In this tutorial, we will learn how to write a Program to implement the Linked List data structure using the Kotlin programming language.

The singly linked list is a linear data structure in which each element of the list contains a pointer that points to the next element in the list. Each element in the singly linked list is called a node. Each node has two components: data and a pointer next which points to the next node in the list.

The first node of the list is called as head, and the last node of the list is called the tail. The last node of the list contains a pointer to the null. Each node in the list can be accessed linearly by traversing through the list from head to tail.

Kotlin Program to Implement Singly Linked List

Let's write the complete program to implement Singly Linked List to perform the below operations:

    getFirst()
    get()
    set()
    removeFirst()
    removeLast()
    removeValue()
    addAll()

Here is the complete Kotlin program with output to implement the Singly Linked List:

class LinkyList<E> {
    private var size = 0
    private var head: Node<E>? = null
    private var tail: Node<E>? = null

    private inner class Node<E> constructor(internal var element: E, internal var next: Node<E>?)

    fun getFirst() = head?.element

    fun getLast() = tail?.element

    fun removeFirst() = unlinkHead()

    fun removeLast() = unlinkTail()

    fun addFirst(element: E) {
        linkHead(element)
    }

    fun addLast(element: E) {
        linkTail(element)
    }

    fun add(element: E) {
        linkTail(element)
    }

    fun <T> addAll(index: Int, arr: Array<T>): Boolean where T : E {
        validatePositionIndex(index)

        val numNew = arr.size
        if (numNew == 0) return false

        var pred: Node<E>?
        var succ: Node<E>?
        when (index) {
            0 -> {
                succ = head
                pred = null
            }
            size -> {
                succ = null
                pred = tail
            }
            else -> {
                pred = node(index - 1)
                succ = pred.next
            }
        }

        for (item in arr) {
            val e = item as E
            val newNode = Node<E>(e, null)
            if (pred == null)
                head = newNode
            else
                pred.next = newNode
            pred = newNode
        }

        if (succ == null) {
            tail = pred
        } else {
            pred!!.next = succ
        }

        size += numNew
        return true
    }

    fun remove(element: E): Boolean {
        var curr = head
        while (curr != null) {
            if (curr.element == element) {
                unlink(curr)
                return true
            }
            curr = curr.next
        }
        return false
    }

    fun clear() {
        var x = head
        while (x != null) {
            val next = x.next
            x.next = null
            x = next
        }
        tail = null
        head = tail
        size = 0
    }

    fun size() = size

    operator fun contains(element: E) = indexOf(element) != -1

    fun get(index: Int): E {
        validateElementIndex(index)
        return node(index).element
    }

    fun set(index: Int, element: E): E {
        validateElementIndex(index)
        val x = node(index)
        val oldVal = x.element
        x.element = element
        return oldVal
    }

    fun add(index: Int, element: E) {
        validatePositionIndex(index)
        if (index == size) {
            linkTail(element)
        } else {
            linkBefore(element, node(index))
        }
    }

    fun addV2(index: Int, element: E) {
        validatePositionIndex(index)
        if (index == 0) linkHead(element)
        else {
            var x = head
            val prevIndex = index - 1
            for (i in 0 until prevIndex) {
                x = x!!.next
            }
            val next = x!!.next
            val newNode = Node(element, next)
            x.next = newNode
            size++
        }
    }

    fun remove(index: Int): E {
        validateElementIndex(index)
        return unlink(node(index))
    }

    fun indexOf(element: E): Int {
        var index = 0
        var x = head
        while (x != null) {
            if (element == x.element)
                return index
            index++
            x = x.next
        }
        return -1
    }

    private fun linkHead(element: E) {
        val h = head
        val newNode = Node<E>(element, h)
        head = newNode
        if (h == null) {
            tail = newNode
        }
        size++
    }

    private fun linkTail(element: E) {
        val t = tail
        val newNode = Node<E>(element, null)
        tail = newNode
        if (t == null) {
            head = newNode
        } else {
            t.next = newNode
        }
        size++
    }

    private fun linkBefore(element: E, succ: Node<E>) {
        val pred = getPrevious(succ)
        val newNode = Node(element, succ)
        if (pred == null) {
            head = newNode
        } else {
            pred.next = newNode
        }
        size++
    }

    private fun unlinkHead() {
        head?.let {
            val next = it.next
            it.next = null
            head = next
            if (next == null) {
                tail = null
            }
            size--
        }
    }

