main.dart

import 'package:flutter/foundation.dart';

extension NumExtension on num {
  bool greaterThan(num other) => this > other;
}

class MyClass {
  final bool a;
  final bool b;
  final bool c;

  MyClass({
    @required this.a,
    @required this.b,
    @required this.c,
  });

  static MyClass fromJson(Map<String, Object> map) {
    return MyClass(
      a: (map['a'] as int)?.greaterThan(0),
      b: (map['b'] as int)?.greaterThan(0),
      c: (map['c'] as int)?.greaterThan(0),
    );
  }

  @override
  String toString() => 'a=$a b=$b c=$c';
}

void main() {
  final map = {
    'a': 0,
    'b': 1,
    'c': null,
  };
  final obj = MyClass.fromJson(map);
  print(obj);
}