code: protected void registerFactorFilter(final FactorFilter<T, V> filter) {
      if (null == filter) {
        throw new IllegalArgumentException("unable to register factor filter. " +
            "The passed comaractor is null or has an invalid weight value.");
      }
  
      // add or replace the filter.
      this.factorFilterList.put(filter.getFactorName(), filter);
      logger.debug(String.format("Factor filter added for '%s'.",
          filter.getFactorName()));
    }
docs: function to register a factorFilter to the internal Map for future reference.
  
  @param factorfilter : the Filter object to be registered.