帮我看看啊，才学不太懂，谢谢！

Question

import java.util.Iterator;

import java.util.Set;

import java.util.TreeSet;

public class Student1 {

String id,name,birthday;

int age;

public Student1(String id1,String name1,String birthday1,int age1){

id=id1;

name=name1;

birthday=birthday1;

age=age1;

}

public String getId() {

return id;

}

public void setId(String id) {

this.id = id;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public String getBirthday() {

return birthday;

}

public void setBirthday(String birthday) {

this.birthday = birthday;

}

public int getAge() {

return age;

}

public void setAge(int age) {

this.age = age;

}

public static void main(String[] args){

Set<Student1>stu=new TreeSet<>();

stu.add(new Student1("001","张珊","1998-05-06", 18));

stu.add(new Student1("002","张明","1994-10-16", 21));

stu.add(new Student1("003","李四","1995-03-04", 20));

stu.add(new Student1("009","钱江","1993-10-10", 22));

stu.add(new Student1("006","小红","1996-12-11", 19));

stu.add(new Student1("005","小明","1995-11-14", 20));

Iterator<Student1>student=stu.iterator();

while(student.hasNext()){

Student1 stu1=student.next();

System.out.println(stu1);

}

}

}