How do you create a custom generic class in Dart, and why would a generic class be preferable to writing several nearly identical, separate classes each hardcoded for a different specific type?

A custom generic class lets you define a reusable class structure once, parameterized with a placeholder type that gets specified later whenever that class is actually used, such as building a generic Result class capable of representing either a successful value of any type or a failure error, and this avoids the need to write and separately maintain several nearly identical classes, each hardcoded for one specific different data type, since the exact same single generic class definition can correctly and safely work with many different concrete types.

class Result<T> {
  final T? value;
  final String? error;
  Result.success(this.value) : error = null;
  Result.failure(this.error) : value = null;
}

final userResult = Result<User>.success(fetchedUser);