JPA - Collection Mapping

package model;

import jakarta.persistence.*;

import java.util.List;

/**
 * Represents a student entity.
 */
@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    /** The list of courses associated with the student. */
    @ManyToMany
    @JoinTable(name = "student_course",
            joinColumns = @JoinColumn(name = "student_id"),
            inverseJoinColumns = @JoinColumn(name = "course_id"))
    private List<Course> courses;

    // Constructors, getters, and setters

    /**
     * Retrieves the student ID.
     */
    public Long getId() {
        return id;
    }

    /**
     * Sets the ID of the student.
     */
    public void setId(Long id) {
        this.id = id;
    }

    /**
     * Retrieves the name of the student.
     */
    public String getName() {
        return name;
    }

    /**
     * Sets the name of the student.
     */
    public void setName(String name) {
        this.name = name;
    }

    /**
     * Retrieves the list of courses associated with the student.
     */
    public List<Course> getCourses() {
        return courses;
    }

    /**
     * Sets the list of courses associated with the student.
     */
    public void setCourses(List<Course> courses) {
        this.courses = courses;
    }
}

package model;

import jakarta.persistence.*;
import java.util.ArrayList;
import java.util.List;

/**
 * Represents a course entity.
 */
@Entity
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    /** The list of students enrolled in the course. */
    @ManyToMany(mappedBy = "courses")
    private List<Student> students = new ArrayList<>();

    // Constructors, getters, and setters

    /**
     * Retrieves the course ID.
     * Sets the ID of the course.
     */
    public Long getId() {
        return id;
    }

    /**
     * Retrieves the course name.
     * Sets the name of the course.
     */
    public String getName() {
        return name;
    }

    /**
     * Retrieves the list of students enrolled in the course.
     * Sets the list of students enrolled in the course.
     */
    public List<Student> getStudents() {
        return students;
    }

    /**
     * Sets the list of students enrolled in the course.
     */
    public void setStudents(List<Student> students) {
        this.students = students;
    }
}

package model;

import jakarta.persistence.*;

/**
 * Represents an employee entity.
 */
@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    /** The name of the employee. */
    private String name;

    /** department to which the employee belongs. */
    @ManyToOne
    @JoinColumn(name = "department_id")
    private Department department;

    // Constructors, getters, and setters

    /**
     * Retrieves the employee iD.
     * Sets the ID of the employee.
     */
    public Long getId() {
        return id;
    }

    /**
     * Retrieves the employee name.
     * Sets the name of the employee.
     */
    public String getName() {
        return name;
    }

    /**
     * Retrieves the department to which the employee belongs.
     * Sets the department to which the employee belongs.
     */
    public Department getDepartment() {
        return department;
    }

    /**
     * Sets the department to which the employee belongs.
     */
    public void setDepartment(Department department) {
        this.department = department;
    }
}

package model;

import jakarta.persistence.*;
import java.util.List;

/**
 * Represents a department entity.
 */
@Entity
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    /** The list of employees belonging to this department. */
    @OneToMany(mappedBy = "department")
    private List<Employee> employees;

    // Constructors, getters, and setters

    /**
     * Retrieves the department ID.
     * Sets the ID of the department.
     */
    public Long getId() {
        return id;
    }

    /**
     * Retrieves the department name.
     * Sets the name of the department.
     */
    public String getName() {
        return name;
    }

    /**
     * Retrieves the list of employees belonging to this department.
     * Sets the list of employees belonging to this department.
     */
    public List<Employee> getEmployees() {
        return employees;
    }

    /**
     * Sets the list of employees belonging to this department.
     */
    public void setEmployees(List<Employee> employees) {
        this.employees = employees;
    }
}

package util;

import jakarta.persistence.EntityManagerFactory;
import jakarta.persistence.Persistence;

/**
 * Utility class for managing JPA operations.
 */
public class JPAUtil {
    /** The name of the persistence unit. */
    private static final String PERSISTENCE_UNIT_NAME = "examplePU";

    /** The entity manager factory. */
    private static EntityManagerFactory entityManagerFactory;

    static {
        try {
            entityManagerFactory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
        } catch (Throwable ex) {
            System.err.println("Initial EntityManagerFactory creation failed: " + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

