I've pretty much finished the rest of my code, but have realised that i need to be able to remove elements from the array I'm using, so if i changed to array list it shouldn't mess with the rest of my sketch. i.e. would i have to change this:

It is a bit of overkill to use ArrayList for everything. If you know in advance the size and if it is fixed, just use arrays, they will use less memory. Array lists of primitives (int, float, etc.) also have a performance penalty. And the syntax is a bit less friendly.

Otherwise, indeed, an array list is actually just an oversized array, which is grown (and reference to data is copied) when the capacity is reached, so performance isn't bad.

beware though if you are deleting entries in checkBoundaryCollision - it'll renumber all the elements after the deleted one and so you'll skip items

for instance:

first time through, i = 0
0 1 2 3
A B C D

delete it
0 1 2
B C D

next loop, i = 1, which now points at C - you've skipped B

so if you're doing this (if! i can't tell that you are) then iterate through the loop backwards so that it's only renumbering items you've already processed.

Sorry for the length,

Wow thanks a lot.