Generic collections

Learn List<T> and Map<K,V> generic collections.

Generic collections

Dart's built-in collections are generic. List<T> gives an ordered sequence, Map<K, V> a key-value store, Set<T> a unique set. The type parameter states which element type the collection holds.

Example: Map<String, int> ages = {'Ayşe': 28, 'Alex': 30};. Here keys are String, values are int. ages['Ayşe'] returns an int; reading the wrong type is prevented by type-safe access. Generic collections fix the element type at compile time; this stops wrong types in loops and method calls.

Nested generic collections

The type parameter of a generic collection can itself be generic. This is the way to build nested structures.

Example: List<List<int>> grid = [[1, 2], [3, 4]];. Here the outer list is a list whose elements are List<int>; a two-dimensional table of numbers. grid[0] gives the first row, grid[0][1] gives the second element of the first row. In the same way structures like Map<String, List<int>> can be built. Nested generics are the powerful tool to express real data models in a type-safe way.

Exercise Work with a generic Map

In main, declare a Map<String, int> ages variable and initialize it with {'Ayşe': 28, 'Alex': 30}. Then call print(ages['Ayşe']).

Show example solution

// Declare a Map<String, int>
void main() {
  Map<String, int> ages = {'Ayşe': 28, 'Alex': 30};
  print(ages['Ayşe']);
}

Test yourself

Which is a generic collection declaration?
- List<String> names = [];
- String s = 'a';
- var z = 10;
- int x = 5;
Show answer

List<String> names = []; — List<String> is a generic collection with a type parameter.

Complete the Map declaration

Map____ ages = {'Ayşe': 28};

Show answer

Map<String, int> ages = {'Ayşe': 28};

Match the collection types

List<T>

Ordered sequence
Map<K, V>

Key-value store
Set<T>

Unique set

Next lesson: Generics vs dynamic

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies