FileBriefDescriptionAdapter.java

FileBriefDescriptionAdapter(Context context, RecyclerView recyclerView, List<FileBriefDescription> data)
{
    this.context = context;
    this.fileBriefDescriptions = data;
    this.recyclerView = recyclerView;
    
    // fileBriefDescriptionsCopy is only read after this and never changed
    this.fileBriefDescriptionsCopy.addAll(fileBriefDescriptions); 
}
    
void filter(String text)
{
    fileBriefDescriptions.clear();
    if (text.isEmpty())
    {
        fileBriefDescriptions.addAll(fileBriefDescriptionsCopy);
    }
    else
    {
        text = text.toLowerCase();
        for (FileBriefDescription item : fileBriefDescriptionsCopy)
        {
            if (item.getFileName().toLowerCase().startsWith(text))
            {
                fileBriefDescriptions.add(item);
            }
        }
    }
}

LibraryActivity.java

    @Override
    public boolean onCreateOptionsMenu(final Menu menu)
    {
        getMenuInflater().inflate(R.menu.library_menu, menu);

        final MenuItem searchItem = menu.findItem(R.id.action_search);
        SearchView searchView = (SearchView) searchItem.getActionView();
        searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener()
        {
            @Override
            public boolean onQueryTextSubmit(String query)
            {
                adapter.filter(query);
                recyclerView.setAdapter(adapter);
                return true;
            }

            @Override
            public boolean onQueryTextChange(String newText)
            {
                adapter.filter(newText);
                recyclerView.setAdapter(adapter);
                return true;
            }
        });


        searchItem.setOnActionExpandListener(new MenuItem.OnActionExpandListener()
        {

            @Override
            public boolean onMenuItemActionExpand(final MenuItem item)
            {
                LibraryActivity.this.setMenuItemsVisibility(menu, searchItem, false);
                return true;
            }

            @Override
            public boolean onMenuItemActionCollapse(final MenuItem item)
            {
                LibraryActivity.this.setMenuItemsVisibility(menu, searchItem, true);

                return true;
            }
        });


        return super.onCreateOptionsMenu(menu);
    }

menu.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <item
        android:id="@+id/action_search"
        android:icon="@drawable/ic_action_search"
        android:title="Search"
        app:showAsAction="always|collapseActionView"
        app:actionViewClass="androidx.appcompat.widget.SearchView"/>
</menu>