f.trim(); would also work in this case - I find it easier to read than substring, though substring is more efficient probably.

Sure. See my other solutions.

Even better if you use lambdas instead.

I'd say instead of returning n-1 just return -1 everytime, The cube may not be made following the construction guidelines therefore falling into the values of
2 iterations of the loop

I like the f.substring(0,f.length()-1) part. Never thought of it I am going to now use it more often.
Kudos for lambdas :)

Not the most readable code but I guess I have not messed around with streams and stuff :P