@Entity
public class User extends Model {
    @Id
    @Constraints.Min(10)
    public Long id;

    @Constraints.Required
    public String username;

    @Constraints.Required
    public String password;

    public static Finder<Long, User> find = new Finder<Long, User>(
        Long.class, User.class
    );
}

User user = User.find.where().eq("username", username).findUnique();

    if(user != null) {
        user.username = "some_new_username";
        user.save();
    }

cannot find symbol [symbol: method setUsername(java.lang.String)] [location: class models.User]

@Entity
public class User extends Model {
    @Id
    @Constraints.Min(10)
    public Long id;

    @Constraints.Required
    public String username;

    public void setUsername(String _username) {
        username = _username;
    }

    @Constraints.Required
    public String password;

    public void setPassword(String _password) {
        password = _password;
    }

    public static Finder<Long, User> find = new Finder<Long, User>(
        Long.class, User.class
    );
}

user.username = "some_new_username";

user.setUsername("some_new_username");