code: public static EnumMap<FacebookParam, CharSequence> extractFacebookParams(Map<CharSequence, CharSequence> reqParams) {
      if (null == reqParams)
        return null;
  
      EnumMap<FacebookParam, CharSequence> result =
        new EnumMap<FacebookParam, CharSequence>(FacebookParam.class);
      for (Map.Entry<CharSequence, CharSequence> entry: reqParams.entrySet()) {
        FacebookParam matchingFacebookParam = FacebookParam.get(entry.getKey().toString());
        if (null != matchingFacebookParam) {
          result.put(matchingFacebookParam, entry.getValue());
        }
      }
      return result;
    }
docs: Out of the passed in <code>reqParams</code>, extracts the parameters that
  are known FacebookParams and returns them.
  @param reqParams a map of request parameters to their values
  @return a map suitable for being passed to verify signature