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?
As per the implementation, for HashMap, the capacity should always be in the power of two. That may be a reason for HashMap's behavior. But in that case the question is, for HashMap why the capacity should always be in power of two?
for HashMap why the capacity should always be in power of two?
I can think of two reasons.
You can quickly determine the bucket a hashcode goes in to. You only need a bitwise AND and no expensive modulo. int bucket = hashcode & (size-1);
Let's say we have a grow factor of 1.7. If we start with a size 11, the next size would be 18, then 31. No problem. Right? But the hashcodes of Strings in Java, are calculated with a prime factor of 31. The bucket a string goes into,hashcode%31, is then determined only by the last character of the String. Bye bye O(1) if you store folders that all end in /. If you use a size of, for example, 3^n, the distribution will not get worse if you increase n. Going from size 3 to 9, every element in bucket 2, will now go to bucket 2,5 or 7, depending on the higher digit. It's like splitting each bucket in three pieces. So a size of integer growth factor would be preferred. (Off course this all depends on how you calculate hashcodes, but a arbitrary growth factor doesn't feel 'stable'.)