Sets and Maps

Alongside List, Dart's two other core collections solve problems a list handles awkwardly: guaranteeing uniqueness, and looking things up by key instead of by position.

Set: unique, unordered values

A Set stores values with no duplicates and, unlike a List, doesn't guarantee any particular iteration order:

Set<String> tags = {'dart', 'flutter', 'mobile'};

tags.add('dart');   // no effect — already present
tags.add('web');

print(tags);              // {dart, flutter, mobile, web}
print(tags.contains('flutter')); // true
print(tags.length);       // 4

The moment you find yourself calling list.contains(x) before list.add(x) just to avoid duplicates, that's a sign you probably want a Set instead — the uniqueness is enforced for you, and .contains() on a Set is also significantly faster than on a List for large collections, since it doesn't need to scan every element.

Sets also support the mathematical operations you'd expect:

final a = {1, 2, 3};
final b = {2, 3, 4};

print(a.union(b));        // {1, 2, 3, 4}
print(a.intersection(b)); // {2, 3}
print(a.difference(b));   // {1}

Map: key-value pairs

A Map associates each key with a value, and looks up by key in constant time rather than scanning:

Map<String, int> ages = {
  'Alice': 30,
  'Bob': 25,
};

ages['Charlie'] = 35;      // add a new entry
ages['Alice'] = 31;        // update an existing one

print(ages['Bob']);        // 25
print(ages['Zoe']);        // null — missing key, not an error
print(ages.containsKey('Alice')); // true

Reading a missing key returns null rather than throwing, which is why ages['Zoe'] has type int?, not int — the compiler is telling you upfront that a lookup might come back empty.

Safe lookups with default values

putIfAbsent and the ?? operator both handle the "get, or fall back" pattern cleanly:

final scores = <String, int>{'Dara': 90};

int darasScore = scores['Dara'] ?? 0;   // 90
int leaScore = scores['Lea'] ?? 0;      // 0 — key doesn't exist

scores.putIfAbsent('Lea', () => 0);     // only inserts if 'Lea' isn't already a key

Iterating a Map

final prices = {'apple': 1.5, 'bread': 3.2};

prices.forEach((item, price) {
  print('$item costs \$$price');
});

for (final key in prices.keys) {
  print(key);
}

for (final value in prices.values) {
  print(value);
}

Choosing between List, Set, and Map

• Reach for List when order matters and duplicates are fine (a queue of tasks, a sequence of steps).
• Reach for Set when you need uniqueness and don't care about order (tags, visited IDs, distinct categories).
• Reach for Map when you need to look something up by a meaningful key rather than by position (a user ID to a user record, a word to its count).

Picking the right one up front usually removes an entire category of bugs — like accidental duplicates in a list that should have been a set — before they can happen.