How to generate a list of random numbers between a certain range, then get numbers not included in that list

I have generated a list of 20 unique random numbers between 1 and 300, and used that for what I need to use it for. However, I also need the numbers that have not made it to the list to be added into another list for use in another function.

Here is the code I have used to generate the random list of 20 numbers:

JToggleButton[][] p = new JToggleButton[5][4];
ArrayList<Integer> list = new ArrayList<Integer>();

    Random rand = new Random();
    for (int i = 0; i < p.length; i++) {
        for (int j = 0; j < p[i].length; j++) {

            int randomNum = rand.nextInt((300 - 1) + 1) + 1;

            while (list.contains(randomNum)) {
                randomNum = rand.nextInt((300 - 1) + 1) + 1;
            }

            list.add(randomNum);

// rest of code that I need the random number list for

I need to get the other 280 numbers not included in this 20 item list into another list, but I am unsure how to actually get these 'unused' numbers