Kotlin: java.lang.UnsupportedOperationException in MutableList add element

Question

I'm implementing a stack algorithm for study purpose in Kotlin

class Stack<T:Comparable<T>>(list:MutableList<T>) {

    var items: MutableList<T> = list


    fun isEmpty():Boolean = this.items.isEmpty()

    fun count():Int = this.items.count()

    fun push(element:T) {
        val position = this.count()
        this.items.add(position, element)
    }

    override  fun toString() = this.items.toString()

    fun pop():T? {
        if (this.isEmpty()) {
            return null
        } else {
            val item =  this.items.count() - 1
            return this.items.removeAt(item)
        }
    }

    fun peek():T? {
        if (isEmpty()) {
            return null
        } else {
            return this.items[this.items.count() - 1]
        }
    }

}

And I'm trying to execute using this code:

fun main(args: Array<String>) {

        var initialValue = listOf<Int>(10) as MutableList<Int>
        var stack = Stack<Int>(initialValue)
          stack.push(22)
        println(stack.count())
        println(stack.isEmpty())
        println(stack.pop())
        println(stack.count())
        println(stack.isEmpty())

    }

When I run the code I receive this error:

Exception in thread "main" java.lang.UnsupportedOperationException
at java.util.AbstractList.add(AbstractList.java:148)
at Stack.push(Stack.kt:17)
at StackKt.main(Stack.kt:45)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

This is related with the follow line that is implemented in push(element:T) method:

 this.items.add(position, element)

The most weird thing is that I used a very similar code implementing a orderedArray and it works perfectly.

Do you have any idea of what I'm doing wrong ?

Answer 1

listOf<Int> is not truly mutable. According to the doc:

fun <T> listOf(vararg elements: T): List<T> (source) Returns a new read-only list of given elements. The returned list is serializable (JVM).

You should use mutableListOf<> instead.

The reason why as MutableList is permitted here is because listOf(10) returns Collections.singletonList(10) which returns a java.util.List (which Kotlin assumes implements the kotlin.collections.MutableList interface). So the compiler does not know it's not really mutable until the mutating method is called at runtime and throws the exception.