apparently there is a X under import androidx.fragment.app.FragmentActivity, when I try to create a fragmentActivity. However, when I look for a sample project, it imports android.support.v4.app.fragment instead. May I know whats the difference, and why do I have this following error when I try to run my own project?:
java.lang.ClassCastException: com.fragmenttest.helloapp.ui.collection.ViewPagerIndicatorActivity cannot be cast to androidx.fragment.app.Fragment.
android.support.v4.app.fragment it's the package for the old Android support fragments.
androidx.fragment.app.FragmentActivity it's where the new fragments are. This came to replace the previous package because it's a much cleaner namespace. This is also known as Android X.
Probably the examples you're seeing are old and still use the old package. If I'm not mistaken Androidx was introduced 1 or 2 years ago.
In most cases you can simply replace the import, but I think the best way is to use Android Studio's migrate to Androidx
First of all you are using different libraries.
android.support.v4.app.* is included in the support libraries.
androidx.fragment.app.* is included in the androidx libraries.
They can't work together. Check the info to migrate.
Then an Activity (or subclasses) is not a Fragment.
You can't cast a Fragment in an Activity.
Activity != Fragment, your java.lang.ClassCastException is suggesting that you want to cast Fragment to Activity
Activity is full screen (mostly), Fragment is only a piece of look, kind of very extended View. One Activity can handle multiple Fragments, e.g. on bigger screens.
FragmentActivity was introduced in support pack for handling all Fragments features in lower OS versions, now you should use AppCompatActivity which extends FragmentActivity