Why ArrayList grows at a rate of 1.5, but for Hashmap it's 2?

Question

As per Sun Java Implementation, during expansion, ArrayList grows to 3/2 it\'s initial capacity whereas for HashMap the expansion rate is double. What is reason behind this?

Answer 1

A general rule to avoid collisions on Maps is to keep to load factor max at around 0.75 To decrease possibility of collisions and avoid expensive copying process HashMap grows at a larger rate.

Also as @Peter says, it must be a power of 2.