Why is the initial capacity in HashMap 16 (power of two) and the initial capacity of Hashtable 11(prime number)?

Question

Please describe the reason if you know. I Googled it, but didn't find well explained answers.

Is it for making index of bucket positive when your hashCode is negative?

Answer 1

For HashMap, the index in the array that stores the entries of the Map is calculated this way (where h is calculated from the hashCode of the key):

static int indexFor(int h, int length) {
    return h & (length-1);
}

Where length is the length of the array.

This only works when length is a power of 2. If length wasn't power of 2, you would have to change this code to the less efficient return h % length.