Use HashSet to weed out duplicate values. List returns either empty or returns duplicate values

I am trying to remove duplicate values from an ArrayList using HashSet, so that city names aren’t returned more than once if they are the same… The List comes back either empty or still shows duplicate values. Was hoping someone could tell me where the error is in my code, so that no duplicate values are returned…

I am using this for reference:

Set<String> set = new HashSet<>(yourList); yourList.clear(); yourList.addAll(set); , but can’t figure out how to make it work in my code. Know it’s probably a simple fix, but I’ve been playing around with it and still hasn’t come back right…

Anyone can tell me where I am going wrong here? According to everyone so far the code looks like it should be working… Although it’s not…

SearchCityFragment

public class SearchCityFragment extends Fragment {

    private List<Post> mPostList;
    private Set<Post> mPostSet;
    private RecyclerView mRecyclerView;
    private CityAdapter mCityAdapter;

    private EditText mSearchBar;
    private RelativeLayout mRelativeLayout;
    private Activity mActivity;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_search_city, container, false);

        mRelativeLayout = view.findViewById(R.id.relative_layout_11);
        mRelativeLayout.setVisibility(View.VISIBLE);

        mRecyclerView = view.findViewById(R.id.recycler_view);
        mRecyclerView.setHasFixedSize(true);
        LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getContext());
        mRecyclerView.setLayoutManager(linearLayoutManager);

        mPostList = new ArrayList<>();
        mCityAdapter = new CityAdapter(getContext(), mPostList);
        mRecyclerView.setAdapter(mCityAdapter);

        mSearchBar = mActivity.findViewById(R.id.search_bar);
        mSearchBar.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
                searchCity(s.toString().toLowerCase());
            }

            @Override
            public void afterTextChanged(Editable s) {

            }
        });

        return view;
    }

    private void searchCity(String s) {
        Query query = FirebaseDatabase.getInstance().getReference("Posts").orderByChild("city").startAt(s).endAt(s + "uf8ff");
        query.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                mPostList.clear();
                for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                    Post post = snapshot.getValue(Post.class);
                    if (s.length() == 0) {
                        mPostList.clear();
                        mRelativeLayout.setVisibility(View.VISIBLE);
                    } else {
                        mRelativeLayout.setVisibility(View.GONE);
                        mPostList.add(post);
                        mPostSet = new HashSet<>(mPostList);
                        mPostList.clear();
                        mPostList.addAll(mPostSet);
                        readCity();
                    }
                }

                mCityAdapter.notifyDataSetChanged();
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }

    private void readCity() {
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Posts");
        reference.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                if (mSearchBar.getText().toString().equals("")) {
                    mPostList.clear();
                    for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                        Post post = snapshot.getValue(Post.class);
                        mPostList.add(post);
                        mPostSet = new HashSet<>(mPostList);
                        mPostList.clear();
                        mPostList.addAll(mPostSet);
                    }

                    mCityAdapter.notifyDataSetChanged();
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }

    @Override
    public void onAttach(@NonNull Context context) {
        super.onAttach(context);
        if (context instanceof Activity) {
            mActivity = (Activity) context;
        }
    }
}

Answer

if you only want to show the post with searched city and you want to show only one such post you can just query the database for post with that city , you don’t need to create any arraylist or hashmap.

                          private void searchCity(String s) {
                               FirebaseDatabase.getInstance()
                                     .getReference("Posts")
                                     .orderByChild("city")
                                     .startAt(s)
                                     .limitToFirst(1).
                                     .addValueEventListener(new ValueEvnetListener){
                                     @Override
                                     public void onDataChange(DataSnapshot 
                                     dataSnapshot) {
                                     // do your rhing
                                      }
                                    @Override
                               public void onCancelled(DatabaseError databaseError) {}
                                    } ;
}