I am working on a sketch which involves creating and moving two different types of geometric shapes about the screen. Currently displayed shapes are being held in two ArrayLists, forms1 and forms2. When two different types overlap, I want them to disappear and I have written the following function to do so:

1. void checkIntersect() {
2.   for (int i = forms1.size() - 1; i >= 0; i--) {
3.     for (int j = forms2.size() - 1; j >= 0; j--) {
4.       Circle form1 = (Circle) forms1.get(i);
5.       Line form2 = (Line) forms2.get(j);
6.       float formActualDist = dist(form1.xpos, form1.ypos, form2.originX, form2.originY);
7.       float formMinDist = (form1.diameter/2) + (form2.diameter/2);
8.       if (formActualDist <= formMinDist) {
9.         forms1.remove(i);
10.         forms2.remove(j);
11.       }
12.     }
13.   }
14. }

I am consistently getting an "IndexOutOfBoundsException: Index: 4, Size: 4" (but with varying equal numerical values) on line 4 at different points after running the sketch. If line 4 is moved to between lines 2 and 3, then the error occurs on line 9. It would be much appreciated if anyone could provide some insight into this situation. If clarification or more of the sketch is required, please let me know. Thank you!