Dart Lesson 35 of 102 2 min read

Queue in Dart: Adding and Removing from Both Ends

Learn the Queue class from dart:collection. Add and remove items at both ends efficiently, and build FIFO queues and stacks.

A Queue is a collection built for adding and removing items at both ends. Removing the first item of a List is slow because every other item has to shift down. A queue does it instantly.

Queue lives in the dart:collection library, so it needs an import.

Creating a queue #

import 'dart:collection';

void main() {
  var queue = Queue<String>();

  queue.add('Asha');       // same as addLast
  queue.addLast('Bimal');
  queue.addFirst('Chandra');

  print(queue);
  print(queue.first);
  print(queue.last);
  print(queue.length);
}

{Chandra, Asha, Bimal}
Chandra
Bimal
3

Removing from either end #

import 'dart:collection';

void main() {
  var queue = Queue.of([10, 20, 30, 40]);

  print(queue.removeFirst());
  print(queue.removeLast());
  print(queue);
}

10
40
{20, 30}

Both methods throw a StateError on an empty queue, so check isNotEmpty first.

First in, first out #

The everyday meaning of a queue: whoever arrives first is served first. Add at the back, remove from the front.

import 'dart:collection';

void main() {
  var printJobs = Queue<String>()
    ..add('report.pdf')
    ..add('photo.png')
    ..add('invoice.docx');

  while (printJobs.isNotEmpty) {
    final job = printJobs.removeFirst();
    print('Printing $job (${printJobs.length} waiting)');
  }
}

Printing report.pdf (2 waiting)
Printing photo.png (1 waiting)
Printing invoice.docx (0 waiting)

Last in, first out: a stack #

Add and remove at the same end and you have a stack, the structure behind an “undo” button.

import 'dart:collection';

void main() {
  var history = Queue<String>();

  history.addLast('typed "Hello"');
  history.addLast('made text bold');
  history.addLast('deleted a line');

  print('Undo: ${history.removeLast()}');
  print('Undo: ${history.removeLast()}');
  print('Remaining: $history');
}

Undo: deleted a line
Undo: made text bold
Remaining: {typed "Hello"}

A plain List with add and removeLast also works well as a stack.

Queue or List? #

Choose a queue when items are processed in arrival order: tasks waiting to run, messages to send, pages to visit in a search. Otherwise stay with a list.

A queue is an Iterable, so for-in, where, map, contains and toList all work on it.

Try it yourself #

Simulate a ticket counter. Add five customer names to a queue. A VIP arrives and goes to the front. Serve customers one at a time, printing who is served and who is next, until the queue is empty.