    private fun unlinkTail() {
        tail?.let {
            val prev = getPrevious(it)
            tail = prev
            if (prev == null) {
                head = null
            } else {
                prev.next = null
            }
            size--
        }
    }

    private fun unlink(curr: Node<E>): E {
        val element = curr.element
        val next = curr.next
        val prev = getPrevious(curr)

        if (prev == null) {
            head = next
        } else {
            prev.next = next
            curr.next = null
        }

        if (next == null) {
            prev?.next = null
            tail = prev
        } else {
            prev?.next = next
            curr.next = null
        }

        size--
        return element
    }

    private fun getPrevious(node: Node<E>): Node<E>? {
        if (head != null && node == head) return null
        var curr = head
        while (curr != null) {
            if (curr.next == node) {
                return curr
            }
            curr = curr.next
        }
        return null
    }

    private fun node(index: Int): Node<E> {
        var x = head
        for (i in 0 until index) {
            x = x!!.next
        }
        return x!!
    }

    private fun validateElementIndex(index: Int) {
        if (index < 0 || index >= size)
            throw IndexOutOfBoundsException(outOfBoundsMsg(index))
    }

    private fun validatePositionIndex(index: Int) {
        if (index < 0 && index > size)
            throw IndexOutOfBoundsException(outOfBoundsMsg(index))
    }

    private fun outOfBoundsMsg(index: Int): String {
        return "Index: $index, Size: $size"
    }

    override fun toString(): String {
        var curr = head
        if (curr == null) return "[]"
        else {
            val sb = StringBuilder()
            sb.append('[')
            while (curr != null) {
                sb.append(curr.element)
                curr = curr.next
                if (curr?.element == null) {
                    sb.append(']')
                } else {
                    sb.append(',').append(' ')
                }
            }
            return sb.toString()
        }
    }
}

fun main(args: Array<String>) {
    val linkyList = LinkyList<String>()
    println("First item of the linky list is - ${linkyList.getFirst()}")
    println("Last item of the linky list is - ${linkyList.getLast()}")

    println()
    linkyList.add("Kotlin")
    println("First item of the linky list is - ${linkyList.getFirst()}")
    println("Last item of the linky list is - ${linkyList.getLast()}")

    println()
    linkyList.add("Java")
    println("First item of the linky list is - ${linkyList.getFirst()}")
    println("Last item of the linky list is - ${linkyList.getLast()}")

    linkyList.add("C#")
    linkyList.add("Python")
    linkyList.add("JavaScript")

    println()
    println("Elements at linkyList - $linkyList")
    linkyList.remove("JavaScript")
    println("Elements at linkyList after removing JavaScript - $linkyList")
    linkyList.remove("Kotlin")
    println("Elements at linkyList after removing Kotlin - $linkyList")
    linkyList.remove("C#")
    println("Elements at linkyList after removing C# - $linkyList")
    linkyList.remove("Java")
    println("Elements at linkyList after removing Java - $linkyList")
    linkyList.remove("Python")
    println("Elements at linkyList after removing Python - $linkyList")

    testGetFirst()
    testAddV2()
    testGet()
    testSet()
    testRemoveFirst()
    testRemoveLast()
    testRemoveValue()
    testAddAll()
}

fun testGetFirst() {
    println()
    println("==================================")
    println("getFirst method testing started")
    val linkyList = LinkyList<String>()
    println(linkyList.getFirst() == null)

    linkyList.add("Kotlin")
    println(linkyList.getFirst() == "Kotlin")

    linkyList.add("Java")
    println(linkyList.getFirst() == "Kotlin")

    linkyList.add("Python")
    println(linkyList.getFirst() == "Kotlin")

    linkyList.add(0, "Python")
    println(linkyList.getFirst() == "Python")

    linkyList.add(1, "JavaScript")
    println(linkyList.getFirst() == "Python")

    linkyList.set(0, "JavaScript")
    println(linkyList.getFirst() == "JavaScript")

    linkyList.set(1, "Kotlin")
    println(linkyList.getFirst() == "JavaScript")

    linkyList.addFirst("Kotlin")
    println(linkyList.getFirst() == "Kotlin")

    linkyList.addLast("JavaScript")
    println(linkyList.getFirst() == "Kotlin")

    println("getFirst method testing ended")
    println("==================================")
    println()
    linkyList.clear()
    println("Elements at LinkyList - $linkyList")

