Filtering causes both lists to delete their Items - Android RecyclerView Filtering Java

Question

So what i want to do is simply filter all of my entries according to a searchString. This works fine but for some reason everytime i filter both lists delete the items. i will only include the (i think) important parts of code here.

Adapter.java:

@Override
    public Filter getFilter() {
        return myFilter;
    }

    private Filter myFilter = new Filter() {
        @Override
        protected FilterResults performFiltering(CharSequence constraint) {
            List<Stellplatz> filteredList = new ArrayList<>();
            Log.d("TEst","Size stellplaetzeall:"+ stellplaetzeall.size());

            Log.d("TEst","Size Stellplaeztze:"+ stellplaetze.size());

            if (constraint == null || constraint.length() == 0) {
                filteredList.addAll(stellplaetzeall);
            } else {
                String filterPattern = constraint.toString().toLowerCase().trim();
                for (Stellplatz item : stellplaetzeall) {
                    if (item.getAnbieter().toLowerCase().contains(filterPattern)) {
                        filteredList.add(item);
                    }
                }
            }
            FilterResults results = new FilterResults();
            results.values = filteredList;
            return results;
        }

        @Override
        protected void publishResults(CharSequence constraint, FilterResults results) {
            stellplaetze.clear();
            stellplaetze.addAll((ArrayList) results.values);
            notifyDataSetChanged();
        }
    };

Fragment.java

@Override
     public void onCreateOptionsMenu(Menu menu, @NonNull MenuInflater inflater){
        inflater.inflate(R.menu.menu_itemlist, menu);
        MenuItem searchItem = menu.findItem(R.id.action_search);
        SearchView searchView = (SearchView) searchItem.getActionView();
        searchView.setImeOptions(EditorInfo.IME_ACTION_DONE);
        searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener() {
            @Override
            public boolean onQueryTextSubmit(String query) {
                return false;
            }
            @Override
            public boolean onQueryTextChange(String newText) {
                adapter.getFilter().filter(newText);
                return false;
            }
        });
     }

I cant see why this would happen. Every example i found is basically the same as my code but for some reason it happens. i would really appreciate any answer that helps me understand this. also im not very good at android programming, first time doing this so an explanation why would be really helpful!

Answer 1

On publish Result you are clearing the List, i think that is wrong:

 @Override
    protected void publishResults(CharSequence constraint, FilterResults results) {
        //stellplaetze.clear();
        stellplaetze.addAll((ArrayList) results.values);
        notifyDataSetChanged();
    }

I have an example of my code i think this will help:

First i create 2 list:

private ArrayList<CountryPhone> objects;
private ArrayList<CountryPhone> filteredData;

 @Override
public int getCount() {
    return filteredData.size();
}

@Nullable
@Override
public CountryPhone getItem(int position) {
    return filteredData.get(position);
}

On Filter Results:

    @Override
    protected FilterResults performFiltering(CharSequence constraint) {

        String filterString = constraint.toString().toLowerCase();
        FilterResults filterResults = new FilterResults();
        ArrayList<CountryPhone> filterlist = new ArrayList<>();

            String name, code;
            for (int i = 0; i < objects.size(); i++) {
                CountryPhone item = objects.get(i);
                name = item.getName().toLowerCase();

                code = item.getCode();

                if (name.contains(filterString) || code.contains(filterString)){
                    filterlist.add(item);
                }
            }

            filterResults.values = filterlist;
            filterResults.count = filterlist.size();

        return filterResults;
    }

    @Override
    protected void publishResults(CharSequence constraint, FilterResults results) {
        filteredData = (ArrayList<CountryPhone>) results.values;

            notifyDataSetChanged();

    }

};