선생님과 학생은: 일대다 관계

package com.collection;

import java.util.HashSet;

//Teacher Student
public class Teacher {
    
    private String name;
    private int age;
    private String sex;
    private HashSet students = new HashSet<>();//Teacher Student ， Teacher Student; students 
    
    
    public HashSet getStudents() {
        return students;
    }
    
    public void setStudents(HashSet 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 + "]";
    }
    
}

Student는 선생님한테 Teacher.

package com.collection;

//Student Teacher
public class Student {
    
    private String name;
    private int age;
    private Teacher teacher; // ， ， 
    
    
    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 + "]";
    }
}

선생님과 학생의 관계

package com.collection;

public class OneToManyDemo {
    public static void main(String[] args) {
        
        Teacher t1 = new Teacher(" ",22," ");
        Student s1 = new Student(" ",10);
        Student s2 = new Student(" ",11);
        Student s3 = new Student(" ",12);
        
        // ： 
        // getStudents() ， add() 
        t1.getStudents().add(s1);
        t1.getStudents().add(s2);
        t1.getStudents().add(s3);
        // ， 
        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);
        }
    }

}