Kotlin: java.lang.UnsupportedOperationException in MutableList add element

Question

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

class Stack>(list:MutableList) {

    var items: Mutabl         


        

Answer 1

You can fix this by calling toMutableList() api rather than using smart cast (as).

Try: var initialValue = listOf< Int >(10).toMutableList()

Below is a working example:

fun main(){
    val x : List<String> = listOf("foo", "bar", "baz")
    //val y: MutableList<String> = x as MutableList<String> throws UnsupportedOperationException
    val y: MutableList<String> = x.toMutableList()
    y.add("sha")
    println(y) // [foo, bar, baz, sha]
}

Answer 2

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.