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In this tutorial, we will write a Program to implement the Queue data structure using the Kotlin programming language.

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.

Queue concepts:

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 Data Structure Implementation in Kotlin

Let's Implement Queue to perform the below operations:

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

Here is the complete Kotlin program with output to implement the Queue:

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]

Related Data Structures and Algorithms in Kotlin

Stack Data Structure Implementation in Kotlin
Queue Data Structure Implementation in Kotlin
Deque Implementation in Kotlin
Singly Linked List Implementation in Kotlin
Doubly Linked List Implementation in Kotlin
Circular Linked List Implementation in Kotlin
Bubble Sort Algorithm in Kotlin
Heap Sort Algorithm in Kotlin
Insertion Sort Algorithm in Kotlin
Merge Sort Algorithm in Kotlin
Quick Sort Algorithm in Kotlin
Selection Sort Algorithm in Kotlin

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