Kotlin Queue tutorial with examples

In this tutorial, we’re gonna look at overview of Kotlin Queue interface and some examples that show how to work with Kotlin Queue such as define, initialize, add or remove items, peek, poll methods.

Related Post: Kotlin Priority Queue tutorial with examples

Overview

A Queue is just like a queue in real-life. It’s a First In First Out (FIFO) data structure. New elements will be added at the back and removed from the front.

• Adding an element at the back is called Enqueue.
• Removing an element from the front is called Dequeue.

Kotlin Queue interface which is part of Collections framework. Diagram below shows hierarchy of Queue interface in Collections.

You can see that the Queue is just an interface, so we need a concrete implementation.
LinkedList class implements the Queue interface => we can use it as a Queue.

Define a Queue in Kotlin

To define a Queue, we create a new LinkedList object.

package com.bezkoder.kotlin.queue

import java.util.Queue
import java.util.LinkedList

fun main(args: Array<String>) {
  
  val nums: Queue<Int> = LinkedList<Int>()
  val names: Queue<String> = LinkedList<String>()
  val persons: Queue<Person> = LinkedList<Person>()
}

Initialize a Queue in Kotlin

val nums: Queue<Int> = LinkedList<Int>(listOf(1, 2, 3))
val names: Queue<String> = LinkedList<String>(listOf("Jack", "Adam", "Katherin"))
val persons: Queue<Person> =
    LinkedList<Person>(listOf(Person("Jack", 27), Person("Adam", 28), Person("Katherin", 19)))

println(nums)
println(names)
println(persons)

Output:

[1, 2, 3]
[Jack, Adam, Katherin]
[Person(name=Jack, age=27), Person(name=Adam, age=28), Person(name=Katherin, age=19)]

Kotlin Queue add items

We add new elements to the Queue (enqueue operation) using add() method

val names: Queue<String> = LinkedList<String>()
  
names.add("Jack")
names.add("Adam")
names.add("Katherin")
names.add("Helen")
names.add("Watson")

println(names)

Output:

[Jack, Adam, Katherin, Helen, Watson]

Kotlin Queue remove items

We can remove an element from the Queue (dequeue operation) using remove() method.
This method will throw NoSuchElementException if the Queue is empty.

// Queue: [Jack, Adam, Katherin]
var name: String?;
  
name = names.remove() // Jack
println(names)
// [Adam, Katherin]

name = names.remove() // Adam
println(names)
// [Katherin]

name = names.remove() // Katherin
println(names)
// []

name = names.remove()
/*
Exception in thread "main" java.util.NoSuchElementException
	at java.util.LinkedList.removeFirst(Unknown Source)
	at java.util.LinkedList.remove(Unknown Source)
	at com.bezkoder.kotlin.queue.AppKt.main(app.kt:42)
*/

Kotlin Queue poll

The poll() method is similar to remove() (dequeue operation), but it returns null if the Queue is empty instead of throwing an exception.

// Queue: [Jack, Adam, Katherin]
var name: String?;

name = names.poll() // Jack
println(names)
// [Adam, Katherin]

name = names.poll() // Adam
println(names)
// [Katherin]

name = names.poll() // Katherin
println(names)
// []

name = names.poll() // null

Kotlin Queue peek

The examples show you how to:

• Check if a Queue is empty using
• Get the size of a Queue
• Check if a Queue contains an element and find its index
• Get the element at the front of the Queue (without removing it) using element() or peek()

The difference between element() and peek() is that, if the Queue is empty, element() throws NoSuchElementException while peek() returns null.

package com.bezkoder.kotlin.queue

import java.util.Queue
import java.util.LinkedList

fun main(args: Array<String>) {
  val names: Queue<String> = LinkedList<String>()

  names.add("Jack")
  names.add("Adam")
  names.add("Katherin")

  println("Queue: " + names)
  println("is Queue empty? : " + names.isEmpty())
  println("Size of Queue : " + names.size)

  val name = "Adam"
  if (names.contains(name)) {
    println("Queue contains " + name);
    println("-at index " + names.indexOf(name));
  } else {
    println("Queue doesn't contain " + name);
  }
  
  var first = names.element()
  println("First item in Queue: " + first);
  
  first = names.peek()
  println("First item in Queue: " + first);
}

Output:

Queue: [Jack, Adam, Katherin]
is Queue empty? : false
Size of Queue : 3
Queue contains Adam
-at index 1
First item in Queue: Jack
First item in Queue: Jack

Iterate a Queue in Kotlin

The examples shows you several ways to iterate over a Queue:

• forEach() method
• simple for loop
• iterator()
• iterator() and forEachRemaining() method

package com.bezkoder.kotlin.queue

import java.util.Queue
import java.util.LinkedList

fun main(args: Array) {
  val names: Queue = LinkedList()

  names.add("Jack")
  names.add("Adam")
  names.add("Katherin")

  println("== forEach() ==")
  names.forEach { name -> print(">$name ") }

  println("\n==  simple for loop ==")
  for (name in names) {
    print(">$name ")
  }

  println("\n==  iterator ==")
  var queueIterator = names.iterator()
  while (queueIterator.hasNext()) {
    val name = queueIterator.next()
    print(">$name ")
  }

  println("\n== iterator with forEachRemaining ==")
  queueIterator = names.iterator()
  queueIterator.forEachRemaining { name -> print(">$name ") }
}

Output:

== forEach() ==
>Jack >Adam >Katherin 
==  simple for loop ==
>Jack >Adam >Katherin 
==  iterator ==
>Jack >Adam >Katherin 
== iterator with forEachRemaining ==
>Jack >Adam >Katherin 

Conclusion

Today we’ve known what is a Queue data structure, how to define, initialize, create a Queue in Kotlin, way to add item to a Queue or remove items from the Queue. You also know some methods to access Queue data and iterate over a Queue.

How about a Queue in that items will be ordered? Let’s explore in the tutorial:
Kotlin Priority Queue tutorial with examples

Happy Learning! See you again.

Further Reading

• Queue
• Collection