I know I have anywhere a loop but I dont know where and why. I try to add trainers to my course (Kurs).
This is my code:

@EqualsAndHashCode( callSuper = false )
@Entity
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Kurs extends PanacheEntityBase implements Serializable {

    @Id
    @GeneratedValue( strategy = GenerationType.AUTO )
    private Long id;

    @Basic
    @Column(name = "name", nullable = false)
    private String name;

    @OneToMany(mappedBy = "kurs", fetch = FetchType.LAZY, cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<Kurs2Trainer> trainers = new HashSet<>();

    public void addTrainer(Kursuser trainer) {
        // Create a Kurs2Trainer object
        Kurs2Trainer kurs2Trainer = new Kurs2Trainer();
        kurs2Trainer.setTrainer(trainer);
        kurs2Trainer.setKurs(this);

        // Add the Kurs2Trainer object to the trainers set
        this.trainers.add(kurs2Trainer);

        // Add this Kurs to the trainer's set
        trainer.getTrainerBei().add(kurs2Trainer);
    }
}

Kursuser:

@Entity
@Data
@EqualsAndHashCode( callSuper = false )
@NoArgsConstructor
@AllArgsConstructor
public class Kursuser extends PanacheEntityBase implements Serializable {

    @Id
    private Long id;

    @OneToOne( fetch = FetchType.LAZY )
    @MapsId
    private User user;

    /*
    Diese Person ist Trainer an den folgenden Kursen
     */
    @OneToMany( mappedBy = "trainer", fetch = FetchType.LAZY, cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<Kurs2Trainer> trainerBei = new HashSet<>();
}

Kurs2Trainer:

@Entity
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Kurs2Trainer implements Serializable {

    @EmbeddedId
    private Kurs2TrainerID id = new Kurs2TrainerID();

    @MapsId("trainerId")
    @ManyToOne( fetch = FetchType.LAZY )
    @JoinColumn( name = "trainer_id" )
    private Kursuser trainer;

    @MapsId("kursId")
    @ManyToOne( fetch = FetchType.LAZY )
    @JoinColumn( name = "kurs_id" )
    private Kurs kurs;
}

Kurs2TrainerID:

@Data
@Embeddable
public class Kurs2TrainerID implements Serializable {
    @Column(name = "trainerId")
    private Long trainerId;
    @Column(name = "kursId")
    private Long kursId;
}

When I try to run this code:

private void addTrainerToKurs(User trainer, Kurs k){
    //Check if we have already the trainer in the course
    if ( CollectionUtils.isNotEmpty( k.getTrainers() ) ) {
        for(Kurs2Trainer kTrainer : k.getTrainers()){
            if(kTrainer.getId().getTrainerId().equals( trainer.getId())){
                return;
            }
        }
    }

    //We dont have this user in our course so add
    Kursuser trainerKursuser = kursuserService.getKursuser(trainer);
    k.addTrainer(trainerKursuser);
}

I get the following error message:

java.lang.StackOverflowError
    at at.tanzauswertung.portal.kurs.entities.Kursuser.$$_hibernate_read_id(Kursuser.java)
    at at.tanzauswertung.portal.kurs.entities.Kursuser.getId(Kursuser.java:25)
    at at.tanzauswertung.portal.kurs.entities.Kursuser.hashCode(Kursuser.java:19)
    at at.tanzauswertung.portal.kurs.entities.Kurs2Trainer.hashCode(Kurs2Trainer.java:11)
    at java.base/java.util.AbstractSet.hashCode(AbstractSet.java:124)

Please show me my mistake. Many thanks