Creating my own Random Number Generator Java

I am trying to implement a Randomized Select Algorithm Quick Sort. I have pretty much implemented the rest of the pseudo-code except for one line.

The pseudo-code for the randomized select algorithm is this: http://ift.tt/1MCMzj5

My main issue is the partition method.

RANDOMIZED-PARTITION(A, p, r)

   i = RANDOM(p, r)
   exchange A[r] ↔ A[i]
   return PARTITION(A, p, r)

I have tried implementing it like this:

Random rand = new Random();
int i = rand.nextInt(r - p) + p + 1;
swap(array, r, i);
return partition(array, p, r);

I know that RANDOM is a method in the pseudo-code, and I know that I shouldn't add 1 at the end of i, but if I don't, it gives me the error of "bounds must be positive". This is only for testing purposes. I know that at some point r reaches -1.

Example Output:

    Please enter a small n: 10
    The unsorted array is:
        51  5   10  70  57  72  98  93  69  51
    Randomized Select: 
        5   10  51  51  57  69  98  93  70  72

It sorts half of the array, but not the other half, and I think the main issue is i since it's used for swapping.

My question is, how do I implement i = RANDOM(p, r)? How would I go about implementing my own RANDOM method for this case?

I can post the whole code if this is not enough info.