@Test
public void whenPasswordIsEmpty_andClickOnLoginButton_shouldDisplayDialog() {
  onView(withId(R.id.login_username)).perform(typeText("admin"));
  onView(withId(R.id.login_button)).perform(click());
  onView(withText(R.string.not_empty_field)).check(matches(isDisplayed()));
  onView(withText(R.string.ok)).perform(click());
}

@Test
public void whenUserNameIsEmpty_andClickOnLoginButton_shouldDisplayDialog() {
  onView(withId(R.id.login_password)).perform(typeText("pass123"));
  onView(withId(R.id.login_button)).perform(click());
  onView(withText(R.string.not_empty_field)).check(matches(isDisplayed()));
  onView(withText(R.string.ok)).perform(click());
}