Why does Java's ArrayList's remove function seem to cost so little?

Question

I have a function which manipulates a very large list, exceeding about 250,000 items. For the majority of those items, it simply replaces the item at position x. However, fo

Answer 1

Interesting and unexpected results. This is just a hypothesis, but...

On average one of your array element removals will require moving half of your list (everything after it) back one element. If each item is a 64-bit pointer to an object (8 bytes), then this means copying 125000 items x 8 Bytes per pointer = 1 MB.

A modern CPU can copy a contiguous block of 1 MB of RAM to RAM pretty quickly.

Compared to looping over a linked list for every access, which requires comparisons and branching and other CPU unfriendly activities, the RAM copy is fast.

You should really try benchmarking the various operations independently and see how efficient they are with various list implementations. Share your results here if you do!