How to set a Fragment tag by code?

Question

Yes. So the only way is at transaction time, e.g. using add, replace, or as part of the layout.

I determined this through an examination of the compatibility sources as I briefly looked for similar at some point in the past.

You can set tag to fragment in this way:

Fragment fragmentA = new FragmentA();
getFragmentManager().beginTransaction()
    .replace(R.id.MainFrameLayout,fragmentA,"YOUR_TARGET_FRAGMENT_TAG")
    .addToBackStack("YOUR_SOURCE_FRAGMENT_TAG").commit(); 

You can provide a tag inside your activity layout xml file.

Supply the android:tag attribute with a unique string.

Just as you would assign an id in a layout xml.

    android:tag="unique_tag"

link to developer guide

You can also get all fragments like this:

For v4 fragmets

List<Fragment> allFragments = getSupportFragmentManager().getFragments();

For app.fragment

List<Fragment> allFragments = getFragmentManager().getFragments();

This is the best way I have found :

   public class MainActivity extends AppCompatActivity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (savedInstanceState == null) {
          // Let's first dynamically add a fragment into a frame container
          getSupportFragmentManager().beginTransaction(). 
              replace(R.id.flContainer, new DemoFragment(), "SOMETAG").
              commit();
          // Now later we can lookup the fragment by tag
          DemoFragment fragmentDemo = (DemoFragment) 
              getSupportFragmentManager().findFragmentByTag("SOMETAG");
        }
    }
}

Nowadays there's a simpler way to achieve this if you are using a DialogFragment (not a Fragment):

val yourDialogFragment = YourDialogFragment()
yourDialogFragment.show(
    activity.supportFragmentManager,
    "YOUR_TAG_FRAGMENT"
)

Under the hood, the show() method does create a FragmentTransaction and adds the tag by using the add() method. But it's much more convenient to use the show() method in my opinion.

You could shorten it for Fragment too, by using a Kotlin Extension :)