Why do we need a helper method inside @OneToMany annotated side in hibernate?

Here I have two entity classes called Course and Instructor which follows bi directional one to many relationship.Source code is given below.I need to know what's the need of the addCourse() method in the Instructor class and can't we create the class without it?

Source code.

Instructor class

@Entity
@Table(name="Instructor")

public class Instructor {
	

	@Id
	@GeneratedValue(strategy =GenerationType.IDENTITY )
	@Column(name="id")
	private int id;

	@Column(name="first_name")
	private String firtName;
	
	@Column(name="last_name")
	private String lastName;
	
	@Column(name="email")
	private String email;
	
	
	
	@OneToMany(mappedBy ="instructor" ,cascade= {CascadeType.DETACH,CascadeType.MERGE,CascadeType.PERSIST,CascadeType.REFRESH}) 
	private List<Course> courses;


    //Getters and Setters + Constructors + toString() method

	
	public void addCourse(Course tempCourse) {//Helper method
		if(courses == null) {
			courses = new ArrayList<>();
		}
		courses.add(tempCourse);
		tempCourse.setInstructor(this);
	}

}

Course class

@Entity
@Table(name="course")
public class Course {
	@Id
	@Column(name = "id")
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	private int id;
	
	@Column(name="title")
	private String title;
	
    @ManyToOne(cascade ={CascadeType.DETACH,CascadeType.MERGE,CascadeType.PERSIST,CascadeType.REFRESH} )
	@JoinColumn(name="instructor_id")
	private Instructor instructor;
	
	//Getters and Setters + Constructors + toString() method	

	
}

Vlad Mihalcea says both sides (parent and child) should be in sync so not breaking Domain Model consistency and avoiding state transition bugs:
https://vladmihalcea.com/jpa-hibernate-synchronize-bidirectional-entity-associations/

There is no need to have this method. What this is instead is a convenience method that allows for quickly adding a course to the List<Course> of courses belonging to an instructor object.

What this does is to initialize the list if not already present, set the Course object to the list of courses as well as set the Instructor object to the passed in Course object.

You could very well replace this method with the following code and still achieve the same results:

public class Instructor {

// misc stuff here

Set<Course> courses = new HashSet<>();
//standard getters/setters

}

And then the desired calling point you could simply do this:

Course course = new Course();
course.setTitle("My Title");
course.setIntructor(instructor);

instructor.getCourses().add(course);

What the above do is essentially what your method does with the exception of not initializing the list object.

The method just checks if the list is null and creates it if it's not there you can leave this out using:

@OneToMany(mappedBy ="instructor" ,
cascade= {CascadeType.DETACH,
CascadeType.MERGE,
CascadeType.PERSIST,
CascadeType.REFRESH}) 
    private List<Course> courses = new ArrayList<>();

The other thing that it does is add reference to the course that makes sure the relation is bidirectional where the course reference the instructor and vise versa.

you can take this out and make these checks in a service method.

to answer your question, you can make the same class without it, but you'll have to do those things somewhere else, this design works, but it's bad design, addCourse should be a service method, putting it there makes you're pojo entity somthing that has logic and control, but the logic of the method is right.