Deleting items in firestore based on timestamp

Question

I'm trying to delete items in firestore based on the documents time stamp, I'm fairly sure the code is right and I'm definitely sure the timestamps match and yet I get a null pointer

  java.lang.NullPointerException: Attempt to invoke virtual method 
  'com.google.android.gms.tasks.Task 
  com.google.firebase.firestore.DocumentReference.delete()' on a null object 
  reference

So here's what I have so far... when the activity starts I run a query and get all the message documents from firestore, I then put these into objects and display them in a recycler view with a custom adapter (basic stuff). What I'm doing now is creating a delete method, so I've created a method in my adapter that adds selected items to a list that I can get to start deleting (getSelectedItems) and I'm using the firestore query below to delete the items

ArrayList<UserMessage> messages = new ArrayList<>();
messages.addAll(mMessageAdapter.getSelectedItems());
final CollectionReference userRef =
            db.collection("users")
                    .document(userId)
                    .collection("contacts")
                    .document(myUser.get_id())
                    .collection("messages");
for (UserMessage message : messages) {

        System.out.println(message.getTime_stamp());
        //this prints like this- Sat Jun 02 12:20:56 GMT+01:00 2018

        Query query = userRef.whereEqualTo("time_stamp", 
        message.getTime_stamp());
        query.get().addOnCompleteListener(new 
        OnCompleteListener<QuerySnapshot>() {
            @Override                                                                   
            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                if (task.isSuccessful()) {
                    System.out.println("found an item to delete");
                    for (DocumentSnapshot doc : task.getResult()) {
                        doc.getDocumentReference(doc.getId())
                                .delete()
                                .addOnSuccessListener(new 
                                    OnSuccessListener<Void>() {
                                    @Override
                                    public void onSuccess(Void aVoid) {
                                     System.out.println("deleted an item");
                                        messagesDeleted[0]++;
                                        prog3Message = ("Deleted " + 
                                        messagesDeleted[0] + " of " + 
                                        allMessages[0] + " messages");
                                        runOnUiThread(changeMessage);
                                    }
                                });
                    }
                    if (messagesDeleted[0] == allMessages[0]) {
                        prog3.dismiss();
                        showMenuItems = false;
                        ///selectedMessageList.clear();
                        hideShowMenu(showMenuItems);
                        mMessageAdapter.clearSelections();
                        //System.out.println("all complete ");
                    }
                } else {

                }
            }
        });
    }

So to start troubleshooting I added a print statement in the first query (getting all messages) for each document I print the timestamp from both the document and the object (just in case) and it prints out like this

I/System.out: object data Sat Jun 02 12:20:56 GMT+01:00 2018
I/System.out: document data Sat Jun 02 12:20:56 GMT+01:00 2018
I/System.out: object data Sat Jun 02 12:23:16 GMT+01:00 2018
I/System.out: document data Sat Jun 02 12:23:16 GMT+01:00 2018

which as you can see matches a print from the delete method above - Sat Jun 02 12:20:56 GMT+01:00 2018 - and yet i get a null pointer when running the query, one interesting fact is when you go to firestore the timestamp shows like this June 1, 2018 at 5:55:49 PM UTC+1 which is a different format and different region so i tried playing with it a little and ended up doing this (not very elegant)

SimpleDateFormat dateFormat = 
new SimpleDateFormat("MMMM dd, yyyy HH:mm:ss aa", Locale.US);
    dateFormat.setTimeZone(TimeZone.getTimeZone("UTC"));
    String formatted = dateFormat.format(message.getTime_stamp());
        String[] parts = formatted.split(" ");
        StringBuilder stringBuilder = new StringBuilder();
        for (int i = 0; i < parts.length; i++){
            if(i == 2){
                stringBuilder.append(parts[i] + " at ");
            }else if (i == parts.length -1){
                stringBuilder.append(parts[i].toUpperCase() + " UTC+1");
            }else{
                stringBuilder.append(parts[i] + " ");
            }
        }
        System.out.println(stringBuilder.toString());
        //June 02, 2018 at 11:20:56 AM UTC+1

this gives me the right pattern but the time is now an hour out can anyone tell me if I'm trying to reinvent the wheel here hopefully theres a better way

Answer 1

I couldnt figure this out and decided to instead write a message to firestore and then update the item with its id so it reads collection (user) document {userId} collection (contacts) document {contactId} collection (messages) document {messageId} document values id = messageId values message = "blah blah" this way i can delete each item by its id