    linkyList.addFirst("Kotlin")
    println("Elements at LinkyList - $linkyList")

    linkyList.addFirst("Kotlin")
    println("Elements at LinkyList - $linkyList")

    linkyList.addFirst("Java")
    println("Elements at LinkyList - $linkyList")

    linkyList.addFirst("Python")
    println("Elements at LinkyList - $linkyList")
}

fun testAddV2() {
    println()
    println("==================================")
    println("testAddV2 method testing started")
    val linkyList = LinkyList<String>()
    linkyList.add("Kotlin")
    linkyList.add("Java")
    linkyList.add("C#")
    linkyList.add("C")
    linkyList.add("C++")
    println("Elements at LinkyList - $linkyList")

    println()
    linkyList.addV2(1, "JavaScript")
    println("Elements at LinkyList - $linkyList")

    println()
    linkyList.addV2(2, "TypeScript")
    println("Elements at LinkyList - $linkyList")

    println()
    linkyList.addV2(3, "CofeeScript")
    println("Elements at LinkyList - $linkyList")

    println()
    linkyList.addV2(7, "MongoDB")
    println("Elements at LinkyList - $linkyList")

    println()
    linkyList.addV2(0, "SQL")
    println("Elements at LinkyList - $linkyList")

    println("testAddV2 method testing started")
    println("==================================")
}

fun testGet() {
    println()
    println("=================================")
    println("Testing get started")
    val linkyList = LinkyList<String>()
    linkyList.add("Kotlin")
    linkyList.add("Java")
    linkyList.add("C#")
    linkyList.add("C")
    linkyList.add("C++")

    println("0th Index - ${linkyList.get(0)}")
    println("1st Index - ${linkyList.get(1)}")
    println("2nd Index - ${linkyList.get(2)}")
    println("3rd Index - ${linkyList.get(3)}")
    println("4th Index - ${linkyList.get(4)}")
    println("Testing get ended")
    println("=================================")
}

fun testSet() {
    println()
    println("=================================")
    println("Testing set started")
    val linkyList = LinkyList<String>()
    linkyList.add("Kotlin")
    linkyList.add("Java")
    linkyList.add("C#")
    linkyList.add("C")
    linkyList.add("C++")

    println("0th Index - ${linkyList.set(0, "Edited Kotlin")}")
    println("1st Index - ${linkyList.set(1, "Edited Java")}")
    println("2nd Index - ${linkyList.set(2, "Edited C#")}")
    println("3rd Index - ${linkyList.set(3, "Edited C")}")
    println("4th Index - ${linkyList.set(4, "Edited C++")}")
    println("Final list - $linkyList")
    println("Testing set ended")
    println("=================================")
}

fun testRemoveFirst() {
    println()
    println("=================================")
    println("Testing removeFirst started")
    val linkyList = LinkyList<String>()
    linkyList.add("Kotlin")
    linkyList.add("Java")
    linkyList.add("C#")
    linkyList.add("C")
    linkyList.add("C++")

    println("List - $linkyList")
    linkyList.removeFirst()
    println("List - $linkyList")
    linkyList.removeFirst()
    println("List - $linkyList")
    linkyList.removeFirst()
    println("List - $linkyList")
    linkyList.removeFirst()
    println("List - $linkyList")
    println("Testing removeFirst ended")
    println("=================================")
}

fun testRemoveLast() {
    println()
    println("=================================")
    println("Testing removeLast started")
    val linkyList = LinkyList<String>()
    linkyList.add("Kotlin")
    linkyList.add("Java")
    linkyList.add("C#")
    linkyList.add("C")
    linkyList.add("C++")

    println("List - $linkyList")
    linkyList.removeLast()
    println("List - $linkyList")
    linkyList.removeLast()
    println("List - $linkyList")
    linkyList.removeLast()
    println("List - $linkyList")
    linkyList.removeLast()
    println("List - $linkyList")
    println("Testing removeLast ended")
    println("=================================")
}

fun testRemoveValue() {
    println()
    println("=================================")
    println("Testing testRemoveValue started")
    val linkyList = LinkyList<String>()
    linkyList.add("Kotlin")
    linkyList.add("Java")
    linkyList.add("C#")
    linkyList.add("C")
    linkyList.add("C++")

    println("JavaScript" in linkyList)
    println("Kotlin" in linkyList)

    println("List - $linkyList")
    linkyList.remove("Java")
    println("List - $linkyList")
    linkyList.remove("Kotlin")
    println("List - $linkyList")
    linkyList.remove("JavaScript")
    println("List - $linkyList")
    linkyList.remove("Python")
    println("List - $linkyList")
    println("Testing testRemoveValue ended")
    println("=================================")
}

fun testAddAll() {
    println()
    println("=================================")
    println("Testing testAddAll started")
    
    val linkyList = LinkyList<String>()

