How Dynamic Lists Work — API Cheat Sheet

This is a reference page, not a graded exercise.

In Python and JavaScript/TypeScript, "array" and "list" are the same built-in type — see the Arrays cheat sheet for list/Array methods like .append/.push. Java and C# are different: their raw arrays (int[], T[]) are fixed-size, so when you need to grow or shrink a collection you reach for a separate generic class — ArrayList<T> in Java, List<T> in C#. This page is about those two.

Java — ArrayList<T>

Want to...	Method	Example
Create	new ArrayList<>()	List<Integer> list = new ArrayList<>();
Add to the end	.add(v)	list.add(5);
Add at an index	.add(i, v)	list.add(0, 5); (shifts everything right)
Read/write an index	.get(i) / .set(i, v)	there's no [] indexing on ArrayList, unlike a raw array
Remove by index	.remove(i)	takes an int
Remove by value	.remove(Object o)	careful: list.remove(5) on a List<Integer> removes index 5, not the value — use list.remove(Integer.valueOf(5)) to remove the value
Check membership	.contains(v)	list.contains(5)
Find an index	.indexOf(v)	-1 if missing
Size	.size()	not .length and not .length()
Sort	Collections.sort(list)	or list.sort(Comparator...)
Iterate	for-each	for (int x : list) { ... }

C# — List<T>

Want to...	Method	Example
Create	new List<T>()	var list = new List<int>();
Add to the end	.Add(v)	list.Add(5);
Add at an index	.Insert(i, v)	list.Insert(0, 5);
Read/write an index	indexer []	list[0] = 5; — unlike Java, C#'s List<T> does support []
Remove by index	.RemoveAt(i)	takes an int
Remove by value	.Remove(v)	removes the first matching value
Check membership	.Contains(v)	list.Contains(5)
Find an index	.IndexOf(v)	-1 if missing
Size	.Count	property, not a method — no parens
Sort	.Sort()	in place
Iterate	foreach	foreach (var x in list) { ... }

The gotcha that catches almost everyone coming from Python/JS: Java's ArrayList has no [] operator at all — it's .get(i)/.set(i, v) only. And list.remove(5) on a List<Integer> is ambiguous-looking but not ambiguous to the compiler: it always resolves to the int (index) overload unless you box the argument yourself.