How can I use stack in Kotlin?

Question

import java.util.ArrayDeque

var stack = ArrayDeque<Int>()
stack.push(1)
stack.push(2)
stack.push(3)
stack.push(4)
println(stack)           // --> [4, 3, 2, 1]
println(stack.isEmpty()) // --> false

println(stack.peek())    // --> 4
println(stack)           // --> [4, 3, 2, 1]

println(stack.pop())     // --> 4
println(stack)           // --> [3, 2, 1]

stack.push(9)
println(stack)           // --> [9, 3, 2, 1]

You can use following:

/**
 * Stack as type alias of Mutable List
 */
typealias Stack<T> = MutableList<T>

/**
 * Pushes item to [Stack]
 * @param item Item to be pushed
 */
inline fun <T> Stack<T>.push(item: T) = add(item)

/**
 * Pops (removes and return) last item from [Stack]
 * @return item Last item if [Stack] is not empty, null otherwise
 */
fun <T> Stack<T>.pop(): T? = if (isNotEmpty()) removeAt(lastIndex) else null

/**
 * Peeks (return) last item from [Stack]
 * @return item Last item if [Stack] is not empty, null otherwise
 */
fun <T> Stack<T>.peek(): T? = if (isNotEmpty()) this[lastIndex] else null

This is done in the same way as you would in Java, but with Kotlin syntax - notably different are the val keyword and lack of new keyword. For example:

import java.util.Stack
...
val someList = ArrayList()
...
val stack = Stack()
stack.addAll(someList)

This is a few years old but I suspect there's room for a different approach. If you want to use a stack structure in Kotlin you certainly don't need to resort to Java. You could easily just create a new class with an internal Kotlin list and stack-like public functions, or use Kotlin's extension methods to give an existing Kotlin collection "stack-like" functionality, for example:

fun <T> MutableList<T>.push(item: T) = this.add(this.count(), item)
fun <T> MutableList<T>.pop(): T? = if(this.count() > 0) this.removeAt(this.count() - 1) else null
fun <T> MutableList<T>.peek(): T? = if(this.count() > 0) this[this.count() - 1] else null
fun <T> MutableList<T>.hasMore() = this.count() > 0 

Then, optionally, you could use a typealias to make it more obvious what you're trying to do when using those functions:

typealias Stack = MutableList<MyClass>

Then create one and use it:

val myStack: Stack = mutableListOf()
myStack.push(MyClass())
myStack.pop()

etc