code: @Nullable
      public DBObject findOne(final DBObject query) {
          return findOne(query, null, null, getReadPreference());
      }
docs: Get a single document from collection.
  
  @param query the selection criteria using query operators.
  @return A document that satisfies the query specified as the argument to this method.
  @mongodb.driver.manual tutorial/query-documents/ Querying