Java 9 Programming By Example

Hash functions

A hash is a mathematical function that assigns a number to an element. Say you work at a university administration and you have to tell if Wilkinson is a student at your class. You can store the names on small papers in envelopes one for each starting letter. Instead of searching through the 10 thousand students, you can look at the papers in the envelope titled W. This very simple hash function assigns the first letter of the name to the name (or the ordinal number of the letter, as we said that a hash function results a number). This is not really a good hash function because it puts only a few elements, if any, into the envelope denoted X and many to A for example.

A good hash function results each possible ordinal number ...