code: public List distinct(final String fieldName, final ReadPreference readPreference) {
          return distinct(fieldName, new BasicDBObject(), readPreference);
      }
docs: Find the distinct values for a specified field across a collection and returns the results in an array.
  
  @param fieldName      Specifies the field for which to return the distinct values
  @param readPreference {@link ReadPreference} to be used for this operation
  @return a List of the distinct values
  @mongodb.driver.manual reference/command/distinct Distinct Command