start of with 2 empty ArrayLists (for flexibility). In the first one you will assign a new random variable and the add it to your first arraylist after checking if the ArrayList contains the new value or not.
Now if you know how many combinations to expect you know the maximum size the first ArrayList needs to be, so we need an if statement to increment a counter that tells us when we are trying to add a value that is already in the array, but we need to make sure we only count it once so we get an accurate idea of the number of combinations we have explored. Thats why I suggest using a second Arraylist. This one shall also only be populated after a second test to see if the second array contains your new value, then finally add a condition to your loop that compares your counter to how big the first or second ArrayList is, if its the size of the predetermined max for the combinations you’ve explored all combinations and the job is done.