Generic collections
- Dart course
- Intermediate
- Lesson 33/116
- 5 min
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.
Practice 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']).
Starter code
// Declare a Map<String, int>
// print ages['Ayşe']
void main() {
}Sample solution
// Declare a Map<String, int>
void main() {
Map<String, int> ages = {'Ayşe': 28, 'Alex': 30};
print(ages['Ayşe']);
}Expected output
28
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which is a generic collection declaration?
- List<String> names = []; Correct answer
- String s = 'a';
- var z = 10;
- int x = 5;
Explanation: List<String> is a generic collection with a type parameter.
2 Complete the Map declaration
Map____ ages = {'Ayşe': 28};Map<String, int> ages = {'Ayşe': 28};Explanation: Map<String, int> makes keys String and values int.
3 Match the collection types
| Term | Meaning |
|---|---|
List<T> |
Ordered sequence |
Map<K, V> |
Key-value store |
Set<T> |
Unique set |