This class extends ArrayList to add a finalize() that
calls delete on the files .
This helps in getting rid of the finalize() in the classes such
as DefaultAbstractBag, and they can be freed up without waiting
for finalize to be called. Only if those classes have spilled to
disk, there will be a (this) class that needs to be finalized.
CAUTION: if you assign a new value for a variable of this type,
the files (if any) in the old object it pointed to will be scheduled for
deletion. To avoid that call .clear() before assigning a new value.