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

I can't give you a reason why this is so (you'd have to ask Sun developers), but to see how this happens take a look at source:

1. HashMap: Take a look at how HashMap resizes to new size (source line 799)

        resize(2 * table.length);
   

2. ArrayList: source, line 183:

   int newCapacity = (oldCapacity * 3)/2 + 1;
   

Update: I mistakenly linked to sources of Apache Harmony JDK - changed it to Sun's JDK.