Funcional Test Android Studio

I'm trying to understand how functional tests work in android applications. But I have difficulties. In the case shown below, I check in database if there are any "work" in the database and if this exists, it goes to the WorkActivity activity, otherwise it goes to CalendarActivity.

MainActivity

     public void btnNotes(View view) {
        if (userDAO.existWork() == false) {
            Intent intentRegisterWork = new Intent(this, WorkActivity.class);
            startActivity(intentRegisterWork);
        } else {
            Intent intent = new Intent(this, CalendarActivity.class);
            startActivity(intent);
    }

MainActivityTest

    @RunWith(AndroidJUnit4.class)
    public class MainActivityTest {
        @Rule
        public ActivityTestRule<MainActivity> mMainActivityTestRule =
                new ActivityTestRule<MainActivity>(MainActivity.class);
        @Test
        public void testChangeIntent() {
            onView(withId(R.id.btnNotes));
            Intent intent = new Intent (mMainActivityTestRule.getActivity(),WorkActivity.class);
            mMainActivityTestRule.launchActivity(intent);
        }
    }

Any suggestions how do I test this?