    /**
     * Retrieves the entity manager factory.
     * @return The entity manager factory.
     */
    public static EntityManagerFactory getEntityManagerFactory() {
        return entityManagerFactory;
    }

    /**
     * Shuts down the entity manager factory.
     */
    public static void shutdown() {
        if (entityManagerFactory != null) {
            entityManagerFactory.close();
        }
    }
}

import jakarta.persistence.EntityManager;
import jakarta.persistence.Query;
import model.Course;
import model.Department;
import model.Employee;
import model.Student;
import util.JPAUtil;

import java.util.List;

/**
 * Main class for demonstrating JPA operations.
 */
public class MainApplication {
    /**
     * Main method to execute JPA operations.
     * @param args Command-line arguments.
     */
    public static void main(String[] args) {
        EntityManager entityManager = JPAUtil.getEntityManagerFactory().createEntityManager();

        // Add data to Department and Employee tables
        entityManager.getTransaction().begin();
        Department department1 = new Department();
        department1.setName("Engineering");
        entityManager.persist(department1);

        Department department2 = new Department();
        department2.setName("Marketing");
        entityManager.persist(department2);

        Employee employee1 = new Employee();
        employee1.setName("Syam");
        employee1.setDepartment(department1);
        entityManager.persist(employee1);

        Employee employee2 = new Employee();
        employee2.setName("Mikel Smith");
        employee2.setDepartment(department1);
        entityManager.persist(employee2);

        Employee employee3 = new Employee();
        employee3.setName("Eswar");
        employee3.setDepartment(department2);
        entityManager.persist(employee3);

        entityManager.getTransaction().commit();

        // Add data to Student and Course tables
        entityManager.getTransaction().begin();
        Student student1 = new Student();
        student1.setName("Mahesh");
        entityManager.persist(student1);

        Student student2 = new Student();
        student2.setName("Sirish");
        entityManager.persist(student2);

        Course course1 = new Course();
        course1.setName("Java Programming");
        course1.getStudents().add(student1);
        course1.getStudents().add(student2);
        entityManager.persist(course1);

        Course course2 = new Course();
        course2.setName("Database Management");
        course2.getStudents().add(student1);
        entityManager.persist(course2);

        entityManager.getTransaction().commit();

        // Print all data from Department and Employee tables
        printAllDepartments(entityManager);
        printAllEmployees(entityManager);

        // Print all data from Student and Course tables
        printAllStudents(entityManager);
        printAllCourses(entityManager);

        entityManager.close();
        JPAUtil.shutdown();
    }

    /**
     * Prints all departments.
     * @param entityManager The entity manager.
     */
    private static void printAllDepartments(EntityManager entityManager) {
        Query query = entityManager.createQuery("SELECT d FROM Department d");
        List<Department> departments = query.getResultList();
        System.out.println("Departments:");
        for (Department department : departments) {
            System.out.println(department.getId() + "\t" + department.getName());
        }
        System.out.println();
    }

    /**
     * Prints all employees.
     * @param entityManager The entity manager.
     */
    private static void printAllEmployees(EntityManager entityManager) {
        Query query = entityManager.createQuery("SELECT e FROM Employee e");
        List<Employee> employees = query.getResultList();
        System.out.println("Employees:");
        for (Employee employee : employees) {
            System.out.println(employee.getId() + "\t" + employee.getName() + "\t" + employee.getDepartment().getName());
        }
        System.out.println();
    }

    /**
     * Prints all students.
     * @param entityManager The entity manager.
     */
    private static void printAllStudents(EntityManager entityManager) {
        Query query = entityManager.createQuery("SELECT s FROM Student s");
        List<Student> students = query.getResultList();
        System.out.println("Students:");
        for (Student student : students) {
            System.out.println(student.getId() + "\t" + student.getName());
        }
        System.out.println();
    }

    /**
     * Prints all courses.
     * @param entityManager The entity manager.
     */
    private static void printAllCourses(EntityManager entityManager) {
        Query query = entityManager.createQuery("SELECT c FROM Course c");
        List<Course> courses = query.getResultList();
        System.out.println("Courses:");
        for (Course course : courses) {
            System.out.println(course.getId() + "\t" + course.getName());
            System.out.println("Students enrolled:");
            for (Student student : course.getStudents()) {
                System.out.println("\t- " + student.getName());
            }
        }
        System.out.println();
    }
}