Java™ How To Program (Early Objects), Tenth Edition

Map Implementation with Hash Tables

When a program creates objects, it may need to store and retrieve them efficiently. Storing and retrieving information with arrays is efficient if some aspect of your data directly matches a numerical key value and if the keys are unique and tightly packed. If you have 100 employees with nine-digit social security numbers and you want to store and retrieve employee data by using the social security number as a key, the task will require an array with over 800 million elements, because nine-digit Social Security numbers must begin with 001–899 (excluding 666) as per the Social Security Administration’s website

http://www.socialsecurity.gov/employer/randomization.html

This is impractical for virtually all ...