← Back to Data Structures and Algorithms in Kotlin

In this source code example, we will write a code to implement the Queue data structure using the Kotlin programming language.

Queue Data Structure

A queue is an ordered list in which insertions are done at one end (rear) and deletions are done at another end (front). The first element to be inserted is the first one to be deleted. Hence, it is called the First in First out (FIFO) or Last in Last out (LILO) list.

A queue is a data structure used for storing data (similar to Linked Lists and Stacks). In a queue, the order in which data arrives is important. In general, a queue is a line of people or things waiting to be served in sequential order starting at the beginning of the line or sequence.

When an element is inserted in a queue, the concept is called EnQueue.

When an element is removed from the queue, the concept is called DeQueue.

DeQueueing an empty queue is called underflow (treat as an Exception).

EnQueuing an element in a full queue is called overflow (treat as an Exception).

Queue Implementation in Kotlin

Let's Implement Queue to perform the below operations:

enqueue()
enqueueAll()
dequeue()
front()
rear()
isEmpty()
isFull()

Here is the complete code to implement Queue using Kotlin:

import java.util.Arrays

class Queue<E> {
    private val minCapacityIncrement = 12

    private var elements: Array<Any?>
    private var size = 0

    constructor() {
        this.elements = arrayOf()
    }

    constructor(initialCapacity: Int) {
        this.elements = arrayOfNulls(initialCapacity)
    }

    constructor(elements: Array<E>) {
        this.elements = elements as Array<Any?>
        size += elements.size
    }

    fun enqueue(element: E) {
        if (size == elements.size) {
            val newArray = arrayOfNulls<Any>(size + if (size < minCapacityIncrement / 2)
                minCapacityIncrement
            else
                size shr 1)
            System.arraycopy(elements, 0, newArray, 0, size)
            elements = newArray
        }
        elements[size++] = element
    }

    fun enqueueAll(newElements: Array<E>) {
        val newSize = size + newElements.size
        if (elements.size < newSize) {
            // New sizing can be of any logic as per requirement
            val newArray = arrayOfNulls<Any>(newSize + minCapacityIncrement)
            System.arraycopy(elements, 0, newArray, 0, size)
            elements = newArray
        }
        System.arraycopy(newElements, 0, elements, size, newElements.size)
        size = newSize
    }

    fun dequeue(): E {
        if (size == 0) throw QueueUnderflowException()
        val oldVal = elements[0]
        elements[0] = null
        System.arraycopy(elements, 1, elements, 0, --size)
        return oldVal as E
    }

    fun dequeue(count: Int) {
        if (size == 0 || size < count) throw QueueUnderflowException()
        System.arraycopy(elements, count, elements, 0, size - count)
        size -= count
        for (i in 0 until count) {
            elements[size + i] = null
        }
    }

    fun front() = try {
        elements[0] as E
    } catch (e: IndexOutOfBoundsException) {
        throw QueueUnderflowException();
    }
    
    fun rear() = try {
        elements[size - 1] as E
    } catch (e: IndexOutOfBoundsException) {
        throw QueueUnderflowException();
    }

    fun isEmpty() = size == 0

    fun isFull() = size == elements.size

    override fun toString(): String {
        if (size == 0) return "[]"
        val length = size - 1
        val builder = StringBuilder(size * 16)
        builder.append('[')
        for (i in 0 until length) {
            builder.append(elements[i])
            builder.append(", ")
        }
        builder.append(elements[length])
        builder.append(']')
        return builder.toString()
    }
}

class QueueUnderflowException : RuntimeException()

inline fun <reified T> queueOf(vararg elements: T) = Queue<T>(elements as Array<T>)

fun main(args: Array<String>) {
    val animals = Queue<String>(10)
    System.out.println("$animals - Empty? -- ${animals.isEmpty()}")

    animals.enqueue("Lion")
    System.out.println("$animals - Empty? -- ${animals.isEmpty()}")

    animals.enqueue("Tiger")
    System.out.println("$animals - Empty? -- ${animals.isEmpty()}")

    animals.enqueue("Crocodile")
    animals.enqueue("Cat")
    animals.enqueue("Dog")
    animals.enqueue("Cow")
    animals.enqueue("Cangaroo")
    animals.enqueue("Rat")
    animals.enqueue("Bull")
    System.out.println("$animals - Empty? -- ${animals.isEmpty()}")
    animals.enqueue("Ox")
    System.out.println("$animals - Empty? -- ${animals.isEmpty()}")
    animals.enqueue("Zebra")
    System.out.println("$animals - Empty? -- ${animals.isEmpty()}")
    animals.dequeue()
    System.out.println("$animals - Empty? -- ${animals.isEmpty()}")

    println()
    val languages = Queue(arrayOf("Kotlin", "Java"))
    println("$languages - Empty? -- ${languages.isEmpty()}")
    languages.enqueue("C")
    println("$languages - Empty? -- ${languages.isEmpty()}")
    languages.dequeue()
    println("$languages - Empty? -- ${languages.isEmpty()}")
    languages.dequeue()
    println("$languages - Empty? -- ${languages.isEmpty()}")
    languages.dequeue()
    println("$languages - Empty? -- ${languages.isEmpty()}")

    // testQueueOf
    val languages1 = queueOf("Kotlin", "Java")
    println(languages1)
    languages1.enqueue("C")
    println(languages1)
    languages1.dequeue()
    println(languages1)
}

Output:

[] - Empty? -- true
[Lion] - Empty? -- false
[Lion, Tiger] - Empty? -- false
[Lion, Tiger, Crocodile, Cat, Dog, Cow, Cangaroo, Rat, Bull] - Empty? -- false
[Lion, Tiger, Crocodile, Cat, Dog, Cow, Cangaroo, Rat, Bull, Ox] - Empty? -- false
[Lion, Tiger, Crocodile, Cat, Dog, Cow, Cangaroo, Rat, Bull, Ox, Zebra] - Empty? -- false
[Tiger, Crocodile, Cat, Dog, Cow, Cangaroo, Rat, Bull, Ox, Zebra] - Empty? -- false

[Kotlin, Java] - Empty? -- false
[Kotlin, Java, C] - Empty? -- false
[Java, C] - Empty? -- false
[C] - Empty? -- false
[] - Empty? -- true
[Kotlin, Java]
[Kotlin, Java, C]
[Java, C]