Implementing a linked list in Kotlin

I recently started learning Kotlin, so I decided to implement some data structures in it. So, I tried implementing a singly linked list:

package datastructures

public class LinkedList {
    private data class Node(var nodeValue: Int, var next: Node? = null)
    private var head: Node? = null

    fun insert(n: Int) {
        if(head == null) head = Node(n)
        else {
            var cur = head
            while(cur?.next != null) {
                cur = cur?.next
            }
            cur?.next = Node(n)
        }
    }

    fun print() {
        var cur = head
        while(cur != null) {
            print("${cur.nodeValue} ")
            cur = cur?.next
        }
    }

}

fun main(args: Array<String>) {
    val n = LinkedList()
    n.insert(5)
    n.insert(3)
    n.print()
}

      

and I got the following error:

Error:(22, 13) Kotlin: [Internal Error] org.jetbrains.jet.codegen.CompilationException: Back-end (JVM) Internal error: cannot store to value org.jetbrains.jet.codegen.StackValue$OnStack@a0a447f
Cause: cannot store to value org.jetbrains.jet.codegen.StackValue$OnStack@a0a447f
File being compiled and position: (22,13) in C:/Users/Khaled/IdeaProjects/Kotlin/src/LinkedList.kt
PsiElement: cur?.next = Node(n)
The root cause was thrown at: StackValue.java:75
    at org.jetbrains.jet.codegen.ExpressionCodegen.genQualified(ExpressionCodegen.java:243)
    at org.jetbrains.jet.codegen.ExpressionCodegen.genStatement(ExpressionCodegen.java:262)
    at ...

      

I've searched here and on google but can't figure out what is the problem causing this error.

Edit: So I tried to re-implement the function insert

and use requireNotNull()

to avoid the compiler about null safety.

Here is the code and now it works:

fun insert(n: Int) {
    if (head == null) head = Node(n)
    else {
        var cur = head!!
        while (cur.next != null) {
            cur = cur.next!!
        }
        cur.next = Node(n)
    }
}

      

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1 answer


I think the problem lies in this line:

cur?.next = Node(n)

      

The problem is, the compiler doesn't know what to do if cur

- null

. This currently results in an internal error, but this may be supported in a future release.



Currently, the best solution is to rewrite the code in such a way that the compiler can check that cur

never null

. The problem is that the compiler assumes that fields declared as var

can change at any time, so their values ​​must be loaded into local variables before checking for null

:

var cur = head
if(cur == null) head = Node(n)
else {
    var next = cur.next
    while(next != null) {
        cur = next
        next = cur.next
    }
    cur.next = Node(n)
}

      

+3


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