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Common Collection Mistakes

Avoid these common pitfalls when working with Dart collections.

1. Modifying a Collection While Iterating​

❌ Bad Code​

final numbers = [1, 2, 3, 4, 5];
for (final n in numbers) {
  if (n.isEven) {
    numbers.remove(n); // Throws ConcurrentModificationError!
  }
}

✅ Solution​

Use removeWhere() or iterate over a shallow copy:

// Option A: removeWhere
numbers.removeWhere((n) => n.isEven);

// Option B: Shallow copy iteration
for (final n in [...numbers]) {
  if (n.isEven) numbers.remove(n);
}

2. Using {} Intending to Create a Set​

❌ Bad Code​

final mySet = {}; // Creates a Map<dynamic, dynamic>, NOT a Set!
print(mySet.runtimeType); // _Map<dynamic, dynamic>

✅ Solution​

final mySet = <String>{}; // Explicit type parameter
final set2 = <int>{1, 2, 3};

3. Assuming List.contains() is Fast​

❌ Bad Code​

final hugeList = List.generate(100000, (i) => i);

// O(n) scan every single check - very slow in loops!
for (var i = 0; i < 1000; i++) {
  if (hugeList.contains(i)) { ... }
}

✅ Solution​

Convert to a Set for O(1) checks:

final hugeSet = hugeList.toSet();
for (var i = 0; i < 1000; i++) {
  if (hugeSet.contains(i)) { ... }
}

4. Misunderstanding List.filled() Reference Sharing​

❌ Bad Code​

// All 3 elements point to the SAME List instance!
final grid = List.filled(3, <int>[]);
grid[0].add(99);

print(grid); // [[99], [99], [99]] - unexpected mutation!

✅ Solution​

Use List.generate() to produce fresh instances:

final grid = List.generate(3, (_) => <int>[]);
grid[0].add(99);

print(grid); // [[99], [], []]

5. Expecting Equality (==) to Check Collection Contents​

❌ Bad Code​

final listA = [1, 2, 3];
final listB = [1, 2, 3];

if (listA == listB) {
  // Never executed! Reference comparison fails.
}

✅ Solution​

Use package:collection:

import 'package:collection/collection.dart';

if (const ListEquality().equals(listA, listB)) {
  // Executed successfully
}

6. Null Safety Pitfalls with Map Lookups​

❌ Bad Code​

final Map<String, int> scores = {'Alice': 100};
int bobScore = scores['Bob']!; // Throws Null check operator used on a null value!

✅ Solution​

int bobScore = scores['Bob'] ?? 0;

// Or test existence
if (scores.containsKey('Bob')) {
  print(scores['Bob']);
}

Summary​

  • Am I mutating a list inside a for-in loop? (Use removeWhere)
  • Did I write {} expecting a Set? (Use <T>{})
  • Am I using List.contains inside a tight loop? (Convert to Set)
  • Did I create nested objects with List.filled? (Use List.generate)
  • Am I comparing collections with ==? (Use package:collection)