Dart: lists of supertype takes subtype only at runtime - dart

Question

I ran into an issue similar to this:
void main() {
_buildMixedList([1,2.3,4,5.6,7.6,8]);
_buildHomogeneousList([1,2,4,5,7,8]);
}
abstract class NumberWrapper {}
class DoubleWrapper extends NumberWrapper{
final double myDouble;
DoubleWrapper(this.myDouble);
}
class IntWrapper extends NumberWrapper{
final int myInt;
IntWrapper(this.myInt);
}
List<NumberWrapper?> _buildMixedList(List<dynamic> numbers) {
List<NumberWrapper?> wrappers = numbers.map((number) {
if(number is int){
return IntWrapper(number);
}
if(number is double){
return DoubleWrapper(number);
}
return null;
}).toList();
wrappers.add(DoubleWrapper(0.2));
return wrappers;
}
List<NumberWrapper?> _buildHomogeneousList(List<dynamic> numbers) {
List<NumberWrapper?> wrappers = numbers.map((number) {
if(number is int){
return IntWrapper(number);
}
return null;
}).toList();
wrappers.add(DoubleWrapper(0.2));
return wrappers;
}
As you can see, the two methods are doing something similar (adding object of different types to a list). The first one adds different objects inside a map() function and the other adds only one type in map() and then adds another after.
The second one throws this error:
: TypeError: Instance of 'DoubleWrapper': type 'DoubleWrapper' is not a subtype of type 'IntWrapper?'Error: TypeError: Instance of 'DoubleWrapper': type 'DoubleWrapper' is not a subtype of type 'IntWrapper?'
As if the list is being changed to List<IntWrapper?> just because we only added IntWrappers in the map().
I wrote this test code after encountering this in one of my projects, so it's not representative of a real case. I tried it on dartPad.
Coming from a java background I was expecting the second method to work. Is it a bug or is it intended? If intended, why is that so?

Answer 1

Your problem is that there are a difference between the type of the variable and the type of the object which you are pointing to.
So in this case:
List<NumberWrapper?> wrappers = numbers.map((number) {
if(number is int){
return IntWrapper(number);
}
return null;
}).toList();
What you are actually are doing is creating a List<IntWrapper?> which you are using a variable of the type List<NumberWrapper?> to point at. Why? Because the type of the variable in this case does not change the type of the returned List from toList() (which type is determined by what type map() returns).
The reason the type is List<IntWrapper?> is because Dart are trying to be smart about automatically assigning the type. In this case, the analyzer can see you List will only contain IntWrapper or null.
I think the best solution here is to rewrite this part to something like this:
List<NumberWrapper?> _buildHomogeneousList(List<num> numbers) {
final wrappers = <NumberWrapper?>[
for (final number in numbers)
if (number is int) IntWrapper(number) else null
];
wrappers.add(DoubleWrapper(0.2));
return wrappers;
}
By using the [] syntax to create the List, it is easier to specify the type you want the List to be.
Alternative, you can do this where we add the expected type to the map method:
List<NumberWrapper?> _buildHomogeneousList(List<num> numbers) {
List<NumberWrapper?> wrappers = numbers.map<NumberWrapper?>((number) {
if (number is int) {
return IntWrapper(number);
}
return null;
}).toList();
wrappers.add(DoubleWrapper(0.2));
return wrappers;
}