Why is vector array doubled?

Question

Why does the classic implementation of Vector (ArrayList for Java people) double its internal array size on each expansion instead of tripling or quadrupling it?

Answer 1

If you were to allocate an unusual-sized block of memory, then when that block gets deallocated (either because you're resizing it or it gets GC'd) there would be an unusual-sized hole in memory which could cause headaches for the memory manager. So it's usually preferred to allocate memory in powers of two. In some cases the underlying memory manager will only give you blocks of certain sizes, and if you request a weird size it will round up to the next larger size. So rather than asking for 470 units, getting back 512 anyway, and then resizing again once you've used all 470 that you've asked for, might as well just ask for 512 to begin with.