code: private String joinEnumNames(String separator) {
      E[] enumTypes = enumClass.getEnumConstants();
      StringBuilder sb = new StringBuilder();
      for(int i = 0; i < enumTypes.length; ++i) {
        if(i > 0) {
          sb.append(separator);
        }
        sb.append(enumTypes[i].name());
      }
      return sb.toString();
    }
docs: Utility method for merging possible values into a string for informational
  messages.
  
  @param separator char sequence to use as a separator for enum values.
  @return <code>{VAL1}{separator}{VAL2}{separator}...</code>