Firestore - Why check if DocumentSnapshot is not null AND call exists?

Question

Take a look at this code example from the Firestore documentation:

DocumentReference docRef = db.collection("cities").document("SF");
docRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
    @Override
    public void onComplete(@NonNull Task<DocumentSnapshot> task) {
        if (task.isSuccessful()) {
            DocumentSnapshot document = task.getResult();
            if (document != null && document.exists()) {
                Log.d(TAG, "DocumentSnapshot data: " + document.getData());
            } else {
                Log.d(TAG, "No such document");
            }
        } else {
           Log.d(TAG, "get failed with ", task.getException());
        }
    }
});

https://firebase.google.com/docs/firestore/query-data/get-data

Why check if document != null? If I read the source code correctly (beginner), the exists method checks for nullity internally.

Which `ifExists` method are you referring to? But the point is, if `document` is null, you cannot call any methods of it or you will get a `NullPointerException`. — Henry, Apr 07 '18 at 08:03
check for null is not necessary here exists will is already doing a job of null. — Nouman Ch, Apr 07 '18 at 08:08
@NoumanCh No, `exists` is not even called if `document` is `null` (an NPE is thrown before that). — Henry, Apr 07 '18 at 08:13
I tried it with a document reference to a non existing document and NO NPE is thrown. — Florian Walther, Apr 07 '18 at 08:23
The answer in this thread implies that my concerns are correct, the null check is not necessary: https://stackoverflow.com/questions/47350129/about-the-firestore-query-data-documentation-specifically-documentsnapshot — Florian Walther, Apr 07 '18 at 08:43
It may be that the null check is not necessary but the reason is certainly not that `exists` does any checks. — Henry, Apr 07 '18 at 08:47
I fixed the snippet here: https://github.com/firebase/snippets-android/commit/58973e33dfdd39a9ded36e7e6887c8feef7493b1 — Sam Stern, Apr 11 '18 at 20:24

score 17 · Accepted Answer · answered Apr 07 '18 at 15:27

17

A successfully completed task will never pass null for the DocumentSnapshot. If the requested document does not exist, you'll get an empty snapshot. This means that:

Calling document.exists() returns false
Calling document.getData() throws an exception

So there is indeed no reason to check if document != null before calling document.exists().

answered Apr 07 '18 at 15:27

Frank van Puffelen

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Thank you. Good to see that my conclusions were right, because I am a beginner. – Florian Walther Apr 07 '18 at 20:20
1

And what about the `onEvent` method in the `EventListener`? If I check for an exception and no exception was thrown, is the `DocumentSnapshot` there always NOT null as well? https://firebase.google.com/docs/firestore/query-data/listen – Florian Walther Apr 08 '18 at 09:40
*Googler here*. I wrote the code snippet in question and you're right, the `null` check is not needed. I guess I was just feeling extra careful that day. I will fix it! – Sam Stern Apr 11 '18 at 20:22

Alex Mamo · Answer 2 · 2018-04-07T12:57:23.697

3

If you execute the following statement:

document != null

It will be evaluated to false, which means that your task is null and the following message will be printed out:

Log.d(TAG, "No such document");

However, if you are calling a method on your task object, (e.g. toString()) it will throw the following error:

java.lang.IllegalStateException: This document doesn't exist. Use DocumentSnapshot.exists() to check whether the document exists before accessing its fields.

This message explicitly tells you to use exists() method rather than to check for nullity.

The official documentation regarding the use of how to get a document method says:

Note: If there is no document at the location referenced by docRef, the resulting document will be null.

edited Apr 07 '18 at 12:57

answered Apr 07 '18 at 08:50

Alex Mamo

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I actually think that documentation may be wrong. As far as I know the `DocumentSnapshot` will never be `null`. See https://jsbin.com/wesareq/1/edit?js,console – Frank van Puffelen Apr 07 '18 at 15:21
Thanks Alex! In the meantime we updated the documentation to be more explicit about the return value. – Frank van Puffelen Apr 10 '18 at 13:06
@FrankvanPuffelen Nice to hear that. I'll take a look right now. Thanks! – Alex Mamo Apr 10 '18 at 13:19

score 3 · Answer 3 · edited Oct 06 '18 at 13:45

3

if documentSnapshot is nullable you should do the following:

if (documentSnapshot != null) {
    if (documentSnapshot.exists()) {
         //exists
    } else {
         //doesn't exist
    }
}

edited Oct 06 '18 at 13:45

AmirReza-Farahlagha

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answered Oct 06 '18 at 12:43

Esmail Elhanash

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7

This works but you dont need multiple if statements. Remove the first statement and replace with this line ``` assert documentSnapshot != null; ``` – Zephania Mwando Nov 12 '20 at 21:02
@esmail What if we want to check if the certain field of the requested document exists or not? I get a bad state exception If I try to get the field of the document if it doesn't exist. Below is the question, kindly give it a minute https://stackoverflow.com/questions/68937605/bad-state-cannot-get-a-field-on-a-documentsnapshotplatform-which-does-not-exist – Faizan Kamal Aug 26 '21 at 11:36

score 0 · Answer 4 · answered Nov 12 '20 at 20:58

Another straightforward fix for such errors would be


DocumentSnapshot documentSnapshot = task.getResult();

 assert documentSnapshot != null;
 if (documentSnapshot.exists()) {

      //Your operation e.g
     ArrayList<String> obj= (ArrayList<String>) documentSnapshot.get("documentObject");
     String s = Objects.requireNonNull(obj).toString();
     Toast.makeText(context, "THIS WORKS "+ s, Toast.LENGTH_SHORT).show();

  } else {
     //Another operation
   Toast.makeText(context, "DOES NOT EXIST", Toast.LENGTH_SHORT).show();
  }

Firestore - Why check if DocumentSnapshot is not null AND call exists?

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