User entity

User.java

@Entity
@Table(name = "users")
public class User implements Identifiable<Long> {
    
    @Id
    @GeneratedValue(generator = "SequencePerEntityGenerator")
    private Long id; // only getter
    
    @Column(length = 16, nullable = false, unique = true)
    private String username; // getter and setter
    
    @Column(name = "first_name", nullable = false)
    private String firstName;// getter and setter
    
    @Column(name = "last_name", nullable = false)
    private String lastName;// getter and setter
    
    @ManyToOne(optional = false, fetch = FetchType.LAZY)
    private Group group;// getter and package-private setter
    
    /**
     * A user may only exist with all first and second names and the username.
     */
    public User(final String username, final String firstName, final String lastName) {
        
        this.username = username;
        this.firstName = firstName;
        this.lastName = lastName;
    }
    
    /**
     * For JPA only.
     */
    protected User() {}
    
    //hashcode & equals
}