Scala Cookbook

11.27. Using a Queue

Problem

You want to use a queue data structure in a Scala application.

Solution

A queue is a first-in, first-out (FIFO) data structure. Scala offers both an immutable queue and mutable queue. This recipe demonstrates the mutable queue.

You can create an empty, mutable queue of any data type:

import scala.collection.mutable.Queue
var ints = Queue[Int]()
var fruits = Queue[String]()
var q = Queue[Person]()

You can also create a queue with initial elements:

scala> val q = Queue(1, 2, 3)
q: scala.collection.mutable.Queue[Int] = Queue(1, 2, 3)

Once you have a mutable queue, add elements to it using +=, ++=, and enqueue, as shown in the following examples:

scala> import scala.collection.mutable.Queue
import scala.collection.mutable.Queue

// create an empty queue
scala> var q = new Queue[String]
q: scala.collection.mutable.Queue[String] = Queue()

// add elements to the queue in the usual ways
scala> var q = new Queue[String]
q: scala.collection.mutable.Queue[String] = Queue()

scala> q += "apple"
res0: scala.collection.mutable.Queue[String] = Queue(apple)

scala> q += ("kiwi", "banana")
res1: scala.collection.mutable.Queue[String] = Queue(apple, kiwi, banana)

scala> q ++= List("cherry", "coconut")
res2: scala.collection.mutable.Queue[String] =
  Queue(apple, kiwi, banana, cherry, coconut)

// can also use enqueue
scala> q.enqueue("pineapple")

scala> q
res3: scala.collection.mutable.Queue[String] =
  Queue(apple, kiwi, banana, cherry, coconut, pineapple)

Because a queue is a FIFO, you ...