Question

How is the rehashing process done in a hashmap or hashtable when the size exceeds the maxthreshold value?

Are all pairs just copied to a new array of buckets?

EDIT:

What happen to the elements in the same bucket (in linked list) after rehashing? I mean will they remain in same bucket after rehashing?

Answer 1

The maximum threshold in the question is called the load factor.

It is advisable to have a load factor of around 0.75. Load factor is defined as (m/n) where n is the total size of the hash table and m is the preferred number of entries which can be inserted before a increment in size of the underlying data structure is required.

Rehashing can be done in two cases:

1. When the present m’/n ratio increases beyond the load factor

2. M’/n ratio falls to a very low value say 0.1

In both the cases m’ is the current number of entries. Also, both the cases demand the shifting of the present entries into a bigger or a smaller hash table.

In the question’s context rehashing is the process of applying a hash function to the entries to move them to another hash table. It is possible to use the hash function which was used earlier or use a new function altogether.

Please note: Rehashing is also done when a collision occurs. (It’s a way of handling collisions too.)

To add some more context and a detailed discussion please visit my blog Hashing Basics