Android：尝试对空对象引用调用虚拟方法

Question

I have an application with one activity and this activity has a fragment. In this fragment I show in a Listview "pending orders". I set up I method to reload the search in my database for the pending order once every 55 seconds. And I reload the Activity once every 60 seconds. The problem is that something when the fragment is being loaded I get this error:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getCacheDir()' on a null object reference

And I am trying to track this problem but it's not very easy because I am still a beginner in Android development.

This is the fragment:

public class PendingOrdersFragment extends Fragment implements AdapterView.OnItemClickListener {

    ListView lstOrders;
    ArrayList<ChefOrderList> orderLists;
    TextView empty;

    private SwipeRefreshLayout mSwipeRefreshLayout;
    private final Handler handler = new Handler();

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_list, null);

    }

    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        doTheAutoRefresh();
        lstOrders = (ListView) view.findViewById(lstFood);
        lstOrders.setOnItemClickListener(this);
        empty = (TextView) view.findViewById(R.id.empty);

        mSwipeRefreshLayout = (SwipeRefreshLayout) view.findViewById(R.id.swipe_refresh);
        mSwipeRefreshLayout.setColorSchemeResources(R.color.colorPrimary);
        mSwipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
            @Override
            public void onRefresh() {
                if (NetworkHelper.isOnline(getActivity())) {
                    callServiceForMenuStatusDetails(PrefernceHelper.getString(getActivity(),Commons.Constants.DATE));
                } else {
                    NetworkHelper.noNetworkToast(getActivity());
                }
            }
        });

        System.out.println(PrefernceHelper.getString(getActivity(), Commons.Constants.DATE));

        if (NetworkHelper.isOnline(getActivity())) {
            callServiceForMenuStatusDetails(PrefernceHelper.getString(getActivity(), Commons.Constants.DATE));
        } else {
            NetworkHelper.noNetworkToast(getActivity());
        }

    }

    //ProgressDialog progressDialog;

    private void callServiceForMenuStatusDetails(String date) {

        //progressDialog = new ProgressDialog(getActivity());
        //progressDialog.setTitle("Loading");
        //progressDialog.setMessage("Please Wait");
        //progressDialog.setCancelable(false);
        //progressDialog.show();

        new VolleyHelper(getActivity()).get("chefOrderDetailsByDate/" + PrefernceHelper.getString(getActivity(), Commons.Constants.USER_ID) + "/" + date, null, new Response.Listener<JSONObject>() {
            @Override
            public void onResponse(JSONObject response) {
                orderLists = new ArrayList<ChefOrderList>();
                mSwipeRefreshLayout.setRefreshing(false);
                try {
                    JSONArray jsonArray = response.getJSONArray("orderlist");
                    int pending = 0, accepted = 0, rejected = 0;

                    for (int i = 0; i < jsonArray.length(); i++) {
                        ChefOrderList orderList = new ChefOrderList();
                        JSONObject jsonObject = jsonArray.getJSONObject(i);


                        orderList.setOrderId(jsonObject.getString("orderid"));
                        orderList.setOrderStatus(jsonObject.getString("order_status"));
                        orderList.setOrderDate(jsonObject.getString("order_date"));
                        orderList.setOrderFrom(jsonObject.getString("orderfrom"));

                        if(jsonObject.getString("order_status").equals("Pending")) {
                            //System.out.println(orderList.getOrderId());
                            orderLists.add(orderList);
                            pending = pending + 1;
                            TabLayoutScreenActivity.orderSizeVariable = TabLayoutScreenActivity.orderSizeVariable + 1;
                        } else if (jsonObject.getString("order_status").equals("Order accepted by chef")){
                            accepted = accepted + 1;

                        } else if (jsonObject.getString("order_status").equals("Order rejected by chef")){
                            rejected = rejected + 1;
                        }
                    }

                    //for (int i= 0; i< orderLists.size();i++) {
                    //    System.out.println(orderLists.get(i).getOrderId());
                    //}

