How to count unique _id with ObjectId field in mongodb using Java driver 3.4+?

Question

I need to count documents unique _id with ObjectId field. My documents are like:

So, I need to count these documents by unique _id and then replace everyone using replaceOne method (without touching _id of the document, because _id field is immutable ) to another documents.

I know that I can realize it with a few ways:

1) Using Iterator of FindIterable to get the count of the documents:

FindIterable findIterable = collection.find(Filters.eq("_id" (this is field), ... (but how I need to write value here if each document has unique _id?) ));
        Iterator iterator = findIterable.iterator();
        int count = 0;
        while (iterator.hasNext()) {
            iterator.next();
            count++;
        }

2) or simply create variable as long count = collection.count(Document.parse("_id : (this is field) " + ...(but how I need to write value here if each document has unique _id?)));

Correct me, please, or advise me the way how can I count these documents properly by unique _id to replace then? I appreciate any help.

Answer 1

As mentioned by @mtj,

Ids are unique by design, so the number of unique _ids is equal to the number of documents in the collection. Or in other words: use collection.count()