findFragmentById always returns null

Question

I\'m defining an ID for my fragment in the xml layout:

Answer 1

I was using android.support.v4.app.Fragment in my layout while calling getFragmentManager() which actually searched for android.app.Fragment subclasses and I got null. So the fix was to call getSupportFragmentManager() instead.

In general make sure the package of a fragment you are subclassing and using in your layout is the same returned by the corresponding FragmentManager which performs search.

Answer 2

Or, you should have instead used :

(MyFragment) getFragmentManager().findFragmentById(R.id.fragment_container);

Answer 3

R.id.test_fragment is not the ID of your fragment but the id of your LinearLayout

Calling add(int containerViewId, Fragment fragment) will add a fragment without a tag. So or you use add(int containerViewId, Fragment fragment, String tag) and you get back your fragment using your tag (as an ID)