Generate a Point object randomly excluding certain values

I have an ArrayList which contains some Point objects.

private List<Point> snakeXY = Arrays.asList(new Point(10,5), new Point(11,5), new Point(12,5),new Point(13,5) );

I want to generate a Point object ( new Point(x, y) ) randomly with x and y between 1 and 30 (exclude) and that is different from any Point object present in the snakeXY ArrayList. For example you can have a Point object with the values (14,5) or (15,29) or (5,12) but not (12,5) or (13,5) because they are already present in snakeXY.

I have the feeling that I have to use streams but I don't see how?

Solution

I don't really see an alternative to creating a pool of candidate points, which will hold all possible points on the 30x30 grid minus the points already in snakeXY, and selecting a random point from this pool. You can minimize the cost by using the int encoding of point position, rather than actually creating Point objects.

We first build a Set of point positions to exclude:

List<Point> snakeXY = Arrays.asList(new Point(10,5), new Point(11,5), new Point(12,5),new Point(13,5) );

int min = 2;
int max = 30;
int n = max - min;
Set<Integer> exclude = new HashSet<>();
for(Point p : snakeXY)
    exclude.add((p.y-min)*n + (p.x-min));

We then use a list to hold a pool of possible point positions:

List<Integer> pool = new ArrayList<>();
for(int i=0; i<n*n; i++) 
    if(!exclude.contains(i)) pool.add(i);

Now we simply select a random position from our pool and create the corresponding Point object:

Random rand = new Random();
int p = rand.nextInt(pool.size());
int pi = pool.get(p);
Point pt = new Point(min + (pi % n), min + (pi / n));
System.out.println(pt);

If we want to generate multiple points we need to remove the selected point position from the pool.

for(int i=0; i<5; i++)
{
    int p = rand.nextInt(pool.size());
    int pi = pool.remove(p);
    Point pt = new Point(min + (pi % n), min + (pi / n));
    System.out.println(pt);
}