generate random long user ID

Question

I am writing an android app giving each client a long user ID through this formula:

long userID = (long) (Math.random() * 2 * Long.MAX_VALUE - Long.MAX_VALUE);

Am I utilizing MAX_VALUE correctly i.e. taking advantage of every possible long value?

What are my chances of having two duplicate user IDs with 10k, 100k or 1m users? How could I calculate this?

Answer 1

You can make use of either one of those mentioned below:

1. Instead of using the random to get a long user id, use current date and time expressed in milliseconds.

2. Use a Random UUID

Answer 2

https://github.com/bigfatsea/SUID Simple Unique Identifier

You may expect duplicated ids on same instance 136 years later, good enough for medium-small projects.

Example:

long id = SUID.id().get();

Answer 3

You can generate a Random UUID using randomUUID()

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