What provides fast element retrieval by index in ArrayList?

Fast element retrieval by index in ArrayList is ensured because it is based on an array.

ArrayList stores elements in a contiguous block of memory, so accessing an element by index is simply an array access by offset, which operates in O(1) time.

Example:

ArrayList<String> list = new ArrayList<>();
list.add("A");
list.add("B");
String element = list.get(1); // Fast access to element "B"

Unlike linked lists, where accessing an element requires traversing links, ArrayList allows instant access to an element by index.