Collections

Creating a new List

Lists can be created in multiple ways.

The recommended way is to use a List literal:

var vegetables = ['broccoli', 'cabbage'];

The List constructor can be used as well:

var fruits = new List();

If you prefer stronger typing, you can also supply a type parameter in one of the following ways:

var fruits = <String>['apples', 'oranges'];
var fruits = new List<String>();

For creating a small growable list, either empty or containing some known initial values, the literal form is preferred. There are specialized constructors for other kinds of lists:

var fixedLengthList1 = new List(8);
var fixedLengthList2 = new List.filled(8, "initial text");
var computedValues = new List.generate(8, (n) => "x" * n);
var fromIterable = new List<String>.from(computedValues.getRange(2, 5));

See also the Effective Dart style guide about collections.

Creating a new Set

Sets can be created via the constructor:

var ingredients = new Set();
ingredients.addAll(['gold', 'titanium', 'xenon']);

Creating a new Map

Maps can be created in multiple ways.

Using the constructor, you can create a new map as follow:

var searchTerms = new Map();

Types for the key and value can also be defined using generics:

var nobleGases = new Map<int, String>();
var nobleGases = <int, String>{};

Maps can otherwise be created using the map literal:

var map = {
    "key1": "value1",
    "key2": "value2"
};

Map each element in the collection.

All collection objects contain a map method that takes a Function as an argument, which must take a single argument. This returns an Iterable backed by the collection. When the Iterable is iterated, each step calls the function with a new element of the collection, and the result of the call becomes the next element of the iteration.

You can turn an Iterable into a collection again by using the Iterable.toSet() or Iterable.toList() methods, or by using a collection constructor which takes an iterable like Queue.from or List.from.

Example:

main() {
  var cats = [
    'Abyssinian',
    'Scottish Fold',
    'Domestic Shorthair'
  ];

  print(cats); // [Abyssinian, Scottish Fold, Domestic Shorthair]

  var catsInReverse =
  cats.map((String cat) {
    return new String.fromCharCodes(cat.codeUnits.reversed);
  })
  .toList(); // [nainissybA, dloF hsittocS, riahtrohS citsemoD]

  print(catsInReverse);
}

See dartpad example here: https://dartpad.dartlang.org/a18367ff767f172b34ff03c7008a6fa1

Filter a list

Dart allows to easily filter a list using where.

var fruits = ['apples', 'oranges', 'bananas'];
fruits.where((f) => f.startsWith('a')).toList(); //apples

Of course you can use some AND or OR operators in your where clause.