FragmentActivity: Cannot cast from Fragment to derived class

You are extending the wrong Fragment class. Import android.support.v4.app.Fragment instead of android.app.Fragment.

As devconsole pointed out in the comment above: The class that extends Fragment needs to import

android.support.v4.app.Fragment;

and not

android.app.Fragment;

I guess it has to do with the Android Compatibility Package. Problem is now resolved!

android casting android-fragments

Just in case people are looking for kotlin equivalent:

myFragmentClass = supportFragmentManager.findFragmentById(R.id.my_fragment) as MyFindOfFragment

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