Collections

Dart has built-in support for list, set, and map collections. To learn more about configuring the types collections contain, check out Generics.

Perhaps the most common collection in nearly every programming language is the array, or ordered group of objects. In Dart, arrays are List objects, so most people just call them lists.

Dart list literals are denoted by a comma separated list of expressions or values, enclosed in square brackets ([]). Here's a simple Dart list:

var list = [1, 2, 3];

You can add a comma after the last item in a Dart collection literal. This trailing comma doesn't affect the collection, but it can help prevent copy-paste errors.

var list = [
  'Car',
  'Boat',
  'Plane',
];

Lists use zero-based indexing, where 0 is the index of the first value and list.length - 1 is the index of the last value. You can get a list's length using the .length property and access a list's values using the subscript operator ([]):

var list = [1, 2, 3];
assert(list.length == 3);
assert(list[1] == 2);

list[1] = 1;
assert(list[1] == 1);

To create a list that's a compile-time constant, add const before the list literal:

var constantList = const [1, 2, 3];
// constantList[1] = 1; // This line will cause an error.

For more information about lists, refer to the Lists section of the dart:core documentation.

A set in Dart is an unordered collection of unique items. Dart support for sets is provided by set literals and the Set type.

Here is a simple Dart set, created using a set literal:

var halogens = {'fluorine', 'chlorine', 'bromine', 'iodine', 'astatine'};

To create an empty set, use {} preceded by a type argument, or assign {} to a variable of type Set:

var names = <String>{};
// Set<String> names = {}; // This works, too.
// var names = {}; // Creates a map, not a set.

Add items to an existing set using the add() or addAll() methods:

var elements = <String>{};
elements.add('fluorine');
elements.addAll(halogens);

Use .length to get the number of items in the set:

var elements = <String>{};
elements.add('fluorine');
elements.addAll(halogens);
assert(elements.length == 5);

To create a set that's a compile-time constant, add const before the set literal:

final constantSet = const {
  'fluorine',
  'chlorine',
  'bromine',
  'iodine',
  'astatine',
};
// constantSet.add('helium'); // This line will cause an error.

For more information about sets, refer to the Sets section of the dart:core documentation.

In general, a map is an object that associates keys and values. Both keys and values can be any type of object. Each key occurs only once, but you can use the same value multiple times. Dart support for maps is provided by map literals and the Map type.

Here are a couple of simple Dart maps, created using map literals:

var gifts = {
  // Key:    Value
  'first': 'partridge',
  'second': 'turtledoves',
  'fifth': 'golden rings'
};

var nobleGases = {
  2: 'helium',
  10: 'neon',
  18: 'argon',
};

You can create the same objects using a Map constructor:

var gifts = Map<String, String>();
gifts['first'] = 'partridge';
gifts['second'] = 'turtledoves';
gifts['fifth'] = 'golden rings';

var nobleGases = Map<int, String>();
nobleGases[2] = 'helium';
nobleGases[10] = 'neon';
nobleGases[18] = 'argon';

Add a new key-value pair to an existing map using the subscript assignment operator ([]=):

var gifts = {'first': 'partridge'};
gifts['fourth'] = 'calling birds'; // Add a key-value pair

Retrieve a value from a map using the subscript operator ([]):

var gifts = {'first': 'partridge'};
assert(gifts['first'] == 'partridge');

If you look for a key that isn't in a map, you get null in return:

var gifts = {'first': 'partridge'};
assert(gifts['fifth'] == null);

Use .length to get the number of key-value pairs in the map:

var gifts = {'first': 'partridge'};
gifts['fourth'] = 'calling birds';
assert(gifts.length == 2);

To create a map that's a compile-time constant, add const before the map literal:

final constantMap = const {
  2: 'helium',
  10: 'neon',
  18: 'argon',
};

// constantMap[2] = 'Helium'; // This line will cause an error.

For more information about maps, refer to the Maps section of the dart:core documentation.

Dart supports the spread operator (...) and the null-aware spread operator (...?) in list, map, and set literals. Spread operators provide a concise way to insert multiple values into a collection.

For example, you can use the spread operator (...) to insert all the values of a list into another list:

var list = [1, 2, 3];
var list2 = [0, ...list];
assert(list2.length == 4);

If the expression to the right of the spread operator might be null, you can avoid exceptions by using a null-aware spread operator (...?):

var list2 = [0, ...?list];
assert(list2.length == 1);

For more details and examples of using the spread operator, see the spread operator proposal.

Dart offers collection if and collection for for use in list, map, and set literals. You can use these operators to build collections using conditionals (if) and repetition (for).

Here's an example of using collection if to create a list with three or four items in it:

var nav = ['Home', 'Furniture', 'Plants', if (promoActive) 'Outlet'];

Dart also supports if-case inside collection literals:

var nav = ['Home', 'Furniture', 'Plants', if (login case 'Manager') 'Inventory'];

Here's an example of using collection for to manipulate the items of a list before adding them to another list:

var listOfInts = [1, 2, 3];
var listOfStrings = ['#0', for (var i in listOfInts) '#$i'];
assert(listOfStrings[1] == '#1');

For more details and examples of using collection if and for, see the control flow collections proposal.