Queue

Learn FIFO collections with Queue<T>.

Queue<T>

Queue<T> is a FIFO (first in first out) collection. It comes from the dart:collection library. Elements are added at the end and removed from the front.

Create a Queue with import 'dart:collection'; var queue = Queue<String>();. The addLast method appends to the end and removeFirst removes and returns the front element.

A Queue is ideal for tasks processed in order. It is used in printer queues, message queues and similar scenarios. This lesson teaches the basic operations.

Queue flow

When you add three elements to a Queue and remove them in turn the FIFO rule shows itself. The first added element leaves first. removeFirst gives one element per call and shrinks the queue.

Example: queue.addLast('A'); queue.addLast('B'); queue.addLast('C');. Now queue.removeFirst() returns 'A', the second call returns 'B'. queue.length shrinks after each removal.

This behavior mirrors real-life queues: the first person in line is served first. A Queue expresses this logic in code.

Exercise Queue operations

Add import 'dart:collection'. Create a Queue<String>(). Add 'A', 'B', 'C' with addLast. Remove two elements with removeFirst and print each. Finally print the length.

Show example solution

import 'dart:collection';

void main() {
  var queue = Queue<String>();
  queue.addLast('A');
  queue.addLast('B');
  queue.addLast('C');
  print(queue.removeFirst());
  print(queue.removeFirst());
  print(queue.length);
}

Test yourself

What does removeFirst return?
```
var q = Queue<String>();
q.addLast('A');
var x = q.removeFirst();
```
- The last element
- The list
- It throws an error
- The first element
Show answer

The first element — removeFirst removes and returns the front element of the queue.
Match the Queue methods

addLast(x)

Append to end
removeFirst()

Remove from front
Queue<String>()

Creation
dart:collection

Library

Add an element to the Queue

var queue = Queue<String>();
queue.____('A');

Show answer

var queue = Queue<String>();
queue.addLast('A');

Next lesson: Iterable

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