RecyclerView Adapter return NullPointerException

Question

I want to make double recyclerview to represent rooms on each floor. but NullPointerException occurs in floorAdapter.

public class FloorAdapter extends RecyclerView.Adapter {

    public ArrayList floors;
    private Context context;
    private LayoutInflater layoutInflater;

    public FloorAdapter(ArrayList floors, Context context) {
        this.floors = floors;
        this.context = context;
        this.layoutInflater = LayoutInflater.from(context);
    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view = layoutInflater.inflate(R.layout.signle_floor, parent, false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        holder.recyclerView.setAdapter(new RoomAdapter(context, floors.get(position).rooms));
        holder.recyclerView.setLayoutManager(new LinearLayoutManager(context, LinearLayoutManager.HORIZONTAL, false));
        holder.recyclerView.setHasFixedSize(true);
        holder.tvFloorNum.setText(floors.get(position).floorNum);
    }

    @Override
    public int getItemCount() {
        return floors.size();
    }

    public class ViewHolder extends RecyclerView.ViewHolder {
        RecyclerView recyclerView;
        TextView tvFloorNum;

        public ViewHolder(View itemView) {
            super(itemView);
            recyclerView = (RecyclerView) itemView.findViewById(R.id.rvFloors);
            tvFloorNum = (TextView) itemView.findViewById(R.id.tvFloorNum);
        }
    }
}

public class RoomAdapter extends RecyclerView.Adapter {
    private Context context;
    private ArrayList rooms;
    private LayoutInflater inflater;

    public RoomAdapter(Context context, ArrayList rooms) {
        this.context = context;
        this.rooms = rooms;
        this.inflater = LayoutInflater.from(context);
    }

    @Override
    public CustomViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view;
        view = inflater.inflate(R.layout.single_room, parent, false);
        return new CustomViewHolder(view);
    }

    @Override
    public void onBindViewHolder(CustomViewHolder holder, int position) {
        RoomData room = rooms.get(position);
        holder.tvRoomNum.setText(room.roomNum);
    }

    @Override
    public int getItemCount() {
        return rooms.size();
    }


    public class CustomViewHolder extends RecyclerView.ViewHolder {
        public TextView tvRoomNum;

        public CustomViewHolder(View itemView) {
            super(itemView);
            tvRoomNum = (TextView) itemView.findViewById(R.id.tvRoomNumber);
        }
    }
}

public class RoomActivity extends AppCompatActivity {

    private RecyclerView rvFloor;
    private FloorAdapter floorAdapter;
    private ArrayList floors;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_room);


        floors = prepareData();

        rvFloor = findViewById(R.id.rvFloors);

        floorAdapter = new FloorAdapter(floors, RoomActivity.this);
        LinearLayoutManager manager = new LinearLayoutManager(RoomActivity.this);
        rvFloor.setLayoutManager(manager);
        rvFloor.setAdapter(floorAdapter);

    }

    private ArrayList prepareData() {
        ArrayList floors = new ArrayList();

        //첫번째 subject 추가
        FloorData floor1 = new FloorData();
        floor1.floorNum = 1;
        floor1.rooms = new ArrayList();

        RoomData room101 = new RoomData();
        room101.roomNum = 101;

        RoomData room102 = new RoomData();
        room102.roomNum = 102;

        RoomData room103 = new RoomData();
        room103.roomNum = 103;

        floor1.rooms.add(room101);
        floor1.rooms.add(room102);
        floor1.rooms.add(room103);


        floors.add(floor1);

        FloorData floor2 = new FloorData();
        floor2.floorNum = 2;
        floor2.rooms = new ArrayList();

        RoomData room201 = new RoomData();
        room201.roomNum = 201;

        RoomData room202 = new RoomData();
        room202.roomNum = 202;

        RoomData room203 = new RoomData();
        room203.roomNum = 203;

        floor2.rooms.add(room201);
        floor2.rooms.add(room202);
        floor2.rooms.add(room203);


        floors.add(floor2);


        return floors;
    }
}

Logcat explain

"java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setAdapter(androidx.recyclerview.widget.RecyclerView$Adapter)' on a null object reference"

I refer to https://medium.com/@ashishkudale/android-list-inside-list-using-recyclerview-73cff2c4ea95. It's almost the same, but I don't know why the error is happening. please help me

Sarah Khan · Accepted Answer

two things. First, in your FloorAdapter's ViewHolder's constructor, you are finding recycler view of activity instead of that of Adapter. Change your code to,

public ViewHolder(View itemView) {
            super(itemView);
            recyclerView = (RecyclerView) itemView.findViewById(R.id.rvRooms); //**This is rvFloors in your code**
            tvFloorNum = (TextView) itemView.findViewById(R.id.tvFloorNum);
        }

in FloorAdapter. This will resolve your crash.

But you will experience two more crashes here

@Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        holder.recyclerView.setAdapter(new RoomAdapter(context, floors.get(position).rooms));
        holder.recyclerView.setLayoutManager(new LinearLayoutManager(context, LinearLayoutManager.HORIZONTAL, false));
        holder.recyclerView.setHasFixedSize(true);
        holder.tvFloorNum.setText(floors.get(position).floorNum); // **Crash will be on this line**
    }

and

@Override
    public void onBindViewHolder(CustomViewHolder holder, int position) {
        RoomData room = rooms.get(position);
        holder.tvRoomNum.setText(room.roomNum);//**Crash will be on this line**
    }

These will happen because setText method takes String as argument but you are passing either Integer or int.

So change both these lines to

holder.tvFloorNum.setText(String.valueOf(floors.get(position).floorNum));
holder.tvRoomNum.setText(String.valueOf(room.roomNum));

Hope this answer helps.

RecyclerView Adapter return NullPointerException

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