老师和学生是：一对多关系

package com.collection;

import java.util.HashSet;

//Teacher是一对多个Student
public class Teacher {
    
    private String name;
    private int age;
    private String sex;
    private HashSet<Student> students = new HashSet<>();//Teacher是第一对多个Student的关系，所以再Teacher这里创建一个集合存储Student;这里students是一个集合
    
    
    public HashSet<Student> getStudents() {
        return students;
    }
    
    public void setStudents(HashSet<Student> students) {
        this.students = students;
    }
    
    public Teacher() {
        super();
    }
    
    public Teacher(String name, int age, String sex) {
        super();
        this.name = name;
        this.age = age;
        this.sex = sex;
    }
    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 getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }

    @Override
    public String toString() {
        return "Teacher [name=" + name + ", age=" + age + ", sex=" + sex + "]";
    }
    
}

Student是对一个老师Teacher

package com.collection;

//Student是对一个老师Teacher
public class Student {
    
    private String name;
    private int age;
    private Teacher teacher; //因为是多个学生对应一个老师，所以这里就不用集合，而是一个属性老师
    
    
    public Teacher getTeacher() {
        return teacher;
    }

    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

    public Student() {
        super();
    }

    public Student(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }

    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;
    }

    @Override
    public String toString() {
        return "Student [name=" + name + ", age=" + age + "]";
    }
}

老师和学生的关联关系

package com.collection;

public class OneToManyDemo {
    public static void main(String[] args) {
        
        Teacher t1 = new Teacher("张老师",22,"女");
        Student s1 = new Student("小李",10);
        Student s2 = new Student("小胡",11);
        Student s3 = new Student("小赵",12);
        
        //老师和学生的：关联关系
        //一个老师getStudents()得到学生的集合，然后add()方法把学生添加进去集合
        t1.getStudents().add(s1);
        t1.getStudents().add(s2);
        t1.getStudents().add(s3);
        //每个学生都设置有一个老师，该老师且是同一个
        s1.setTeacher(t1);
        s2.setTeacher(t1);
        s3.setTeacher(t1);
        
        print1(t1);
        
    }

    private static void print1(Teacher t1) {
        System.out.println(t1.getName());
        for(Student s:t1.getStudents()) {
            System.out.println(s);
        }
    }

}