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Dart – Collections
  • Last Updated : 23 Feb, 2021
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Collections are groups of objects that represent a particular element. The dart::collection library is used to implement the collection in dart. There are a variety of collections available in dart.

Some of the classes of Dart Collection are –

  • List<E>: A List is an ordered group of objects.
  • Set<E>: Collection of objects in which each of the objects occurs only once
  • Map<K, V>: Collection of objects that has a simple key/value pair-based object. The key and value of a map can be of any type.
  • Queue<E>: A Queue is a collection that can be manipulated at both ends. One can iterate over a queue with an Iterator or using forEach.
  • DoubleLinkedQueue<E>: Doubly-linked list based on the queue data structure.
  • HashMap<K, V>: Map based on hash table i.e unordered map.
  • HashSet<E>: Set based on hash table i.e unordered set.
  • LinkedHashMap<K, V>: Similar to HashMap but based on LinkedList.
  • LinkedHashSet<E>: Similar to HashSet but based on LinkedList.
  • LinkedList<E extends LinkedListEntry<E>>: It is a specialized double-linked list of elements.
  • LinkedListEntry<E extends LinkedListEntry<E>>: An element of a LinkedList.
  • MapBase<K, V>: This is the base class for Map.
  • UnmodifiableListView<E>: An unmodifiable List view of another List.
  • UnmodifiableMapBase<K, V>: Basic implementation of an unmodifiable Map.
  • UnmodifiableMapView<K, V>: Unmodifiable view of the map.

We will be discussing the 4 basic collections with examples here.

1. List<E>

The list is an ordered group of objects where each object is from one specific type. To define a list in dart, specify the object type inside the angled brackets (<>) as shown below:

List<String> fruits = ["Mango", "Apple", "Banana"]

Example: 



Here we have defined a list and performed some common operations along with some basic commonly used methods.

Dart




void main() {
  // creating a new empty List 
  List geekList = new List();
    
  // We can also create a list with a predifined type
  // as List<int> sampleList = new List()
  // and also define a list of fixed length as
  // List<int> sampleList = new List(5)
    
  // Adding an element to the geekList
  geekList.addAll([1,2,3,4,5,"Apple"]);
  print(geekList);
    
  // Looping over the list
  for(var i = 0; i<geekList.length;i++){
    print("element $i is ${geekList[i]}");
  }
    
  // Removing an element from geekList by index
  geekList.removeAt(2);
    
  // Removing an element from geekList by object
  geekList.remove("Apple");
  print(geekList);
    
  // Return a reversed version of the list
  print(geekList.reversed);
    
  // Checks if the list is empty
  print(geekList.isEmpty);
    
  // Gives the first element of the list
  print(geekList.first);
    
  // Reassigning the geekList and creating the
  // elements using Iterable 
  geekList = Iterable<int>.generate(10).toList();
  print(geekList);
}

Output:

[1, 2, 3, 4, 5, Apple]
element 0 is 1
element 1 is 2
element 2 is 3
element 3 is 4
element 4 is 5
element 5 is Apple
[1, 2, 4, 5]
(5, 4, 2, 1)
false
1
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

2. Set<E>

Sets are one of the essential part of Dart Collections. A set is defined as an unordered collection of unique objects. To define a set follow the below:

Set fruits = Set.from("Mango", "Apple", "Banana")

Example:

As discussed earlier a set stores a group of objects that are not repeating. A sample program is shown below.

Dart




void main() {
    
  // Initializing the Set and Adding the values
  Set geekSet = new  Set(); 
  geekSet.addAll([9,1,2,3,4,5,6,1,1,9]);
    
    
  // Looping over the set
  for(var el in geekSet){
    print(el);
  }
    
  // length of the set.
  print('Length: ${geekSet.length}');
    
  // printing the first element in the set
  print('First Element: ${geekSet.first}');
    
  // Deleting an element not present. No Change
  geekSet.remove(10);
    
  // Deleting an element 9
  geekSet.remove(9);
  print(geekSet);
}

Output:



9
1
2
3
4
5
6
Length: 7
First Element: 9
{1, 2, 3, 4, 5, 6}

3. Map<K, V>

In Dart, Maps are unordered key-value pair collection that sets an associate key to the values within. To define a Map, specify the key type and the value type inside the angle brackets(<>) as shown below:

Map<int, string> fruits = {1: "Mango", 2:"Apple", 3:"Banana"}

Example:

The map collection stores the objects as a key-value pair. An example is shown below.

Dart




void main() {
    
  // Intializing the map with sample values.
  var geekMap = {1:"Apple",2:"Mango",3:"Banana"};
  print(geekMap);
    
  // Adding elements by different methods.
  geekMap.addAll({4:'Pineapple',2:'Grapes'});
  geekMap[9]="Kiwi";
  print(geekMap);
    
  // printing key and values
  print('Keys: ${geekMap.keys} \nValues: ${geekMap.values}');
    
  // removig an element from the map by its key
  geekMap.remove(2);
    
  // printing the map and its length
  print('{$geekMap} length is ${geekMap.length}');
}

Output:

{1: Apple, 2: Mango, 3: Banana}
{1: Apple, 2: Grapes, 3: Banana, 4: Pineapple, 9: Kiwi}
Keys: (1, 2, 3, 4, 9) 
Values: (Apple, Grapes, Banana, Pineapple, Kiwi)
{{1: Apple, 3: Banana, 4: Pineapple, 9: Kiwi}} length is 4

4. Queue<E>

Queues are used to implement FIFO(First in First Out) collection. This collection can be manipulated from both ends. A queue in dart is defined as follows:

Queue<String> queue = new Queue("Mango", "Apple","Banana")

Example:

Dart




import 'dart:collection'
void main() {
    
  // Initializing the Set and Adding the values
  // We can also initialize a queue of a specific type
  // as Queue<int> q = new Queue();
  var geekQueue = new  Queue(); 
  geekQueue.addAll([9,1,2,3,4,5,6,1,1,9]);
    
  // Adds Element to the Start of the Queue
  geekQueue.addFirst("GFG"); 
    
  // Adds Element to the End of the Queue
  geekQueue.addLast("GFG2"); 
  print(geekQueue);
    
  // Removes the first Element
  geekQueue.removeFirst();
  print(geekQueue);
    
  // Removes the Last Element
  geekQueue.removeLast();
  print(geekQueue);
    
  // printing the first element in the set
  print('First Element: ${geekQueue.first}');
    
  // Looping over the set
  for(var el in geekQueue){
    print(el);
  }
    
  // Other Operations
  // length of the set.
  print('Length: ${geekQueue.length}');
    
  // Deleting an element not present. No Change
  geekQueue.remove(10);
    
  // Deleting an element 9
  geekQueue.remove(2);
  print(geekQueue);
    
    
}

Output:

{GFG, 9, 1, 2, 3, 4, 5, 6, 1, 1, 9, GFG2}
{9, 1, 2, 3, 4, 5, 6, 1, 1, 9, GFG2}
{9, 1, 2, 3, 4, 5, 6, 1, 1, 9}
First Element: 9
9
1
2
3
4
5
6
1
1
9
Length: 10
{9, 1, 3, 4, 5, 6, 1, 1, 9}
Documentation

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