                    System.out.println("Variable: " + TabLayoutScreenActivity.orderSizeVariable);
                    System.out.println("Fixed: " + TabLayoutScreenActivity.orderSizeFixed);

                    if (TabLayoutScreenActivity.orderSizeVariable > TabLayoutScreenActivity.orderSizeFixed){
                        TabLayoutScreenActivity.orderSizeFixed = TabLayoutScreenActivity.orderSizeVariable;
                        PushNotification();
                    }
                    TabLayoutScreenActivity.orderSizeVariable = 0;



                    if(jsonArray.length() > 0 && pending!=0) {
                        lstOrders.setVisibility(View.VISIBLE);
                        empty.setVisibility(View.INVISIBLE);
                        ChefOrderListAdapter adpater = new ChefOrderListAdapter(getActivity(), orderLists);
                        lstOrders.setAdapter(adpater);
                    }else {
                        lstOrders.setEmptyView(getView().findViewById(android.R.id.empty));
                        empty.setVisibility(View.VISIBLE);
                        lstOrders.setVisibility(View.INVISIBLE);
                    }

                    //progressDialog.dismiss();


                } catch (JSONException e) {
                    e.printStackTrace();
                }

            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {

            }
        });
    }

    public void PushNotification()
    {
        NotificationManager nm = (NotificationManager)getContext().getSystemService(NOTIFICATION_SERVICE);
        Notification.Builder builder = new Notification.Builder(getContext());
        Intent notificationIntent = new Intent(getContext(), TabLayoutScreenActivity.class);
        PendingIntent contentIntent = PendingIntent.getActivity(getContext(),0,notificationIntent,0);

        //set
        builder.setContentIntent(contentIntent);
        builder.setSmallIcon(R.drawable.chef_hat);
        builder.setContentText("Click here to open the app.");
        builder.setContentTitle("You have a new order!");
        if(android.os.Build.VERSION.SDK_INT>=21) {
            builder.setColor(Color.parseColor("#D8540D"));
        }
        builder.setAutoCancel(true);
        builder.setDefaults(Notification.DEFAULT_ALL);

        Notification notification = builder.build();
        nm.notify((int)System.currentTimeMillis(),notification);
    }

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

        System.out.println("Test");
        Intent intent = new Intent();
        intent.putExtra("orderID", orderLists.get(position).getOrderId());
        intent.putExtra("orderFrom", orderLists.get(position).getOrderFrom());
        intent.setClass(getActivity(), OrderDetailsActivity.class);
        startActivity(intent);
        getActivity().finish();

    }

    private void doTheAutoRefresh() {
        handler.postDelayed(new Runnable() {
            @Override
            public void run() {
                System.out.println("Auto refresh: Pending");
                //if (!(PrefernceHelper.getString(getActivity(), Commons.Constants.DATE).equals(null))) {
                Calendar c = Calendar.getInstance();
                SimpleDateFormat postFormater = new SimpleDateFormat("yyyy-MM-dd");
                callServiceForMenuStatusDetails(postFormater.format(c.getTime()));
                //}
                doTheAutoRefresh();
            }
        }, 55000);
    }
}

This is the full log cat for the error:

Volley.java 43 is:

Volley.java 78 is:

VolleyHelper.java 34 is:

For the Pending order fragment: Line 47 is:

public class PendingOrdersFragment extends Fragment implements AdapterView.OnItemClickListener {

Line 105 is:

new VolleyHelper(getActivity()).get("chefOrderDetailsByDate/" + PrefernceHelper.getString(getActivity(), Commons.Constants.USER_ID) + "/" + date, null, new Response.Listener<JSONObject>() {

Line 222 is:

callServiceForMenuStatusDetails(postFormater.format(c.getTime()));

Answer 1

Your Error means that the variable that the function getCacheDir () uses is null. In this case your variable context. You can get your Application Context like this:

public void onCreate() {
    super.onCreate();
    Context mContext = getApplicationContext();
}

Answer 2

In my case, the crash was happening because of activity reference, when I was trying to finish Activity, to restart the App. The following error was coming for me:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.Activity.finish()' on a null object reference

I solved it like this, NameOfActivity.this.finish() .

Answer 3

This Error is because the mContext which you are passing is null

//Put an if condition to check if mContext is null or not

     if(mContext()!=null) {
        requestQueue = Volley.newRequestQueue(mContext());               
        }