Method that accepts an instance of the concrete class - dart

Question

I have an abstract class Filter that is implemented by various transaction filters (e.g. DateFilter, AccountFilter, etc)
abstract class TransactionFilter {
String asString();
bool operator >(TransactionFilter other);
}
// Concrete implementations
class DateFilter implements TransactionFilter {
DateTime startDate, endDate;
DateFilter(this.startDate, this.endDate);
// ⏹ ERROR
// 'DateFilter.>' ('bool Function(DateFilter)') isn't a valid override of
// 'TransactionFilter.>' ('bool Function(TransactionFilter)')
bool operator >(DateFilter other) =>
startDate.isBefore(other.startDate) && endDate.isAfter(other.endDate);
}
class AccountFilter implements TransactionFilter {
List<int> accounts;
DateFilter(this.accounts);
// Same error as above
bool operator >(AccountFilter other) =>
other.any((e) => !accounts.contains(e))
}
This seems to be because Dart doesn't consider DateFilter and AccountFilter to be equivalent to TransactionFilter.
EDIT: I'd like the > operator to strictly be used by classes of the same subtype (e.g. comparing two DateFilter instances). How do I do this in the given example?

Answer 1

What you are doing does not make type-safe sense. Your TransactionFilter specifies that all classes that inherit from TransactionFilter do have a > operator which can compare against any object of the type TransactionFilter.
So when you in a subclass define the operator as:
bool operator >(DateFilter other)
Or:
bool operator >(AccountFilter other)
This is not allowed since DateFilter and AccountFilter is more restrictive than TransactionFilter (your DateFilter class are only allowed to compare against another DateFilter and not e.g. AccountFilter).
Update
Here is an example of how to do it instead using generics:
abstract class TransactionFilter<T extends TransactionFilter<T>> {
String asString();
bool operator >(T other);
}
// Concrete implementations
class DateFilter implements TransactionFilter<DateFilter> {
DateTime startDate, endDate;
DateFilter(this.startDate, this.endDate);
bool operator >(DateFilter other) =>
startDate.isBefore(other.startDate) && endDate.isAfter(other.endDate);
#override
String asString() {
// TODO: implement asString
throw UnimplementedError();
}
}
class AccountFilter implements TransactionFilter<AccountFilter> {
List<int> accounts;
AccountFilter(this.accounts);
bool operator >(AccountFilter other) =>
other.accounts.any((e) => !accounts.contains(e));
#override
String asString() {
// TODO: implement asString
throw UnimplementedError();
}
}