toString method
A string representation of this object.
Some classes have a default textual representation,
often paired with a static parse function (like int.parse).
These classes will provide the textual representation as
their string representation.
Other classes have no meaningful textual representation
that a program will care about.
Such classes will typically override toString to provide
useful information when inspecting the object,
mainly for debugging or logging.
Implementation
@override
String toString() {
  var stringBuffer = StringBuffer();
  stringBuffer.write(_value);
  if (parameters != null && parameters!.isNotEmpty) {
    _parameters!.forEach((name, value) {
      stringBuffer
        ..write('; ')
        ..write(name)
        ..write('=')
        ..write(value);
    });
  }
  return stringBuffer.toString();
}