unique numbers : Java Glossary

The traditional way to handle it is to have a central database the assigns numbers and increments a counter. The problem with is the central database may be inaccessible. Further the central database becomes a crucial bottleneck.

You can also have an in-ram unique number server with a thread safe access.

I suggest a partially decentralised variant in my Unique Number Server Student Project.

Another way to do it is to use large pseudorandom numbers and count on the improbability of two numbers being picked the same. This technique is fully decentralised approach. No communication between number assigners is needed. This is how java.util. UUID (Universally Unique Identifier) works.

Ticket Numbers

Let’s say you needed to assign ticket numbers. The easiest way is to have a central counter you increment to serve ascending integers. If you want a discourage counterfeiting ticket numbers, other than my making duplicates of actual tickets, you need to make the set of number you use sparce, so if someone randomly picks a number it will be an invalid number. You want a simple mathematical way of knowing if a number is valid.

There are two simple algorithms.

• Multiply the sequential integer by a large prime. To test, divide by the prime and see if it divides evenly. To further disguise what you are doing apply some reversible tweak such as adding a constant.
• Encrypt the sequential number by any JCE (Java Cryptography Extension) algorthm. To check, decrypt the ticket number and see it if is a small integer. This will likely generate you a longer value than would be practical, other than say as a bar code.

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