Usecase for dynamic queries

UserLogin.java

/**
 * Interactor to login the user using username and password
 */
public class UserLogin extends UseCase {

    private final UserRepository userRepository;
    private String password;
    private String username;

    public UserLogin(ThreadExecutor threadExecutor,
                     PostExecutionThread postExecutionThread,
                     UserRepository userRepository) {
        super(threadExecutor, postExecutionThread);
        this.userRepository = userRepository;
    }

    /**
     * Initializes the interactor with the username and password to use for the authentication.
     *
     * @param username - username for the user
     * @param password - password for the user.
     */
    public UserLogin init(@NonNull String username, @NonNull  String password) {
        this.username = username;
        this.password = password;

        return this;
    }

    /**
     * Builds the {@link UseCase} observable
     * @return an {@link} Observable that will emit the logged in {@link UserProfile}
     **/
    @Override
    public Observable buildUseCaseObservable() {
        if (this.username == null || this.password == null) {
            throw new IllegalArgumentException("init(username,password) not called, or called with null argument.");
        }

        return Observable.concat(validate(), this.userRepository.user(this.username, this.password));
    }

    private Observable validate() {
        return Observable.create(new Observable.OnSubscribe<Object>() {
            @Override
            public void call(Subscriber<? super Object> subscriber) {
                if (UserLogin.this.username.isEmpty()) {
                    subscriber.onError(new InvalidEmailException());
                } else if (UserLogin.this.password.isEmpty()) {
                    subscriber.onError(new InvalidLoginPasswordException());
                } else {
                    subscriber.onCompleted();
                }
            }
        });
    }
}

//So you would execute it like:
//this.userLogin.init("myUser", "myPassword").execute(new UserLoginSubscriber())