Android 应用程序中的空对象引用

Question

大家好

所以我试图让下面的代码部分用于购物清单应用程序的运行，但是我收到一个导致应用程序崩溃的错误。

我的代码：

public void setProduct(Context context, View shoppingListViewFragment, String userEmail, String userName, ListModel shoppingListModel, ItemModel productModel) {
    String listID = shoppingListModel.getListID();
    String listName = shoppingListModel.getListName();
    String itemID = productModel.getItemID();
    String itemName = productModel.getItemName();
    Boolean currentList = productModel.getCurrentList();
    productNameContainer.setText(itemName);

    FirebaseFirestore dbRef = FirebaseFirestore.getInstance();
    DocumentReference itemIdRef = dbRef.collection("products").document(listID)
            .collection("shoppingListProducts").document(itemID);

    itemView.setOnClickListener(view -> {
        Map<String, Object> map = new HashMap<>();
        if (currentList) {
            map.put("currentList", false);
        } else {
            map.put("currentList", true);
        }
        itemIdRef.update(map).addOnSuccessListener(aVoid -> {
            if (currentList) {
                dbRef.collection("shoppingLists").document(userEmail)
                        .collection("userShoppingLists").document(listID).get().addOnCompleteListener(task -> {
                            Map<String, Object> map1 = (Map<String, Object>) task.getResult().get("users");
                            String notificationMessage = userName + " has just bought " + itemName + " from " + listName + "'s list!";
                            UserNotificationsModel userNotificationsModel = new UserNotificationsModel(notificationMessage, userEmail);

                            for (Map.Entry<String, Object> entry : map1.entrySet()) {
                                String sharedUserEmail = entry.getKey();

                                if (!sharedUserEmail.equals(userEmail)) {
                                    dbRef.collection("notifications").document(sharedUserEmail)
                                            .collection("userNotifications").document()
                                            .set(userNotificationsModel);
                                }
                            }
                        });
            }
        });
    });

    itemView.setOnLongClickListener(view -> {
        AlertDialog.Builder builder = new AlertDialog.Builder(context);
        builder.setTitle("Edit / Delete Item");

        EditText editText = new EditText(context);
        editText.setInputType(InputType.TYPE_CLASS_TEXT | InputType.TYPE_TEXT_FLAG_CAP_WORDS);
        editText.setText(itemName);
        editText.setSelection(editText.getText().length());
        editText.setHint("Please type item name");
        editText.setHintTextColor(Color.GRAY);
        builder.setView(editText);

        builder.setPositiveButton("Update", (dialogInterface, i) -> {
            String newItemName = editText.getText().toString().trim();
            Map<String, Object> map = new HashMap<>();
            map.put("itemName", newItemName);
            itemIdRef.update(map);
        });

        builder.setNegativeButton("Delete", (dialogInterface, i) -> {
            itemIdRef.delete().addOnSuccessListener(aVoid -> Snackbar.make(shoppingListViewFragment, "Item deleted!", Snackbar.LENGTH_LONG).show());
        });

        AlertDialog alertDialog = builder.create();
        alertDialog.show();

        return true;
    });
}

我收到以下错误：

FATAL EXCEPTION: main
Process: example.com.shoppinghelper, PID: 17150
java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Set java.util.Map.entrySet()' on a null

基本上，代码会尝试将数据写入通知集合，前提是某个列表已被某人共享。但是，如果没有人与我分享特定的购物清单，那么我就会收到上述错误。

我可以指出如何处理此类空指针异常的正确方向吗？

感谢大家的帮助。

Answer 1

你的map1对象被赋予了getResult()的值，但是你会发现getResult()将返回null

Map<String, Object> map1 = (Map<String, Object>) task.getResult().get("users");

导致这个 for 声明

for (Map.Entry<String, Object> entry : map1.entrySet()){}

要导致空指针异常，您应该在其之前添加检查 map1 != null