code: @Nullable
      public DBObject findOne(final DBObject query, final DBObject projection, final DBObject sort) {
          return findOne(query, projection, sort, getReadPreference());
      }
docs: Get a single document from collection.
  
  @param query      the selection criteria using query operators.
  @param projection specifies which fields MongoDB will return from the documents in the result set.
  @param sort       A document whose fields specify the attributes on which to sort the result set.
  @return A document that satisfies the query specified as the argument to this method.
  @mongodb.driver.manual tutorial/query-documents/ Querying