// racy single-check idiom, safe because String is immutable
    String result = toString;
    if (result == null) {
      result = computeToString();
      toString = result;
    }
    return result;
  }

  private String computeToString() {
    StringBuilder builder = new StringBuilder().append(type).append('/').append(subtype);
    if (!parameters.isEmpty()) {
      builder.append("; ");