【解決方法】仮想メソッド「void android.widget.textview.append(java.lang.charsequence)」を呼び出そうとしています

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.append(java.lang.CharSequence)' on a null object reference

Cart.java

<pre>
public class Cart extends AppCompatActivity {

    TextView textViewCartData;
    TextView itemWeight;
    TextView itemPrice;
    TextView textViewDisDur;
    SharedPreferences sharedPreferences;
    Button buttonConfirm;
    ImageView buttonRemove;

    RecyclerView recyclerView;
    ArrayList<ListData> arrayList;

    int pricePerKM = 5;
    String urlConfirm =
            "link/confirm?user_email=";

    String urlRemove =
            "link/cart/clear?user_email=";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_cart);
        recyclerView = findViewById(R.id.recyclerView);
        LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getApplicationContext());
        recyclerView.setLayoutManager(linearLayoutManager);


        textViewDisDur = findViewById(R.id.textDisDur);
        sharedPreferences = getSharedPreferences("CustomerApp", MODE_PRIVATE);
        buttonConfirm = findViewById(R.id.btnConfirmOrder);
        buttonRemove = findViewById(R.id.btnClearCart);
        fetchData();
        buttonConfirm.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                sendRequest(view, urlConfirm + sharedPreferences.getString("email", ""));
            }
        });

        buttonRemove.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                sendRequest(view, urlRemove + sharedPreferences.getString("email", ""));
            }
        });
    }

    public void sendRequest(View v, String apiUrl) {
        Log.e("url", apiUrl);
        v.setEnabled(false);
        RequestQueue queue = Volley.newRequestQueue(getApplicationContext());
        StringRequest stringRequest = new StringRequest(Request.Method.GET, apiUrl, new Response.Listener<String>() {
            @Override
            public void onResponse(String response) {
                v.setEnabled(true);
                if (response.equals("success")) {
                    Toast.makeText(getApplicationContext(), "Operation success", Toast.LENGTH_SHORT).show();
                    finish();
                } else
                    Toast.makeText(getApplicationContext(), "Operation failed", Toast.LENGTH_SHORT).show();
            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                v.setEnabled(true);
                error.printStackTrace();
            }
        });
        queue.add(stringRequest);
    }

    public void parseJSON(String data) {
        try {
            JSONArray jsonArray = new JSONArray(data);
            JSONObject jsonObject = jsonArray.getJSONObject(0);
            JSONArray jsonArray1 = jsonObject.getJSONArray("cart");
            for (int i = 0; i < jsonArray1.length(); i++) {
                JSONObject stu = jsonArray1.getJSONObject(i);
                String id = stu.getString("id");
                String name = stu.getString("name");
                String des = stu.getString("description");
                String numItem = stu.getString("numItem");
                String price = stu.getString("price");
                textViewCartData.append("Item: " + name + "");
                itemWeight.append("Ceki: " + numItem + "");
                itemPrice.append("" + price + "");
            }
            String distance = String.valueOf(jsonObject.getInt("distance") / 1000);
            String duration = String.valueOf((jsonObject.getInt("duration") / 60));
            int total_price = Integer.parseInt(distance) * pricePerKM;
            textViewDisDur.setText("Distance: " + distance + " KM\nDelivery Fee: "
                    + total_price + " $" + "\nDuration: " + duration + "Minutes");
        } catch (JSONException e) {
            Log.e("error", e.getLocalizedMessage());
            e.printStackTrace();
        }
        CartListAdapter cartListAdapter = new CartListAdapter(arrayList, Cart.class);
        recyclerView.setAdapter(cartListAdapter);
    }

    public void fetchData() {
        RequestQueue queue = Volley.newRequestQueue(this);
        StringRequest stringRequest = new StringRequest(Request.Method.GET,
                "link/cart/list?user_email="
                        + sharedPreferences.getString("email", ""), new Response.Listener<String>() {
            @Override
            public void onResponse(String response) {
                parseJSON(response);
            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                error.printStackTrace();
            }
        });
        queue.add(stringRequest);
    }
}

CartListAdapter.java

<pre lang="Java"><pre>
public class CartListAdapter extends RecyclerView.Adapter<CartListAdapter.MyViewClass> {

    ArrayList<ListData> listdata;
    Context context;
    SharedPreferences sharedPreferences;

    public static class MyViewClass extends RecyclerView.ViewHolder {
        TextView textViewCartData;
        TextView itemWeight;
        TextView itemPrice;
        public CardView cardView;


        public MyViewClass(View itemView) {
            super(itemView);
            this.textViewCartData = itemView.findViewById(R.id.textCartData);
            this.itemWeight = itemView.findViewById(R.id.itemWeight);
            this.itemPrice = itemView.findViewById(R.id.itemPrice);
            cardView = itemView.findViewById(R.id.cardView);
        }
    }

    public CartListAdapter(ArrayList<ListData> listdata, Class<Cart> cartClass) {
        this.listdata = listdata;
    }

    @NonNull
    @Override
    public MyViewClass onCreateViewHolder(ViewGroup parent, int viewType) {
        context = parent.getContext();
        sharedPreferences = context.getSharedPreferences("CustomerApp", MODE_PRIVATE);
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.row,parent,false);
        MyViewClass myViewClass = new MyViewClass(view);
        return myViewClass;
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewClass holder, int position) {
        ListData list = listdata.get(position);
        holder.itemPrice.setText(list.getPrice() + "$");
        holder.textViewCartData.setText(list.getName());
        holder.itemWeight.setText(list.getDes());
    }

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

}

問題は何ですか？

私が試したこと：

I don't know exactly where the problem is.
It says empty value. I understand the problem, I just don't understand.
  Please tell me the correct solution.