MainViewModel.java

public static class MainViewModel {

    public ObservableField<String> firstName = new ObservableField<>();
    public ObservableField<String> lastName = new ObservableField<>();
    public ObservableField<String> helloText = new ObservableField<>();
    public ObservableBoolean helloButtonEnabled = new ObservableBoolean(false);

    public MainViewModel() {

        Observable.combineLatest(toObservable(firstName), toObservable(lastName), (firstName, lastName) -> StringUtils.isNotNullOrEmpty(firstName) && StringUtils.isNotNullOrEmpty(lastName))
                .subscribe(result -> {
                    helloButtonEnabled.set(result);
                    if (!result) {
                        helloText.set(StringUtils.EMPTY);
                    }
                }, Throwable::printStackTrace);
    }

    public void buttonClicked() {
        helloText.set(String.format("Hello %s %s !", firstName.get(), lastName.get()));
    }
}