Replacing the 100 in the previous line of code
for(int i = 0; i<100; i++)
for(int i = 0; i<ballCollection.size(); i++)
means it will keep adding another class instance every frame indefinitely. It ends in a very trippy visual.
I continued to play around with arraylists changing the location of the starting points, the speed, shapes, colour and even reversing the gravity.