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Android’s Java and Kotlin arrays have a fixed length, so they do not have a universal push() or pop() method. For a resizable sequence, use Kotlin’s MutableList or Java’s ArrayList. If the data is specifically a last-in-first-out stack, use ArrayDeque. Keep a real array only when an API requires array semantics; appending then means creating a larger copy.
What ActionScript’s methods actually do
In ActionScript, push(value) appends to the end and pop() removes and returns the last item:
items.push("blue");
var removed:* = items.pop();The Android equivalent depends on whether your variable is a list, a stack, or a fixed-size array.
Kotlin: use a MutableList for a changing sequence
Kotlin’s Array<T> has a size fixed when it is created. A MutableList can grow and shrink and is usually the practical replacement for an ActionScript array used as a dynamic sequence. Kotlin’s standard mutable-list implementation is effectively an array-backed, resizable list. See Kotlin arrays and the collections overview.
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Append one item
val items = mutableListOf("red", "green")
items.add("blue") // ActionScript push()
println(items) // [red, green, blue]add(element) appends to the end. On a mutable list, items += "blue" is also commonly used; the exact behavior of +, -, +=, and -= depends on whether the collection is mutable and whether the variable can be reassigned. Details are documented in Kotlin’s plus/minus guide.
Remove and return the last item
Guard against an empty list when an empty pop is possible:
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val removed = if (items.isNotEmpty()) {
items.removeAt(items.lastIndex)
} else {
null
}lastIndex is size - 1. On an empty list it is -1, so calling removeAt(items.lastIndex) without a check is invalid. In newer Kotlin code, removeLastOrNull() is a convenient non-throwing alternative:
val removed = items.removeLastOrNull()If the list is guaranteed to be non-empty, this shorter form removes and returns the final element:
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val removed = items.removeAt(items.lastIndex)Complete Kotlin example
fun main() {
val items = mutableListOf("one", "two")
items.add("three")
val popped = items.removeLastOrNull()
println(items) // [one, two]
println(popped) // three
}Java: use ArrayList
Java’s ArrayList is a resizable-array implementation. add(E) appends, while remove(int) removes and returns the element at an index. The Android reference documents these operations at ArrayList.
Append and pop safely
import java.util.ArrayList;
ArrayList<String> items = new ArrayList<>();
items.add("red");
items.add("green");
items.add("blue"); // ActionScript push()
String removed = items.isEmpty()
? null
: items.remove(items.size() - 1); // ActionScript pop()
System.out.println(items); // [red, green]
System.out.println(removed); // blueWhen emptiness is impossible by design, items.remove(items.size() - 1) is sufficient. Otherwise, check isEmpty() as shown.
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Use ArrayDeque when it is truly a stack
If the only meaningful operations are adding and removing at one end—for example undo history, navigation history, or parser state—ArrayDeque communicates LIFO intent more clearly than a general-purpose list. Android’s implementation has been available since API level 9; removeLast() throws NoSuchElementException when empty. See the Android ArrayDeque reference.
Kotlin stack
import java.util.ArrayDeque
val stack = ArrayDeque<String>()
stack.addLast("one")
stack.addLast("two")
stack.addLast("three")
val popped = if (stack.isEmpty()) null else stack.removeLast()
println(stack) // [one, two]
println(popped) // threeJava stack
import java.util.ArrayDeque;
ArrayDeque<String> stack = new ArrayDeque<>();
stack.addLast("one");
stack.addLast("two");
stack.addLast("three");
String popped = stack.isEmpty() ? null : stack.removeLast();Use MutableList/ArrayList when indexed access, middle insertion, or row-oriented list operations matter. Use ArrayDeque when LIFO behavior is the data structure’s defining rule.
