Hashing

Hashing is designed to solve the problem of needing to efficiently find or store an item in a collection. It is a technique to narrow down the search space.
Hashing means using some function or algorithm to map object data to some representative integer value.
One basic approach. Calculate a hash for each key and store the hash value in an array index of the same value. Store the key, value pair in the same index positions in other arrays.
However, this approach suffers from wasted space in the array - for those array indexes for which corresponding hash values are never generated.
Another problem is collisions - a single hash value can represent many keys.
To solve wasted space use : hashcode modulo arraysize.
To solve collisions use linkedlists to store key->value pairs for each hash value.

Hash function properties

Be able to distribute the keys evenly over the range of positions in the array.
Distribute the keys over a larger range of values.
Combining the above two, Ideally a hash function should distribute hash codes evenly over the range of integers.