Programming Random numbers without repetition of numbers in java?

Answer 1

Since you (hopefully) can't predict random numbers in Java, we'll need to store the numbers that we've generated so far. A Set seem ideal for this, as it automatically handles keeping itself free of duplicates. The problem is that the order in which elements are removed from a set are not guaranteed to be in the same as the order in which they were added. I would suggest using a List of some sort:

// returns a List of n random integers from 0 to max

static List<Integer> getUniqueRandomInts(final int n, final int max) {

// if n > max, this function would never return, so remove the possibility

if (n > max) {

return null;

}

// new list of values

final List<Integer> list = new ArrayList<Integer>();

// seed our random generator with the current time

final Random rnd = new Random(System.currentTimeMillis());

// keep trying to add numbers until we have the proper number

while (list.size() < n) {

int num = rnd.nextInt(max);

if (!list.contains(num)) {

list.add(num);

}

}

return list;

}