Dart generics method not recognizing type - flutter

Question

I have an abstract base class validator with a method which takes a generic type as parameter.
I will be passing generic type parameter to base class from the subclass inheriting the base class.
Base Class:
abstract class BaseValidator {
bool isValid<T>(T obj);
}
Child Class:
class IPv4Validator extends BaseValidator{
final IPV4_REGEX = "^((25[0-5]|(2[0-4]|1d|[1-9]|)d).?\b){4}\$";
#override
bool isValid<String>(String obj) {
bool hasMatch = RegExp(IPV4_REGEX).hasMatch(obj);
return hasMatch;
}
}
Here hasMatch takes in non nullable string. When I directly pass some string hasMatch doesn't throw an error.
But when I try to pass the generic value in the method parameter, it shows an error.
The argument type 'String' can't be assigned to the parameter type
'String'.
I couldn't able to understand why generic type is not accepting, even though its compile-time type.

Answer 1

The following code solves this particular problem. But it may be different from what you intended to implement. On the other hand, the code will be cleaner if you create a new concrete class for different data types.
abstract class BaseValidator<T> {
bool isValid(T obj);
}
class IPv4Validator extends BaseValidator<String>{
final IPV4_REGEX = "^((25[0-5]|(2[0-4]|1d|[1-9]|)d).?\b){4}\$";
#override
bool isValid(String obj) {
bool hasMatch = RegExp(IPV4_REGEX).hasMatch(obj);
return hasMatch;
}
}
Explanation.
In the line class IPv4Validator extends BaseValidator<String> we are not declaring a new class BaseValidator, it is already declared as BaseValidator<T>. Here we are inheriting the specialization of the existing generic class BaseValidator. While in the line bool isValid<String>(String obj), we declare a new function, so the compiler understands it as if we were declaring a new generic function with a parameter type named String. So, here bool isValid<String>(String obj) is equivalent to bool isValid<T>(T obj), just instead of name T we used name String, which is not an object String.

Answer 2

another fix that you can do is to use the covariant keyword, to implement that, try this:
abstract class BaseValidator<T> {
bool isValid(T obj);
}
class IPv4Validator extends BaseValidator {
final IPV4_REGEX = "^((25[0-5]|(2[0-4]|1d|[1-9]|)d).?\b){4}\$";
#override
bool isValid(covariant String obj) {
bool hasMatch = RegExp(IPV4_REGEX).hasMatch(obj);
return hasMatch;
}
}