一对多
生活中常见一对多关系的例子，如一个老师教多个学生，一个学生属于一个老师，那么这就是一个典型的一对多关系，可以通过集合进行关系的表示。
定义一个学生类
public class Student {
private Integer id;
private String name;
private byte age;
private Teacher teacher;
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
public Student(Integer id, String name, byte age) {
this.id = id;
this.name = name;
this.age = age;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public byte getAge() {
return age;
}
public void setAge(byte age) {
this.age = age;
}
@Override
public String toString() {
return "Student{" +
"id=" + id +
", name='" + name + '\'' +
", age=" + age +'}';
}
}
在以上的 Student 类中包含了一个 Teacher 属性，表示一个学生属于一个老师。在程序运行时，只需要传入 Teacher 类的引用就可以完成这样的关系。
定义一个老师类
import java.util.HashSet;
public class Teacher {
private Integer id;
private String name;
private byte sex;
private HashSet<Student> students = new HashSet<>();
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public byte getSex() {
return sex;
}
@Override
public String toString() {
return "Teacher{" +
"id=" + id +
", name='" + name + '\'' +
", sex=" + sex +
", students=" + students +
'}';
}
public Teacher(Integer id, String name, byte sex) {
this.id = id;
this.name = name;
this.sex = sex;
}
public void setSex(byte sex) {
this.sex = sex;
}
public HashSet<Student> getStudents() {
return students;
}
public void setStudents(HashSet<Student> students) {
this.students = students;
}
}
逻辑关系
//一对多
public class OneToManyDemo {
public static void main(String[] args) {
Student heyi1 = new Student(1, "heyi1", (byte) 2);
Student heyi2 = new Student(2, "heyi2", (byte) 4);
Teacher teacher = new Teacher(1, "heyi老师", (byte) 12);
HashSet<Student> students = teacher.getStudents();
students.add(heyi1);
students.add(heyi2);
heyi2.setTeacher(teacher);
heyi1.setTeacher(teacher);
print(teacher);
}
private static void print(Teacher t){
System.out.println(t.getName());
for (Student s: t.getStudents()) {
System.out.println(s);
}
}
}
多对多
使用集合不仅可以表示一对一的关系，也可以表示多对多的关系。例如，一个学生可以选多门课程，一门课程可以有多个学生参加，那么这就是一个典型的多对多关系。
要完成上面要求，首先应该定义两个类，分别是学生信息（Student）类、课程信息（Course）类。在学生类中存在一个集合，保存全部的课程。同样，在课程类中也要存在一个集合，保存全部的学生。
定义学生类
package model.vo;
import java.util.HashSet;
public class Student {
private Integer id;
private String name;
private byte age;
private Teacher teacher;
private HashSet<Course> allCourse = new HashSet<Course>();
public HashSet<Course> getAllCourse() {
return allCourse;
}
public void setAllCourse(HashSet<Course> allCourse) {
this.allCourse = allCourse;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
public Student(Integer id, String name, byte age) {
this.id = id;
this.name = name;
this.age = age;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public byte getAge() {
return age;
}
public void setAge(byte age) {
this.age = age;
}
@Override
public String toString() {
return "Student{" +
"id=" + id +
", name='" + name + '\'' +
", age=" + age +'}';
}
}
定义课程类
package model.vo;
import java.util.HashSet;
public class Course {
private int id;
//课程名称
private String name;
private HashSet<Student> allStudents = new HashSet<>();
public Course(int id, String name) {
this.id = id;
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public HashSet<Student> getAllStudents() {
return allStudents;
}
public void setAllStudents(HashSet<Student> allStudents) {
this.allStudents = allStudents;
}
@Override
public String toString() {
return "Course{" +
"id=" + id +
", name='" + name + '\'' +
'}';
}
}
逻辑关系
import model.vo.Course;
import model.vo.Student;
import model.vo.Teacher;
public class ManyToManyDemo {
public static void main(String[] args) {
Course c1 = new Course(3, "英语");
// 实例化课程对象
Course c2 = new Course(5, "计算机");
// 实例化课程对象
Student s1 = new Student(1, "张三", (byte) 20);
// 实例化学生对象
Student s2 = new Student(2, "李四", (byte) 21);
Student s3 = new Student(3, "王五", (byte) 22);
// 向课程增加学生信息
c1.getAllStudents().add(s1);
c1.getAllStudents().add(s2);
// 向学生信息增加课程
s1.getAllCourse().add(c1);
s1.getAllCourse().add(c2);
coursePrint(c1);
studentPrint(s1);
}
private static void coursePrint(Course course) {
System.out.println(course.getName());
for (Student s : course.getAllStudents()) {
System.out.println(s);
}
}
private static void studentPrint(Student student) {
System.out.println(student.getName());
for (Course course : student.getAllCourse()) {
System.out.println(course);
}
}
}