private void initUpAction() {
        final ActionBar actionBar = getSupportActionBar();
        if (actionBar != null) {
            // don't display a back button it the activity is running as top-level activity with bottom navigation attached
            actionBar.setDisplayHomeAsUpEnabled(!isTaskRoot() || !(this instanceof AbstractNavigationBarActivity));
        }
    }
Classes actually having an ActionBar (as opposed to the Dialog activities)
main/src/main/java/cgeo/geocaching/activity/AbstractActionBarActivity.java