You could create your own ArrayList-Class (derived from the original) and override the Add-Method
Then you could check for null when Adding.

publicbooleanAdd(E e){if(e ==null)returnfalse;elsereturnsuper.Add(e)}

As Mark stated in the comments you perhaps want to override all other possibilties of Adding values too. (

• add(E e)
• add(int index, E element)
• addAll(Collection c)
• addAll(int index, Collection c)
• set(int index, E element)