In the given example, what is the logic behind the use of the Random class?

The method below creates a one-dimensional array of type int of a given size and randomly assigns the value 1 to a given percentage of the array. I don't completely understand the use of nextInt in this example. How is it made sure that in every iteration of the while loop a different index is chosen and not an index that was already chosen in a previous iteration.

import java.util.Random;

...

    int[] randomIntArray(int size, double percent) {
        int n = (int) (size * percent);
        int[] array = new int[size];
        Random r = new Random();

        while(n > 0) {
            int next = r.nextInt(size);
            if (array[next] != 0) continue;
            array[next] = 1;
            n--;
        }

        return array;
    }