Question

I have a simple code which generates random numbers

SecureRandom random = new SecureRandom();
...
public int getRandomNumber(int maxValue) {
    return random.nextInt(maxValue);
}

The method above is called about 10 times (not in a loop). I want to ensure that all the numbers are unique (assuming that maxValue > 1000).

Can I be sure that I will get unique numbers every time I call it? If not, how can I fix it?

EDIT: I may have said it vaguely. I wanted to avoid manual checks if I really got unique numbers so I was wondering if there is a better solution.

Answer 1

There are different ways of achieving this and which is more appropriate will depend on how many numbers you need to pick from how many.

• If you are selecting a small number of random numbers from a large range of potential numbers, then you’re probably best just storing previously chosen numbers in a set and “manually” checking for duplicates. Most of the time, you won’t actually get a duplicate and the test will have practically zero cost in practical terms. It might sound inelegant, but it’s not actually as bad as it sounds.
• Some underlying random number generation algorithms don’t produce duplicates at their “raw” level. So for example, an algorithm called a XORShift generator can effectively produce all of the numbers within a certain range, shuffled without duplicates. So you basically choose a random starting point in the sequence then just generate the next n numbers and you know there won’t be duplicates. But you can’t arbitrarily choose “max” in this case: it has to be the natural maximum of the generator in question.

• If the range of possible numbers is small-ish but the number of numbers you need to pick is within a couple of orders of magnitude of that range, then you could treat this as a random selection problem. For example, to choose 100,000 numbers within the range 10,000,000 without duplicates, I can do this:

  Let m be the number of random numbers I’ve chosen so far

  For i = 1 to 10,000,000

  Generate a random (floating point) number, r, in the range 0-1

  If (r < (100,000-m)/(10,000,000-i)), then add i to the list and increment m

  Shuffle the list, then pick numbers sequentially from the list as required

But obviously, there’s only much point in choosing the latter option if you need to pick some reasonably large proportion of the overall range of numbers. For choosing 10 numbers in the range 1 to a billion, you would be generating a billion random numbers when by just checking for duplicates as you go, you’d be very unlikely to actually get a duplicate and would only have ended up generating 10 random numbers.