参考资料：
java新特性stream流的相关操作（一）
groupingBy的综合使用：
package com.example.worddemo.test.jacob;import com.alibaba.fastjson2.JSON;import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.concurrent.ConcurrentMap;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;/*** @author 军哥* @version 1.0* @description: TODO* @date 2022/8/1 19:20*/public class Student {//班级private String classNumber;//姓名private String name;//年龄private int age;//地址private String address;//数学成绩private int mathScores;//语文成绩private int chainessScores;public Student(String classNumber, String name, int age, String address, int mathScores, int chainessScores) {this.classNumber = classNumber;this.name = name;this.age = age;this.address = address;this.mathScores = mathScores;this.chainessScores = chainessScores;}public String getClassNumber() {return classNumber;}public void setClassNumber(String classNumber) {this.classNumber = classNumber;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public String getAddress() {return address;}public void setAddress(String address) {this.address = address;}public int getMathScores() {return mathScores;}public void setMathScores(int mathScores) {this.mathScores = mathScores;}public int getChainessScores() {return chainessScores;}public void setChainessScores(int chainessScores) {this.chainessScores = chainessScores;}//https://blog.csdn.net/sugelachao/article/details/117736155public static void main(String[] args) {//--1 添加数据Student student1 = new Student("701","张三",16,"北京",78,90);Student student2 = new Student("700","李四",17,"北京",78,90);Student student3 = new Student("703","王五",16,"上海",78,90);Student student4 = new Student("701","赵六",16,"上海",78,90);Student student5 = new Student("700","钱七",18,"",78,90);Student student6 = new Student("701","老八",17,"",78,90);//高二年级的成绩单List<Student> students = Stream.of(student1,student2,student3,student4,student5,student6).collect(Collectors.toList());//--2 按照班级分组Map<String, List<Student>> collect1 = students.stream().collect(Collectors.groupingBy(Student::getClassNumber));System.out.println(JSON.toJSONString(collect1));//--2-1 按照班级 和 地区分组Function<Student, List<Object>> compositeKey = student ->Arrays.asList(student.getClassNumber(), student.getAddress());Map<List<Object>, List<String>> collect2_1 = students.stream().collect(Collectors.groupingBy(compositeKey, Collectors.mapping(Student::getName, Collectors.toList())));System.out.println(JSON.toJSONString(collect2_1));//--2-2 按照班级 和 地区分组Map<List<Object>, List<Student>> collect3_1 = students.stream().collect(Collectors.groupingBy(compositeKey));System.out.println(JSON.toJSONString(collect3_1));//--3 按照班级分组得到每个班级的同学姓名Map<String, List<String>> collect2 = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.mapping(Student::getName, Collectors.toList())));System.out.println(JSON.toJSONString(collect2));//--4 统计每个班级人数Map<String, Long> collect3 = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.counting()));System.out.println(JSON.toJSONString(collect3));//--5 求每个班级的数学平均成绩Map<String, Double> collect4 = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.averagingDouble(Student::getMathScores)));System.out.println(JSON.toJSONString(collect4));//--6 按班级分组求每个同学的总成绩Map<String, Map<String, Integer>> collect5 = students.stream().collect(Collectors.groupingBy(Student::getClassNumber,Collectors.toMap(Student::getName, student -> student.getMathScores() + student.getChainessScores())));System.out.println(JSON.toJSONString(collect5));//--7 嵌套分组，先按班级分组，再按年龄分组Map<String, Map<Integer, List<Student>>> collect6 = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.groupingBy(Student::getAge)));System.out.println(JSON.toJSONString(collect6));//--8 分组后得到一个线程安全的ConcurrentMapConcurrentMap<String, List<Student>> collect7 = students.stream().collect(Collectors.groupingByConcurrent(Student::getClassNumber));System.out.println(JSON.toJSONString(collect7));//--9 根据年龄分组并小到大排序,TreeMap默认为按照key升序TreeMap<Integer, List<String>> collect8 = students.stream().collect(Collectors.groupingBy(Student::getAge, TreeMap::new, Collectors.mapping(Student::getName, Collectors.toList())));System.out.println(JSON.toJSONString(collect8));//{16:["张三","王五","赵六"],17:["李四","老八"],18:["钱七"]}//--10 根据年龄分组并大到小排序,因为TreeMap默认为按照key升序，所以排完顺序再反转一下就OK了TreeMap<Integer, List<String>> collect9 = students.stream().collect(Collectors.groupingBy(Student::getAge, TreeMap::new, Collectors.mapping(Student::getName, Collectors.toList())));Map<Integer, List<String>> collect10 = collect9.descendingMap();System.out.println(JSON.toJSONString(collect10));//{18:["钱七"],17:["李四","老八"],16:["张三","王五","赵六"]}}
}
