findFragmentById always returns null

Question

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

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/test_fragment"
...

Then I add this fragment in the activity's onCreate method:

MyFragment myFragment = new MyFragment();
fragmentTransaction.add(R.id.fragment_container, myFragment);
fragmentTransaction.commit();

This is all working fine. Replacing fragments and is also working.

Later I'm trying to retrieve this fragment by its ID in one of the activity's methods:

MyFragment myFragment = (MyFragment) getFragmentManager().findFragmentById(R.id.test_fragment);

Doing so leads to myFragment being null. Always.

When I try to do the same with tags instead of IDs I can retrieve the fragment by its tag without any problems:

MyFragment myFragment = new MyFragment();
fragmentTransaction.add(R.id.fragment_container, myFragment, "testfragment");
fragmentTransaction.commit();

...

MyFragment myFragment = (MyFragment) getFragmentManager().findFragmentByTag("testfragment");

Why can't findFragmentById find the fragment, but findFragmentByTag does so? Am I missing something?

Try `(MyFragment) getFragmentManager().findFragmentById(R.id.container);` — offset, Jan 27 '15 at 07:42

score 44 · Accepted Answer · edited Mar 14 '13 at 03:35

44

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)

edited Mar 14 '13 at 03:35

Javier

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answered May 31 '12 at 12:37

StErMi

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This is a valid workaround but did not answer the question. Have the look at the docs for a proper description. The docs say the id is either assigned when inflating the fragment from xml or is the container id when added using a transaction... http://developer.android.com/reference/android/app/FragmentManager.html#findFragmentById(int) – speedynomads Jul 24 '13 at 09:11
See this answer for a good description on programatically assigning id's: http://stackoverflow.com/a/13241629/857681 – speedynomads Jul 24 '13 at 09:13
6

If a Fragment is added without an id, can't you find it using the id of the container it's placed in? I'm pretty sure that's a thing. – Mike T Aug 06 '13 at 09:16
5

Mike T is right. The docs explicitly say you are supposed to be able to use the ID of the container of the fragment in `getFragmentById()`. – Timmmm May 22 '14 at 09:49
1

You don't have to use `add(...)` with tag. And the docs are right. You just have to use the correct `FragmentManager` instance to make `getFragmentById()` work in the way you expect it to work. Check my answer below for additional information. – vir us Oct 12 '15 at 23:20

score 9 · Answer 2 · edited Aug 22 '13 at 13:15

9

Use the <FrameLayout> tag as a container in your layout file. Later to replace the fragments from your activity dynamically, you can use the ID of the <FrameLayout> container to replace the fragments in your activity.

<FrameLayout
            android:id="@+id/test_fragment"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />

edited Aug 22 '13 at 13:15

Tommy

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answered Jun 20 '13 at 07:26

Amit Bhandari

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Blackbelt · Answer 3 · 2012-05-31T13:19:49.223

4

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/test_fragment"

it should be a fragment not a LinearLayout

<fragment android:name="com.example.yourfragment"
            android:id="@+id/test_fragment"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />

edited May 31 '12 at 13:19

answered May 31 '12 at 12:32

Blackbelt

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So what do I have to change? I can't define my layout with as top node. I can't add the fragments in the xml layout of the activity since I have to remove them later on. – Viper May 31 '12 at 12:57
Ok than you can not define fragment in the layout. If you need to retrive the linerarlayout you have to use the inflater parameter in the onCreateView – Blackbelt May 31 '12 at 17:00

vir us · Answer 4 · 2015-10-12T23:26:18.230

4

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.

edited Oct 12 '15 at 23:26

answered Oct 12 '15 at 23:13

vir us

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score 3 · Answer 5 · edited Mar 14 '13 at 03:34

3

Or, you should have instead used :

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

edited Mar 14 '13 at 03:34

Javier

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answered Mar 14 '13 at 03:14

user2168109

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score 2 · Answer 6 · edited Sep 27 '16 at 13:17

2

R.id.test_fragment is your LinearLayout ID not your Fragment.

You can define and id on a fragment when it is inflated from an xml like in this sample http://developer.android.com/guide/topics/fundamentals/fragments.html#Adding

edited Sep 27 '16 at 13:17

Piyush

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answered May 31 '12 at 12:34

GBouerat

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score 2 · Answer 7 · answered Nov 12 '19 at 04:26

The reasons mentioned above are valid but another possible reason is that you are trying to search for the fragment while being in a fragment this also results in fragmentManager.findFragmentById to return null. We should use childFragmentManager.findFragmentById to find the fragment inside a fragment. According to the official documentation.

public final androidx.fragment.app.FragmentManager getChildFragmentManager()

Return a private FragmentManager for placing and managing Fragments inside of this Fragment.

I know it's against stackoverflow rules, but just upvoting is not enough, you are legend. — Amr, Apr 17 '21 at 10:13

score 1 · Answer 8 · answered Mar 31 '17 at 06:53

1

FragmentB fragmentB = 
(FragmentB) getSupportFragmentManager().findFragmentById(R.id.containerb);

if(fragmentB != null)
{
   fragmentB.showuserResponse(msg);
}

Use container id as fragment id. And then check for null reference.

answered Mar 31 '17 at 06:53

Aman Srivastava

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score 0 · Answer 9 · edited Jun 20 '20 at 09:12

fragmentTransaction.add(R.id.fragment_container, myFragment);

MyFragment myFragment = (MyFragment) getFragmentManager().findFragmentById(R.id.test_fragment);

Please notice the difference.

You should pass R.id.fragment_container as the argument to findFragmentById, as you pass it to the add function, instead of R.id.test_fragment

By the way , according to the inner implementation of the two functions, it should be right that the id can be that of its container view.

findFragmentById always returns null

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