TopolearnCollections hold almost all app data, and collection-if/for is how Flutter builds lists of widgets.
Dart's core collections:
List<T>— ordered, zero-indexed:var nums = [1, 2, 3];.Set<T>— unique items:var tags = {'dart', 'flutter'};.Map<K, V>— key/value pairs:var ages = {'ana': 31, 'bo': 25};.
An empty {} is a Map; write <String>{} for an empty Set. Type arguments can be explicit: <int>[].
Lists support add, addAll, insert, removeAt, length, first, last, contains, indexOf, sort. Out-of-range indexing throws a RangeError. Map lookup ages['zoe'] returns null (the value type becomes nullable); use containsKey, putIfAbsent or update(key, f, ifAbsent: ...). Iterate maps with for (final MapEntry(:key, :value) in ages.entries) or ages.forEach.
Collection literals have special syntax:
- Spread:
[...a, ...b], and null-aware...?maybeList. - Collection if:
[ 'home', if (loggedIn) 'profile' ]. - Collection for:
[for (final n in nums) n * n].
This lets Flutter build widget lists declaratively: children: [Header(), if (showBanner) Banner(), for (final m in messages) Tile(m)].
Collections are mutable unless created const or with List.unmodifiable. A final variable can still have its list contents changed; only reassignment is blocked.
Example
void main() {
final fruits = ['apple', 'pear'];
fruits.add('kiwi'); // final list, mutable contents
print(fruits.length); // 3
final emptySet = <String>{}; // {} alone would be a Map
emptySet.addAll(['a', 'a', 'b']);
print(emptySet); // {a, b}
final stock = {'apple': 3, 'pear': 0};
stock.update('kiwi', (n) => n + 1, ifAbsent: () => 1);
print(stock['plum']); // null
print(stock); // {apple: 3, pear: 0, kiwi: 1}
const showAll = false;
final menu = [
'home',
if (showAll) 'admin',
for (final f in fruits) f.toUpperCase(),
];
print(menu); // [home, APPLE, PEAR, KIWI]
}Remember
[]List,{a, b}Set,{k: v}Map;{}alone is a Map- Collection if/for and spread
...build lists declaratively - Map lookup returns a nullable value
Watch out for
- Declaring
var s = {};expecting a Set and getting aMap<dynamic, dynamic>.