请按以下步骤实现代码
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创建一个新的 JDK 1.8 项目，能运行 main 方法即可
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新建一个 Record 类，类中包含以下属性 level, teacher, student
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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
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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;
}
@Override
public 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}}