How is the difficulty of accessing an element by index in an ArrayList determined?

Access to an element by index in an ArrayList has a complexity of O(1), which means constant time. This is because ArrayList is implemented based on an array, where elements are stored in a contiguous memory area.

Therefore, to get an element by index, it is enough to calculate the address of the element as the base address of the array plus the offset, which takes a fixed amount of time regardless of the size of the list.