how and when Rehashing is done in HashMap

Question

i have some confusion about Hashing and Rehashing. below is my understanding please correct me if i am wrong.

as per picture, bucket is actually the array of En

Answer 1

Rehashing is the process of re-calculating the hashcode of already stored entries (Key-Value pairs), to move them to another bigger size hashmap when Load factor threshold is reached.

When the number of items in map, crosses the Load factor limit at that time hashmap doubles its capacity and hashcode is re-calculated of already stored elements for even distribution of key-value pairs across new buckets.

Why Rehashing is required?

After doubling the capacity, what to do with the key-value pairs already present in buckets?

If we keep the existing key-value pairs as it is, then doubling the capacity may not help, because O(1) complexity will be achieved only if items are evenly distributed across all buckets.

So for each existing key-value pairs, hashcode is calculated again with increased hashmap capacity as a parameter, which results in either placing the item in same bucket or in different bucket.

Rehashing is done to distribute items across the new length hashmap, so that get and put operation time complexity remains O(1).

NOTE:

Hashmap maintain complexity of O(1) while inserting data in and getting data from hashmap, but for 13th key-value pair, put request will no longer be O(1), because as soon as map will realize that 13th element came in, that is 75% of map is filled.

It will first double the bucket(array) capacity and then it will go for Rehash. Rehashing requires re-computing hashcode of already placed 12 key-value pairs again and putting them at new index which requires time.

you should read this it might help

http://javabypatel.blogspot.co.il/2015/10/what-is-load-factor-and-rehashing-in-hashmap.html