Lists & Collections

Real-world analogy

What is it?

Dart has three main collection types: List (ordered, indexed, allows duplicates), Map (key-value pairs, unique keys), and Set (unordered, unique values). Lists are the most common. All three support iteration, transformation, and null-safe access.

Real-world relevance

Lists of products in a shopping app, Maps of user profiles keyed by ID, Sets of unique tags or categories. Collections are the backbone of data management in every app. The team_mvp_kit project uses List for API responses, Map for JSON parsing, and Set for unique identifiers.

Key points

• Creating Lists — A List is an ordered collection of items. Create with square brackets []. Items are accessed by index starting at 0. Lists can grow and shrink. You can specify the type with List for safety.
• Adding and Removing — Use add() to append one item, addAll() to append multiple, insert() to add at a specific position. Remove with remove() (by value), removeAt() (by index), removeLast(), and clear() (remove all).
• List Methods — Lists have many useful methods: contains() checks membership, indexOf() finds position, sort() orders items, reversed returns a reversed view, sublist() extracts a portion, and join() combines into a string.
• map, filter, where — map() transforms each item. where() (Dart's filter) keeps items that pass a test. These return Iterables, so call .toList() to get a List back. These functional methods are powerful and widely used.
• Maps - Key-Value Pairs — A Map stores key-value pairs. Each key is unique and maps to a value. Create with curly braces {}. Access values with map[key]. Perfect for lookups, configurations, and structured data.
• Map Operations — Add entries with map[key] = value. Remove with remove(). Check existence with containsKey() and containsValue(). Get all keys with .keys and all values with .values.
• Sets - Unique Collections — A Set is an unordered collection with NO duplicates. Adding an existing item is silently ignored. Perfect for tracking unique tags, visited pages, or selected items.
• Spread Operator and Collection If/For — The spread operator (...) merges collections. Collection if adds items conditionally. Collection for adds items from a loop. These make building collections clean and expressive.
• Null-Safe Collection Access — Use the ?[] operator for null-safe map access and firstOrNull/lastOrNull for safe list access. This prevents crashes when dealing with potentially missing data.

Code example

void main() {
  // === LISTS ===
  var fruits = ['apple', 'banana', 'cherry', 'date'];

  // Access and modify
  print(fruits[0]);           // 'apple'
  fruits.add('elderberry');
  fruits.removeAt(1);         // removes 'banana'
  print(fruits);              // [apple, cherry, date, elderberry]

  // Transform with map and where
  var upperFruits = fruits.map((f) => f.toUpperCase()).toList();
  print(upperFruits);         // [APPLE, CHERRY, DATE, ELDERBERRY]

  var longNames = fruits.where((f) => f.length > 5).toList();
  print(longNames);           // [cherry, elderberry]

  // === MAPS ===
  var student = <String, dynamic>{
    'name': 'Ali',
    'age': 14,
    'grades': [90, 85, 92],
  };

  print(student['name']);     // 'Ali'
  student['school'] = 'ABC Academy';  // add new key
  print(student.keys.toList());

  // Iterate a Map
  student.forEach((key, value) {
    print('$key: $value');
  });

  // === SETS ===
  var visited = <String>{'home', 'about', 'contact'};
  visited.add('home');        // ignored - duplicate!
  print(visited.length);     // 3

  // Collection if and for
  var isVIP = true;
  var menu = [
    'Dashboard',
    'Profile',
    if (isVIP) 'VIP Lounge',
  ];
  print(menu);

  var doubled = [for (var i = 1; i <= 5; i++) i * 2];
  print(doubled);             // [2, 4, 6, 8, 10]
}

Line-by-line walkthrough

1. 1. The main function starts
2. 2. A comment header for the Lists section
3. 3. Creating a list of four fruits
4. 4.
5. 5. A comment for access and modify operations
6. 6. Accessing the first item at index 0 -- prints 'apple'
7. 7. Adding 'elderberry' to the end of the list
8. 8. Removing the item at index 1 (banana)
9. 9. Printing the updated list
10. 10.
11. 11. A comment for transformations
12. 12. map() creates a new list with each fruit in UPPERCASE
13. 13. Printing the uppercased list
14. 14.
15. 15. where() keeps only fruits with more than 5 characters
16. 16. Printing the filtered result
17. 17.
18. 18. A comment header for the Maps section
19. 19. Creating a Map with String keys and dynamic values
20. 20. Storing the name 'Ali'
21. 21. Storing the age 14
22. 22. Storing a list of grades as a value
23. 23. Closing the map literal
24. 24.
25. 25. Accessing the 'name' key -- prints 'Ali'
26. 26. Adding a new key 'school' with value 'ABC Academy'
27. 27. Printing all keys of the map
28. 28.
29. 29. A comment for iterating a Map
30. 30. forEach loops through every key-value pair
31. 31. Printing each key and value using string interpolation
32. 32. Closing the forEach
33. 33.
34. 34. A comment header for the Sets section
35. 35. Creating a Set with three unique page names
36. 36. Adding 'home' again -- ignored because it already exists!
37. 37. Printing the length -- still 3, duplicate was ignored
38. 38.
39. 39. A comment for collection if and for
40. 40. A boolean flag for VIP status
41. 41. Creating a list with collection if
42. 42. Always includes 'Dashboard'
43. 43. Always includes 'Profile'
44. 44. Only includes 'VIP Lounge' if isVIP is true
45. 45. Closing the list
46. 46. Printing the menu
47. 47.
48. 48. Collection for generates a list of doubled numbers
49. 49. Printing the result: [2, 4, 6, 8, 10]
50. 50. Closing the main function

Spot the bug

void main() {
  var fruits = ['apple', 'banana'];
  print(fruits[2]);

  var scores = {'math': 90, 'science': 85};
  scores.add('english', 92);
}

Explain like I'm 5

Fun fact

Hands-on challenge

More resources

• Dart Collections (Dart Official)
• Dart List Class (Dart API)
• Iterable Collections (Dart Official)