code: public static String getBundle (String qualifiedKey)
      {
          if (!qualifiedKey.startsWith(QUAL_PREFIX)) {
              throw new IllegalArgumentException(
                  qualifiedKey + " is not a fully qualified message key.");
          }
  
          int qsidx = qualifiedKey.indexOf(QUAL_SEP);
          if (qsidx == -1) {
              throw new IllegalArgumentException(
                  qualifiedKey + " is not a valid fully qualified key.");
          }
  
          return qualifiedKey.substring(QUAL_PREFIX.length(), qsidx);
      }
docs: Returns the bundle name from a fully qualified message key.
  
  @see #qualify