in dart, which is better List.generate or List.of or List.from

Question

in Dart Documentation we have three iterable options to choose from to create lists .which one is best suited for what purpose and which is more effiecient for what purpose?

Answer 1

Basically, you should never use any of those constructors.

Each has its use, but most of those uses can be written using list literals now.

The List.generate is a pendant to List.filled. The latter creates a list filled with the same value in each slot, the former allows you to compute a new value for each slot. With collection-for, I'd probably write:

 var newList = [for (var i = 0; i < 10; i++) compute(i)];

instead of

 var newList = List.generate(10, compute);

(even more so if I can inline the compute function). The one case where generate makes sense is to create a fixed-length list. The literal cannot do that.

I'd also say that you should never use List.of(something). Use something.toList() or [...something] instead. If you need to up-cast, say create a List<num> from an Iterable<int>, you can do <num>[...something], but you can't use toList. If you need to make a fixed-length List<num>, ... then I think List<num>.of(something, growable: false) is actually the simplest solution.

The only reason to use List.from is when the original iterable does not have as tight a type as needed. If you know that your Iterable<num> contains only integers, you might want to do List<int>.from(iterable). You can also do iterable.cast<int>().toList() or [for (var v in iterable) v as int], but List.from can be shorter. Always provide a type variable to List.from.

So, in general: Only use one of these constructors if you need a fixed-length list (passing growable: false), or if you want to down-cast the elements using List.from, and then always provide the type argument.

Otherwise use list literals. That's probably going to be more efficient too because it avoids some function calls.