Get the Latest Fragment in Backstack

Get the last fragment in back stack

If you're using BackStackEntry.getName(), it should be:

ft.replace(android.R.id.tabcontent, fragment, tag);
ft.addToBackStack(tag);

Get fragment from backstack

getFragmentManager().findFragmentByTag("tag")

is only used when you have added a fragment with specific tag for e.g.

fragmentTransaction.add(R.id.order_container,mProfileFragment,"profile");

or

fragmentTransaction.replace(R.id.order_container,mProfileFragment,"sometag");

Then you will be able to find this fragment by the tag.

In your case you are adding a transaction to backstack so you will not be able to find that fragment by tag. You just adding a transaction to backstack thats it not a fragment. And also your fragment was removed from the activity and destroyed so you have to revert the transaction by popping backstack instead of finding that fragment by tag.
You have to call

getFragmentManager().popBackStack("profile");

to get that fragment back to the activity and make it visible on screen.

Is there a way to get fragment from top of stack?

Some thing like this should help your activity figure this out on backpress:

private Fragment getCurrentFragment(){
    FragmentManager fragmentManager = getSupportFragmentManager();
    String fragmentTag = fragmentManager.getBackStackEntryAt(fragmentManager.getBackStackEntryCount() - 1).getName();
    Fragment currentFragment = fragmentManager.findFragmentByTag(fragmentTag);
    return currentFragment;
}

@Override
public void onBackPressed() {
    Fragment fragmentBeforeBackPress = getCurrentFragment();
    // Perform the usual back action
    super.onBackPressed();
    Fragment fragmentAfterBackPress = getCurrentFragment();
}

Hope this helps.

How to get a list of backstack fragment entries in android?

The FragmentManager has methods:

getBackStackEntryCount()

getBackStackEntryAt (int index)

FragmentManager fm = getFragmentManager();

for(int entry = 0; entry<fm.getBackStackEntryCount(); entry++){
   Log.i(TAG, "Found fragment: " + fm.getBackStackEntryAt(entry).getId());
}

How to resume existing Fragment from BackStack

Reading the documentation, there is a way to pop the back stack based on either the transaction name or the id provided by commit. Using the name may be easier since it shouldn't require keeping track of a number that may change and reinforces the "unique back stack entry" logic.

Since you want only one back stack entry per Fragment, make the back state name the Fragment's class name (via getClass().getName()). Then when replacing a Fragment, use the popBackStackImmediate() method. If it returns true, it means there is an instance of the Fragment in the back stack. If not, actually execute the Fragment replacement logic.

private void replaceFragment (Fragment fragment){
  String backStateName = fragment.getClass().getName();

  FragmentManager manager = getSupportFragmentManager();
  boolean fragmentPopped = manager.popBackStackImmediate (backStateName, 0);

  if (!fragmentPopped){ //fragment not in back stack, create it.
    FragmentTransaction ft = manager.beginTransaction();
    ft.replace(R.id.content_frame, fragment);
    ft.addToBackStack(backStateName);
    ft.commit();
  }
}

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EDIT

The problem is - when i launch A and then B, then press back button, B
is removed and A is resumed. and pressing again back button should
exit the app. But it is showing a blank window and need another press
to close it.

This is because the FragmentTransaction is being added to the back stack to ensure that we can pop the fragments on top later. A quick fix for this is overriding onBackPressed() and finishing the Activity if the back stack contains only 1 Fragment

@Override
public void onBackPressed(){
  if (getSupportFragmentManager().getBackStackEntryCount() == 1){
    finish();
  }
  else {
    super.onBackPressed();
  }
}

Regarding the duplicate back stack entries, your conditional statement that replaces the fragment if it hasn't been popped is clearly different than what my original code snippet's. What you are doing is adding to the back stack regardless of whether or not the back stack was popped.

Something like this should be closer to what you want:

private void replaceFragment (Fragment fragment){
  String backStateName =  fragment.getClass().getName();
  String fragmentTag = backStateName;

  FragmentManager manager = getSupportFragmentManager();
  boolean fragmentPopped = manager.popBackStackImmediate (backStateName, 0);

  if (!fragmentPopped && manager.findFragmentByTag(fragmentTag) == null){ //fragment not in back stack, create it.
    FragmentTransaction ft = manager.beginTransaction();
    ft.replace(R.id.content_frame, fragment, fragmentTag);
    ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
    ft.addToBackStack(backStateName);
    ft.commit();
  } 
}

The conditional was changed a bit since selecting the same fragment while it was visible also caused duplicate entries.

Implementation:

I highly suggest not taking the the updated replaceFragment() method apart like you did in your code. All the logic is contained in this method and moving parts around may cause problems.

This means you should copy the updated replaceFragment() method into your class then change

backStateName = fragmentName.getClass().getName();
fragmentPopped = manager.popBackStackImmediate(backStateName, 0);
if (!fragmentPopped) {
            ft.replace(R.id.content_frame, fragmentName);
}
ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
ft.addToBackStack(backStateName);
ft.commit();

so it is simply

replaceFragment (fragmentName);

---

EDIT #2

To update the drawer when the back stack changes, make a method that accepts in a Fragment and compares the class names. If anything matches, change the title and selection. Also add an OnBackStackChangedListener and have it call your update method if there is a valid Fragment.

For example, in the Activity's onCreate(), add

getSupportFragmentManager().addOnBackStackChangedListener(new OnBackStackChangedListener() {

  @Override
  public void onBackStackChanged() {
    Fragment f = getSupportFragmentManager().findFragmentById(R.id.content_frame);
    if (f != null){
      updateTitleAndDrawer (f);
    }

  }
});

And the other method:

private void updateTitleAndDrawer (Fragment fragment){
  String fragClassName = fragment.getClass().getName();

  if (fragClassName.equals(A.class.getName())){
    setTitle ("A");
    //set selected item position, etc
  }
  else if (fragClassName.equals(B.class.getName())){
    setTitle ("B");
    //set selected item position, etc
  }
  else if (fragClassName.equals(C.class.getName())){
    setTitle ("C");
    //set selected item position, etc
  }
}

Now, whenever the back stack changes, the title and checked position will reflect the visible Fragment.

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