The first way to perform an update to a document would be (from my point of view) something like this

   Document tempDoc = new Document();
   tempDoc.put("value", "new value");
   collection.updateOne(Filters.eq("_id", 1), tempDoc);

but this leads to

Exception in thread "main" java.lang.IllegalArgumentException: Invalid BSON field name value

I have to wrap my update in another document

Document tempDoc = new Document();
    tempDoc.put("value", "new value");
    Document tempUpdateOp = new Document("$set", tempDoc);
    collection.updateOne(Filters.eq("_id", 1), tempUpdateOp);

This is strange because there is no need to wrap it. For the purpose of replacing a document there is another method replaceOne. Why do I have to wrap my update into another document for the updateOne call?