请按以下步骤实现代码

• 创建一个新的 JDK 1.8 项目，能运行 main 方法即可

• 新建一个 Record 类，类中包含以下属性 level, teacher, student

• 构建一个 List 集合，集合中包含 8 个 Record 对象， Record 对象包含以下属性：
  Level(年级)，Teacher(老师姓名)，Student(学生姓名)

  每个对象的属性值如下：

Level	Teacher	Student
L1	Li	S1
L1	Wang	S2
L1	Wang	S3
L2	Li	S4
L2	Li	S5
L2	Wang	S6
L3	Wang	S7
L3	Wang	S8

然后实现以下数据统计，要求使用 Java 8 的 Stream 和 Lambda 表达式

• 使用 Stream 集合操作统计每个年级有多少名学生，打印在输出窗口即可，输出效果：
  {L1=3, L2=3, L3=2}

• 使用 filter 方法进行过滤，列出老师 Li 的所有学生姓名，输出效果：
  {Li=[S1, S4, S5]}

• 列出每个老师，每个年级的学生数量，输出效果：
  {Wang={L1=2, L2=1, L3=2}, Li={L1=1, L2=2}}

可参考 Java 文档：https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html

答案

新建一个 TestRecord 类，把以下代码赋值复制进入

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;public class TestRecord {public static void main(String[] args) {List<Record> list = new ArrayList<>();list.add(new Record("L1", "Li", "S1"));list.add(new Record("L1", "Wang", "S2"));list.add(new Record("L1", "Wang", "S3"));list.add(new Record("L2", "Li", "S4"));list.add(new Record("L2", "Li", "S5"));list.add(new Record("L2", "Wang", "S6"));list.add(new Record("L3", "Wang", "S7"));list.add(new Record("L3", "Wang", "S8"));// 使用 Stream 集合操作统计每个年级有多少名学生，打印在输出窗口即可，输出效果：  {L1=3, L2=3, L3=2}Map<String, Long> record1 = list.stream().collect(Collectors.groupingBy(Record::getLevel, Collectors.counting()));System.out.println(record1);// 使用 filter 方法进行过滤，列出老师 Li 的所有学生姓名，输出效果：  {Li=[S1, S4, S5]}Map<String, List<String>> result = list.stream().filter(record -> "Li".equals(record.getTeacher())).collect(Collectors.groupingBy(Record::getTeacher, Collectors.mapping(Record::getStudent, Collectors.toList())));System.out.println(result);// 列出每个老师，每个年级的学生数量Map<String,Map<String,Long>> collect = list.stream().collect(Collectors.groupingBy(Record::getTeacher,Collectors.groupingBy(Record::getLevel,Collectors.counting())));System.out.println(collect);}
}class Record {private String level;private String teacher;private String student;public Record() {}public Record(String level, String teacher, String student) {this.level = level;this.teacher = teacher;this.student = student;}public String getLevel() {return level;}public void setLevel(String level) {this.level = level;}public String getTeacher() {return teacher;}public void setTeacher(String teacher) {this.teacher = teacher;}public String getStudent() {return student;}public void setStudent(String student) {this.student = student;}@Overridepublic String toString() {return "Record{" +"level='" + level + '\'' +", teacher='" + teacher + '\'' +", student='" + student + '\'' +'}';}
}

结果

{L1=3, L2=3, L3=2}
{Li=[S1, S4, S5]}
{Wang={L1=2, L2=1, L3=2}, Li={L1=1, L2=2}}