Dart how can i override a method that in the extension

Question

what I did:

extension BigDate on DateTime {
  String get locatedWeekDay {
    switch (weekday) {
      case DateTime.sunday:
        return "Sun";
      case DateTime.monday:
        return "Mon";
      case DateTime.tuesday:
        return "Tue";
      ......
      default:
        throw Exception();
    }
  }
}
class JapanDate extends DateTime {
  @override
  String get locatedWeekDay {
    switch (weekday) {
      case DateTime.sunday:
        return "日";
      case DateTime.monday:
        return "月";
      ......
      default:
        throw Exception();
    }
  }
}

now I just run this:

DateTime d = JapanDate(2022, 3, 2);
print(d.locatedWeekDay);

it returns me "Wed" oh, can you help me to fix it?

I tried: to add @override to the get method, add the import to the first line.

now I try extend the DateTime and use typedef to rename it. – normidar Mar 02 '22 at 13:14 — normidar, Mar 02 '22 at 13:14

jamesdlin · Accepted Answer · 2023-01-21T19:05:35.330

Extension methods are static; they are compile-time syntactic sugar for an equivalent freestanding function. As such, they are wholly dependent on the static type of the variable (whether explicitly declared or inferred), which in your case is DateTime. They cannot be overridden since overrides involve runtime polymorphism.

What you could do instead is:

Create a base class (or mixin) that provides the locatedWeekDay interface.
Make JapanDate derive from that interface.
Make your extension method check if this implements that interface, falling back to a default implementation if it's an ordinary DateTime object.

abstract class HasLocatedWeekDay {
  String get locatedWeekDay;
}

extension BigDate on DateTime {
  String get locatedWeekDay {
    // Type-promotion for `this` is not yet supported.
    // See: <https://github.com/dart-lang/language/issues/1397>
    final self = this;
    if (self is HasLocatedWeekDay) {
      return self.locatedWeekDay;
    }

    switch (weekday) {
      case DateTime.sunday:
        return "Sun";
      case DateTime.monday:
        return "Mon";
      case DateTime.tuesday:
        return "Tue";
      ......
      default:
        throw Exception();
    }
  }
}

class JapanDate extends DateTime implements HasLocatedWeekDay {
  @override
  String get locatedWeekDay {
    switch (weekday) {
      case DateTime.sunday:
        return "日";
      case DateTime.monday:
        return "月";
      ......
      default:
        throw Exception();
    }
  }
}

score 0 · Answer 2 · answered Mar 02 '22 at 13:16

0

Just don't cast it to DateTime leave it as JapanDate and the code above should work as you expect it to.

JapanDate d = JapanDate(2022, 3, 2);
print(d.locatedWeekDay);

answered Mar 02 '22 at 13:16

Ante Bule

1,834
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but this is not what I wanted. – normidar Mar 02 '22 at 13:19
So what did you want? – Ante Bule Mar 02 '22 at 13:25
I'd suggest just using ```var``` – Andrew Mar 02 '22 at 13:56

Md. Yeasin Sheikh · Answer 3 · 2022-03-02T14:22:54.197

There is no locatedWeekDay method on DateTime. Therefore, it can't be overridden, but you can create a custom class extending this or use an extension as you did for BigDate.

To create a custom class, you need to pass data to super class.

class JapanDate extends DateTime {
  JapanDate(int year,
      [int month = 1,
      int day = 1,
      int hour = 0,
      int minute = 0,
      int second = 0,
      int millisecond = 0,
      int microsecond = 0])
      : super(
            year,
            month = month,
            day = day,
            hour = hour,
            minute = minute,
            second = second,
            millisecond = millisecond,
            microsecond = microsecond);

  String get locatedWeekDay {
    switch (this.weekday) {
      case DateTime.sunday:
        return "日";
      case DateTime.monday:
        return "月";
      //....
      default:
        return "N";
    }
  }
}

Now you can use JapanDate class with locatedWeekDay.

  JapanDate d = JapanDate(2022, 3, 7);

  print(d.locatedWeekDay); ///月

About .weekday on DateTime. It is defined as

 external int get weekday;

External Functions An external function is a function whose body is provided separately from its declaration. An external function may be a top-level function (17), a method

You can follow this What does external mean in Dart?

Check about [External](https://spec.dart.dev/DartLangSpecDraft.pdf#External%20Functions) — Md. Yeasin Sheikh, Mar 02 '22 at 14:34
I know what is external means (I think), I want to extend the locatedWeekDay get method on DateTime for English users. — normidar, Mar 03 '22 at 12:37
You can test the answers on your question, hope some will answer. I thought you like to extend the class, and I've followed that way. — Md. Yeasin Sheikh, Mar 03 '22 at 14:10

Dart how can i override a method that in the extension

3 Answers3