How many times can I use randomGenerator.nextDouble() before I need to reseed?

Question

I'm using the Random class in Java as a pseudo-random number generator. I'm using the function nextDouble a lot of times (~10^5). How many times before I have to reseed to prevent from getting the same numbers? Is reseeding needed?

    Random generator = new Random();
    double[] numbers = new double[n];
    for (int i = 0; i < n; i++) numbers[i] = generator.nextDouble();

This is for an experiment, the numbers will be used as coordinates for points on a space, so I want the distribution to be as uniform as possible.

Also how do I reseed? Where do I get the int seed from?

Answer 1

The random number generator will produce a random double from two random int values. The internal seed has 48 bit, so the random sequence repeats after at most 2^48 int values, or 2^47 double values.

Answer 2

You don't need to worry about reseeding etc if you use a Set (which guarantees unique values):

Random generator = new Random();
Set<Double> numbers = new HashSet<Double>();
while (numbers.size() < n)
    numbers.add(generator.nextDouble());

Despite what you might think, this executes quite quickly: 60 milliseconds for 100000 numbers on a my (typical) PC.

If you really want an array, you can extract it from the set.
If you want to maintain the order they were generated in, use a LinkedHashSet (it had similar performance)