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

listOf is not truly mutable. According to the doc:

fun listOf(vararg elements: T): List (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.