code: @Nullable
      public DBObject findOne(final DBObject query, final DBObject projection, final ReadPreference readPreference) {
          return findOne(query, projection, null, readPreference);
      }
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 readPreference {@link ReadPreference} to be used for this operation
  @return A document that satisfies the query specified as the argument to this method.
  @mongodb.driver.manual tutorial/query-documents/ Querying