DatabaseException: Class java.util.List has generic type parameters, please use GenericTypeIndicator instead [duplicate]

Question

Trying to retrieve a list of string from Firebase Database but i am keep getting this error "DatabaseException: Class java.util.List has generic type parameters, please use GenericTypeIndicator instead". I also have gone through other answers but none of them helped.

My code:


private void incrementCount(){
        reference = FirebaseDatabase.getInstance().getReference("devices").child("deviceIdList");
        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                List<String> list = new ArrayList<>();
                for(DataSnapshot ds : snapshot.getChildren()) {

                    list = ds.getValue(List.class);
                    Log.d("TAG", "size of deviceId Array" + list.size());
                }

                if (!list.contains(androidId)){
                    FirebaseDatabase database = FirebaseDatabase.getInstance();
                    DatabaseReference myRef = database.getReference("posts").child(postId);

                    HashMap<String, Object> hashMap = new HashMap<>();

                    hashMap.put("counter", counter++);
                    myRef.updateChildren(hashMap);
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });


    }

json structure of my db. It only have one item for now:

"devices" : {
    "deviceIdList" : [ "b5ac388d1c1bf0c3", "j4l322n4ş2nş3n3ğ" ]
  },

@FrankvanPuffelen as i mentioned in my question i have already looked up all the answers and none of them worked for me. so this is not a duplicate. — Dinotic
– Dinotic, Commented Jun 16, 2021 at 15:47
You'll need to use a generic type indicator, as shown in here: stackoverflow.com/a/37688361 (and in Abhinav's answer below). If you've also tried that, edit your question to show the code for that please. — Frank van Puffelen
– Frank van Puffelen, Commented Jun 16, 2021 at 16:29

Abhinav Pandey · Accepted Answer · 2021-06-16 15:30:06Z

1

You cannot convert any object to generic List object. You need to define which type of objects the list accepts.

Firebase provides a GenericTypeIndicator to resolve this.

Usage:

GenericTypeIndicator<List<String>> typeIndicator = new GenericTypeIndicator<List<String>>() {};

And then pass typeIndicator as the type argument in your getValue() E g.

list = ds.getValue(typeIndicator)

Should solve your problem.

Thanks

answered Jun 16, 2021 at 15:30

Abhinav Pandey

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2 Comments

Dinotic Over a year ago

Thanks for the ansvver. it now gives this = DatabaseException: Expected a List while deserializing, but got a class java.lang.String.

Abhinav Pandey Over a year ago

Sorry I missed the loop. You ideally don't need to iterate over children of snapshot. You can convert it directly to list.

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