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ActionScript-to-Kotlin-and-Java mapping
| Operation | Kotlin | Java |
|---|---|---|
| Append | list.add(value) |
list.add(value) |
| Pop last item | list.removeAt(list.lastIndex) |
list.remove(list.size() - 1) |
| Insert at index | list.add(index, value) |
list.add(index, value) |
| Remove by index | list.removeAt(index) |
list.remove(index) |
| Remove by value | list.remove(value) |
list.remove(value) |
| Append many | list.addAll(values) |
list.addAll(values) |
| Clear all | list.clear() |
list.clear() |
| Explicit stack push | deque.addLast(value) |
deque.addLast(value) |
| Explicit stack pop | deque.removeLast() |
deque.removeLast() |
Removing by value versus by position
Kotlin
items.remove("green") // removes one matching value
items.removeAll { it == "green" } // removes every matching value
items.removeAt(2) // removes the item at index 2Indexed insertion shifts later elements to the right. Invalid indexes produce IndexOutOfBoundsException. Kotlin’s write-operation rules are covered in collection write operations.
Java’s numeric overload trap
ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(10);
numbers.add(20);
numbers.remove(0); // removes index 0
numbers.remove(Integer.valueOf(20)); // removes the value 20Java overloads remove(int index) and remove(Object value). Wrapping a number with Integer.valueOf selects value removal.
If you must keep a genuine array
An array’s existing elements can be changed, but its length cannot. Appending therefore allocates a new array and copies the old contents.
Kotlin append and pop-copy
var values = intArrayOf(1, 2, 3)
values = values.copyOf(values.size + 1)
values[values.lastIndex] = 4
if (values.isNotEmpty()) {
val removed = values.last()
values = values.copyOf(values.size - 1)
}Kotlin also supports values += 5, which creates an expanded array. Reference arrays work the same way with arrayOf and copyOf. Repeated copying is appropriate when an API specifically requires an array, not for frequent push/pop operations. Primitive arrays such as IntArray avoid boxed numeric elements, but they still require copying to resize.
Common Android and Kotlin mistakes
- Choosing
listOf()for mutable data:listOfis read-only from the caller’s perspective. UsemutableListOfwhen adding or removing. - Assuming
valmeans immutable:val items = mutableListOf("a")prevents replacing the reference, butitems.add("b")remains valid. - Using
Arrays.asListas a growable Java list: its size is fixed. Createnew ArrayList<>(Arrays.asList("a", "b"))before structural changes. - Using
ArrayList.removeLast()everywhere: Android documents that convenience method as added in API level 35. For broad compatibility, useremove(size - 1)in Java orremoveAt(lastIndex)in Kotlin; check the Android ArrayList API. - Ignoring empty pops: list index removal can fail for an empty list, and
ArrayDeque.removeLast()throwsNoSuchElementException. Decide whether to guard, returnnull, or deliberately allow an exception. - Expecting a list mutation to redraw the UI: changing a collection does not automatically notify a
RecyclerView, Compose,LiveData, orStateFlow. Publish or notify new state using the mechanism required by your UI architecture. - Assuming ordinary mutable collections are thread-safe: Kotlin’s
MutableListimplementations generally need explicit synchronization when accessed concurrently. Follow your Android architecture’s thread rules.
Choosing the right type
| Requirement | Recommended type | Why |
|---|---|---|
| Resizable, indexed sequence | MutableList/ArrayList |
Supports append, indexed access, and insertion/removal. |
| LIFO push/pop semantics | ArrayDeque |
The operation names and structure express a stack directly. |
| Fixed-size API or primitive storage | Array, IntArray, or another primitive array |
Matches an API requiring an array; resizing requires copying. |
| Read-only public exposure | Private MutableList plus public List |
Code inside the class can mutate while callers only read. |
private val _items = mutableListOf<String>()
val items: List<String>
get() = _itemsAppending to an array-backed list is generally amortized constant time, although occasional capacity growth copies elements. Removing the last list element is efficient; inserting or removing near the beginning shifts later elements. Performance depends on operation pattern, collection size, runtime, and workload, so treat ArrayDeque primarily as the clearer stack abstraction rather than a universal speed winner. Android describes ArrayList as a resizable-array implementation at its API reference.
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