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Dart Collections

Built-in Collections

Dart language has special support for the following collections:

• lists
• sets
• maps

Lists

A list is an enumeration of values. The list literal is an enumeration of literal values or expressions separated by comma and enclosed in square brackets. It looks like a List in python or like an Array in JavaScript:

//declare a List object
void main() {
    var lst = [1,2,3,];
    print(lst);    // [1,2,3]
    print(lst[0]); // 1 (first element)
    print(lst[1]); // 2 (second element)
    print(lst[2]); // 3 (last element)

    // check length property
    assert(list.length == 3);

    // check each element
    assert(lst[0] == 1); // check
    assert(lst[1] == 2); // check
    assert(lst[2] == 3); // check
}

Notes:

• Elements of list above will be all integers;
• Element index start from 0 to length-1;
• List index can not be negative;
• Last coma is optional, but is good to know compiler ignores it.

Sets

This is a special collection that has a wel know theory we learn in school. We know that a "set" is a collection of unique elements. That is, each element is usually a number or a string.

Pattern:

There are several ways to define a set. Most common is to use curly brackets {} and separate elements by comma:

//an empty set of strings
var persons  = <Strings>{};

//also an empty set of strings
Set<Strings> names = {};

//a set of strings with 3 elements
var fruits = {'apple', 'orange', 'banana'};

//testing some methods
assert(fruits.length == 3);

fruits.add("lemon")
assert(fruits.length == 4);

Maps

A map is a special set that store (key:value) pairs. Therefore in some other languages a map is sometimes called dictionary and sometimes hash table. The secret of maps is that "key" is unique, but unlike a set, the key point to some useful data. This property makes maps more useful than sets.

Pattern 1:

As for "sets" also we have several ways to define a map.

//an empty map
var gifts = Map();

//new elements
var gifts['doru']   = "pony";
var gifts['marian'] = "bike";
var gifts['dani']   = "bike";

//testing some methods
assert(gifts.length == 3);

//testing some methods
print(gifts['doru']);   //expected pony
print(gifts['marian']); //expected bike
print(gifts['dani']);   //expected bike

Notes:

• In this example, "marian" and "dani" both get a "bike". That is, the value is not unique, only the key must be unique.
• Though is not a vector or a list, you can put strings in brackets [''] to assign new elements into the map.

Pattern 2:

Second way to define a set is by using curly bracket literals {...}.

//an empty map
var gifts = {
    'doru'   : "pony",
    'marian' : "bike"
    'dani'   : "scutter"
};

Pattern 3:

Since Map is a generic class, we can use the more explicit notation that should rarely be used. Dart does a good job using type inference so we don't have to use this notation that is actually the only one available in Java for example.

//legacy map declarations
Map<String,String> gifts1 = {};
var gifts2 = <String,String> {};

Note: In Java we need "new" keyword to create an object. In Dart this keyword is optional. So we can call a constructor like a normal function, using name of the class follow by arguments.

Read next: Control Flow