Java Tutorial/JPA/Many To Many Mapping

Bidirectional Many To Many Mapping

File: Department.java

import java.util.ArrayList;
import java.util.Collection;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
@Entity
public class Department {
    @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;
    private String name;
    @ManyToMany(mappedBy="departments")
    private Collection<Student> students;
    
    public Department(){
      students = new ArrayList<Student>();
    }
    
    public int getId() {
        return id;
    }
    
    public void setId(int id) {
        this.id = id;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String deptName) {
        this.name = deptName;
    }
    public void addStudent(Student student) {
      if (!getStudents().contains(student)) {
          getStudents().add(student);
      }
      if (!student.getDepartments().contains(this)) {
          student.getDepartments().add(this);
      }
    }
    public Collection<Student> getStudents() {
      return students;
    }
    public void setStudent(Collection<Student> students) {
      this.students = students;
    }
    public String toString() {
        return "Department id: " + getId() + 
               ", name: " + getName();
    }
}

JPA Creates Join Table For Many To Many Mapping

File: Student.java

import java.util.ArrayList;
import java.util.Collection;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
@Entity
public class Student {
  @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
  private int id;
  private String name;
  @ManyToMany 
  private Collection<Department> departments;
  public Student() {
    departments = new ArrayList<Department>();
}
  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 void addDepartment(Department department) {
    if (!getDepartments().contains(department)) {
        getDepartments().add(department);
    }
    if (!department.getStudents().contains(this)) {
      department.getStudents().add(this);
    }
  }

  public Collection<Department> getDepartments() {
    return departments;
  }
  public void setDepartment(Collection<Department> departments) {
    this.departments = departments;
  }
  public String toString() {
    return "\n\nID:" + id + "\nName:" + name ;
  }
}

Many To Many based on Collection

File: Project.java

import java.util.ArrayList;
import java.util.Collection;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
@Entity
public class Project {
    @Id
    protected int id;
    protected String name;
    @ManyToMany(mappedBy="projects")
    private Collection<Student> students;
    public Project() {
        students = new ArrayList<Student>();
    }
    public void setId(int i){
      id = i;
    }
    public int getId() {
        return id;
    }
    
    public String getName() {
        return name;
    }
    
    public Collection<Student> getStudents() {
        return students;
    }
    
    public String toString() {
        return "Project id: " + getId() + ", name: " + getName();
    }
}

Many To Many Join Table Join Inverse Join Column

File: Student.java

import java.util.ArrayList;
import java.util.Collection;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
@Entity
public class Student {
  @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
  private int id;
  private String name;
  @ManyToMany 
  @JoinTable(name="Student_Dept", 
      joinColumns=@JoinColumn(name="Stut_ID"),
      inverseJoinColumns=@JoinColumn(name="DEPT_ID"))  
  private Collection<Department> departments;
  public Student() {
    departments = new ArrayList<Department>();
}
  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 void addDepartment(Department department) {
    if (!getDepartments().contains(department)) {
        getDepartments().add(department);
    }
    if (!department.getStudents().contains(this)) {
      department.getStudents().add(this);
    }
  }

  public Collection<Department> getDepartments() {
    return departments;
  }
  public void setDepartment(Collection<Department> departments) {
    this.departments = departments;
  }
  public String toString() {
    return "\n\nID:" + id + "\nName:" + name ;
  }
}

Many To Many Mapping and Remove Operation

File: Project.java

import java.util.ArrayList;
import java.util.Collection;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
@Entity
public class Project {
    @Id
    protected int id;
    protected String name;
    @ManyToMany(mappedBy="projects")
    private Collection<Student> students;
    public Project() {
        students = new ArrayList<Student>();
    }
    public void setId(int i){
      id = i;
    }
    public int getId() {
        return id;
    }
    
    public String getName() {
        return name;
    }
    
    public Collection<Student> getStudents() {
        return students;
    }
    
    public String toString() {
        return "Project id: " + getId() + ", name: " + getName();
    }
}