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

The expensive part at increasing the capacity of an ArrayList is copying the content of the backing array a new (larger) one.

For the HashMap, it is creating a new backing array and putting all map entries in the new array. And, the higher the capacity, the lower the risk of collisions. This is more expensive and explains, why the expansion factor is higher. The reason for 1.5 vs. 2.0? I consider this as "best practise" or "good tradeoff".

for HashMap why the capacity should always be in power of two?

I can think of two reasons.

1. 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);

2. 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'.)

The way HashMap is designed/implemented its underlying number of buckets must be a power of 2 (even if you give it a different size, it makes it a power of 2), thus it grows by a factor of two each time. An ArrayList can be any size and it can be more conservative in how it grows.