我想了解Java中的收集器

Question

我想用java收集数据（“男性”）。 但是，我无法理解这个过程。 为什么要使用分类器？在这段代码中？

Collector<Student,?,Map<Student.Gender,List<student>>> collector = Collectors.groupingBy(classifier);

public class GroupingBYExample{
    public static void main(String[] args){
List<Student> totalList = new ArrayList<>();
totalList.add(new Student("Nafla",100,Student.Gender.Male);
totalList.add(new Student("Jacki",99,Student.Gender.FEMale);

Stream<Student> totalStream=  totalList.stream();
Function<Student,Student.Gender> classifier = Student:: getGender;

Collector<Student,?,Map<Student.Gender,List<student>>> collector = Collectors.groupingBy(classifier);

Map<Student.Gender,List<Student>> mapByGender = totalStream.collect(collector);
System.out.println("Male Student:");
mapByGender.get(Student.Gender.MALE).stream().forEach(s->System.out.println(s.getName()+" "));
}
}

public class Student{
public enum Gender{MALE,FEMALE}

    private String name;
    private int score;
    private Gedner gender;
    private City city;
    Student(String name,int score,Gender gender){
        this.name=name;
        this.score=score;
        this.gender=gender;}
    public String getName() {return name;}
    public void setName(String name) {this.name = name;}
    public int getScore() {return score;}
    public void setScore(int score) {this.score = score;}
    public Gender getGender() {return gender;}
    public void setGender(Gender gender) {this.gender = gender;}
}