Java: What is the difference between @ManyToMany and @OneToMany?

@ManyToMany — many-to-many. @OneToMany — one-to-many.

@ManyToMany:

• Both sides have collections.
• Join table required.
• No owning side (or both own).
• More complex to manage.

1@Entity
2public class Student {
3    @Id
4    private Long id;
5
6    @ManyToMany
7    @JoinTable(
8        name = "student_course",
9        joinColumns = @JoinColumn(name = "student_id"),
10        inverseJoinColumns = @JoinColumn(name = "course_id")
11    )
12    private Set<Course> courses;
13}

@OneToMany:

• Parent has collection of children.
• Foreign key in child table.
• Bidirectional with @ManyToOne.
• More common pattern.

1@Entity
2public class User {
3    @Id
4    private Long id;
5
6    @OneToMany(mappedBy = "user")
7    private List<Order> orders;
8}

How it works:

1. @ManyToMany creates a join table to manage the relationship.
2. @OneToMany uses a foreign key in the child table.
3. @ManyToMany is for many-to-many relationships.
4. @OneToMany is for one-to-many relationships.

Key difference:

• @ManyToMany — join table.
• @OneToMany — foreign key.

Best practice:

• Use @ManyToMany sparingly (complex to manage).
• Prefer @OneToMany with @ManyToOne for most relationships.
• Use Set instead of List for @ManyToMany to avoid duplicates.