activity_classic.xml

<TextView
    android:id="@+id/first_name_label"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/first_name_hint"
    android:visibility="invisible"
    android:theme="@style/Label.Small"/>

<EditText
    android:id="@+id/first_name"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginTop="20dp"
    android:hint="@string/first_name_hint"
    android:inputType="textPersonName"/>

<TextView
    android:id="@+id/error"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_above="@id/register"/>

<Button
    android:id="@+id/register"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="@string/register"
    android:layout_alignParentBottom="true"/>

RegisterActivityClassic.java

interface Action2<T,Throwable> {
    void call(T t, Throwable error);
}
interface RegisterApi {
    class RegistrationModel {
        String firstName;
    }
    void register(RegistrationModel data, Action2<RegistrationModel, Throwable> result);
}

public class RegisterActivityClassic extends Activity {
    RegisterApi registerApi; // inject or create remote api
    RegisterApi.RegistrationModel data = new RegisterApi.RegistrationModel();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_register_classic);

        final EditText firstName = (EditText) findViewById(R.id.first_name);
        final TextView firstNameLabel = (TextView) findViewById(R.id.first_name_label);
        firstName.addTextChangedListener(new TextWatcherBase() {
            @Override
            public void afterTextChanged(Editable s) {
                data.firstName = s.toString();
                firstNameLabel.setVisibility(TextUtils.isEmpty(s) ? View.INVISIBLE : View.VISIBLE);
            }
        });

        findViewById(R.id.register).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                registerApi.register(data, new Action2<RegisterApi.RegistrationModel, Throwable>() {
                    @Override
                    public void call(RegisterApi.RegistrationModel registrationModel, Throwable error) {
                        TextView text = (TextView) findViewById(R.id.error);
                        text.setText(error.getLocalizedMessage());
                    }
                });
            }
        });
    }
}