The problem is in the spawn method below - in line 9 you have
size.add(size);
Although you have a linked list called size you also have an int parameter called size. so it is trying to add size to an int. The same applies to offset change lines 9 and 10 to
this.size.add(size); this.offset.add(offset);
In future I suggest that you use a unique name for each variable/parameter