You should read NotACodeMill and then update your post.

NB - this was a reply to the original post which was just a question copied from a book/exam paper/whatever and if i remember properly had nothing to do with random numbers.

You could implement your own random number generator. But beware that it is not easy to create "good" random numbers (random numbers which are truly random, or which at least have a statistical distribution that comes close to being really random). Several algorithms have been invented to generate pseudo-random numbers; do some research to learn more about them.

Or just use Math.random(). It generates a double value between 0.0 (inclusive) and 1.0 (exclusive), so multiply it by X and cast to int to get an int value between 0 (inclusive) and X (exclusive).