true)
public class Student implements Serializable {

    private Integer id;

    private String name;

    private Integer age;
}

②、数据准备

package com.cfay.demo;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Objects;
import java.util.stream.Collectors;

/**
 * @author lcb
 * @since 2023-07-19
 */
public class StudentData {

    private StudentData() {
    }

    public static List<Student> buildStudents() {
        List<Student> students = new ArrayList<>();
        students.add(new Student(1, "小扇", 20));
        students.add(new Student(1, "小雨", 22));
        students.add(new Student(1, "小钰", 19));
        students.add(new Student(1, null, 19));
        students.add(new Student(2, "小蓝", 21));
        students.add(new Student(2, "小天", 20));
        //过滤掉名字为空的数据，并且按照年龄升序排序
        return students.stream().filter(student ->
                Objects.nonNull(student.getName())).sorted(Comparator.comparing(Student::getAge)).collect(Collectors.toList());
    }
}

2、核心代码：

package com.cfay.demo;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

/**
 * @author lcb
 * @since 2023-07-19
 */

public class Test {

    public static void main(String[] args) {
        //将数据按照id进行分组，并把相同id的对象取名称用逗号拼接起来测试
        handleStudentIdNameSplicteMap();
        System.out.println("===================================");
        //将数据按照id进行分组，并把相同id的对象取名称放到集合中去测试
        handleStudentIdNamesMap();
    }

    /**
     * 将数据按照id进行分组，并把相同id的对象取名称用逗号拼接起来
     */
    private static void handleStudentIdNameSplicteMap() {
        //toMap
        System.out.println("toMap ......");
        Map<Integer, String> studentIdNameMap1 = StudentData.buildStudents().stream().collect(Collectors.toMap(Student::getId,
                Student::getName, (pre, act) -> pre + "," + act));
        System.out.println(studentIdNameMap1);

        //groupingBy
        System.out.println("groupingBy ......");
        Map<Integer, String> studentIdNameMap2 = StudentData.buildStudents().stream().collect(Collectors.groupingBy(Student::getId,
                Collectors.mapping(Student::getName, Collectors.joining(","))));
        System.out.println(studentIdNameMap2);
    }

    /**
     * 将数据按照id进行分组，并把相同id的对象取名称放到集合中去
     */
    private static void handleStudentIdNamesMap() {
        //toMap
        System.out.println("toMap ......");
        Map<Integer, List<String>> studentIdNamesMap1 = StudentData.buildStudents().stream().collect(Collectors.toMap(Student::getId, student ->
                new ArrayList<>(Collections.singletonList(student.getName())), (pre, act) -> {
            pre.addAll(act);
            return pre;
        }));
        System.out.println(studentIdNamesMap1);

        //groupingBy
        System.out.println("groupingBy ......");
        Map<Integer, List<String>> studentIdNamesMap2 = StudentData.buildStudents().stream().collect(Collectors.groupingBy(Student::getId,
                Collectors.mapping(Student::getName, Collectors.toList())));
        System.out.println(studentIdNamesMap2);
    }
}

3、运行结果