Creating a Queue in Scala

Question

I am trying to create a Queue in Scala by doing:

import scala.collection.immutable.Queue

val empty = new Queue[Int]

However I am getting an error stating that the Queue constructor is protected. If this is the case, am I missing something? All the Queue methods seem to be defined and working. Must I really extend the Queue class for no reason just to use a Queue?

om-nom-nom · Accepted Answer · 2011-10-02 20:05:41Z

15

For empty Queue use companion object:

val empty = Queue.empty[Int]

answered Oct 2, 2011 at 20:05

om-nom-nom

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5 Comments

providence Over a year ago

Ahh thanks. Hopefully this answer will come up above the articles saying to use new in google..

retronym Over a year ago

or Queue[Int](). If the context of the expression (more precisely, the Expected Type) makes it clear to the compiler that you're after a collection of Ints , you can omit the type parameter, e.g. val x: Queue[Int] = Queue().

providence Over a year ago

@retronym It is using Queue[Int]() that was not working due to protected constructor. Using Scala 2.9.1.final.

Matthew Farwell Over a year ago

@providence retronym is saying to use the companion object, val empty = Queue[Int]()

providence Over a year ago

@Matthew Farwell Oh, sorry. I see that now.

Daniel C. Sobral · Accepted Answer · 2011-10-02 23:42:08Z

3

Use one of the factories:

scala.collection.immutable.Queue()
scala.collection.immutable.Queue.empty

Note that immutable queues are co-variant, so you usually don't need to define a type for it. One exception would be var declarations.

answered Oct 2, 2011 at 23:42

Daniel C. Sobral

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Comments

user unknown · Accepted Answer · 2011-10-03 07:39:03Z

0

scala> val empty = Queue [Int]()
empty: scala.collection.immutable.Queue[Int] = Queue()

answered Oct 3, 2011 at 7:39

user unknown

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