java.util.ArrayDeque vs kotlin.collections.ArrayDeque

Question

I was trying to solve the advent of code 2021 challenge day 10 in Kotlin when I needed to use a stack. After finding out that you shouldnt use Stack anymore but ArrayDeque instead I got provided with two options: java.util.ArrayDeque and kotlin.collections.ArrayDeque. I tried out both but only the Java one acted like intended (stack.push(), stack.pop(), etc). The Kotlin one does not provide those methods.

So I was wondering which one I should use in which case.

"The Kotlin one does not provide those methods." -- Sure it does, it just doesn't call them by those names. addFirst and removeFirst add and remove elements from the head of the Deque just fine. — azurefrog
– azurefrog, Commented Dec 30, 2021 at 23:18

Thiyanesh · Accepted Answer · 2021-12-30 23:46:12Z

5

Kotlin methods that resemble the required operations are

Stack.push = addLast()
Stack.pop = removeLast()

answered Dec 30, 2021 at 23:46

Thiyanesh

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5 Comments

René Over a year ago

Alright, I get that the method names are different but why are those two different in first place?

Thiyanesh Over a year ago

@René In java, ArrayDeque inherits these methods from Deque. But Kotlin, the corresponding methods are available only in ArrayDeque (it inherits only List specific methods). I don't have much knowledge on the reasons for this name choice. Happy New Year :)

Silvio Mayolo Over a year ago

The Java class also defines the addLast, etc. names. And in my mind, those are less ambiguous, to be fair. Once you have a two-sided container, the push/pop/enqueue/dequeue/shift/unshift/etc metaphors start to get confusing. A clear-cut addLast and removeLast makes it quite plain and obvious which side is being interacted with.

cactustictacs Over a year ago

If you look at the docs for the Deque interface (docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/…) you'll see they refer to e.g. pop() as the Stack method, and removeFirst() as the equivalent Deque method. So at a guess, the Stack methods are included for backwards compatibility because Deque is meant to be used as a drop-in replacement - but the "real" methods are the Deque ones, which are mirrored in the Kotlin implementation. Kotlin doesn't bother with the "pretend it's a stack" methods - it's a queue and you can just use one end of it!

BladeCoder Over a year ago

You actually only need a MutableList to be able to call add() and removeLast() for the stack functionality. ArrayDeque is useful when you need a queue, O(1) complexity to add or remove at both start and end.

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