    // Add few elements at begining of the linkedlist
    linkyList.addAll(0, arrayOf<String>("C", "C++"))
    println("List - $linkyList")
    linkyList.addAll(0, arrayOf<String>("Java", "Kotlin"))
    println("List - $linkyList")
    // Add few elements at middle of the linkedlist
    linkyList.addAll(2, arrayOf<String>("Python", "R"))
    println("List - $linkyList")
    // Add few elements at end of the linkedlist
    linkyList.addAll(linkyList.size(), arrayOf<String>("C#", "MATLAB"))
    println("List - $linkyList")
    println("Testing testAddAll ended")
    println("=================================")
}

Output:

First item of the linky list is - null
Last item of the linky list is - null

First item of the linky list is - Kotlin
Last item of the linky list is - Kotlin

First item of the linky list is - Kotlin
Last item of the linky list is - Java

Elements at linkyList - [Kotlin, Java, C#, Python, JavaScript]
Elements at linkyList after removing JavaScript - [Kotlin, Java, C#, Python]
Elements at linkyList after removing Kotlin - [Java, C#, Python]
Elements at linkyList after removing C# - [Java, Python]
Elements at linkyList after removing Java - [Python]
Elements at linkyList after removing Python - []

==================================
getFirst method testing started
true
true
true
true
true
true
true
true
true
true
getFirst method testing ended
==================================

Elements at LinkyList - []
Elements at LinkyList - [Kotlin]
Elements at LinkyList - [Kotlin, Kotlin]
Elements at LinkyList - [Java, Kotlin, Kotlin]
Elements at LinkyList - [Python, Java, Kotlin, Kotlin]

==================================
testAddV2 method testing started
Elements at LinkyList - [Kotlin, Java, C#, C, C++]

Elements at LinkyList - [Kotlin, JavaScript, Java, C#, C, C++]

Elements at LinkyList - [Kotlin, JavaScript, TypeScript, Java, C#, C, C++]

Elements at LinkyList - [Kotlin, JavaScript, TypeScript, CofeeScript, Java, C#, C, C++]

Elements at LinkyList - [Kotlin, JavaScript, TypeScript, CofeeScript, Java, C#, C, MongoDB, C++]

Elements at LinkyList - [SQL, Kotlin, JavaScript, TypeScript, CofeeScript, Java, C#, C, MongoDB, C++]
testAddV2 method testing started
==================================

=================================
Testing get started
0th Index - Kotlin
1st Index - Java
2nd Index - C#
3rd Index - C
4th Index - C++
Testing get ended
=================================

=================================
Testing set started
0th Index - Kotlin
1st Index - Java
2nd Index - C#
3rd Index - C
4th Index - C++
Final list - [Edited Kotlin, Edited Java, Edited C#, Edited C, Edited C++]
Testing set ended
=================================

=================================
Testing removeFirst started
List - [Kotlin, Java, C#, C, C++]
List - [Java, C#, C, C++]
List - [C#, C, C++]
List - [C, C++]
List - [C++]
Testing removeFirst ended
=================================

=================================
Testing removeLast started
List - [Kotlin, Java, C#, C, C++]
List - [Kotlin, Java, C#, C]
List - [Kotlin, Java, C#]
List - [Kotlin, Java]
List - [Kotlin]
Testing removeLast ended
=================================

=================================
Testing testRemoveValue started
false
true
List - [Kotlin, Java, C#, C, C++]
List - [Kotlin, C#, C, C++]
List - [C#, C, C++]
List - [C#, C, C++]
List - [C#, C, C++]
Testing testRemoveValue ended
=================================

=================================
Testing testAddAll started
List - [C, C++]
List - [Java, Kotlin, C, C++]
List - [Java, Kotlin, Python, R, C, C++]
List - [Java, Kotlin, Python, R, C, C++, C#, MATLAB]
Testing testAddAll ended
=================================

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