java - How to track an object inside an ArrayList becoming null? -

the structure have is:

map<string, arraylist<bean>> 

this arraylist modified (added/removed) different locations within different threads.

at times, bean inside arraylist becoming null. how can track when becomes null? want track makes bean null, fix bug.

this happens when scenario tested huge data set.

here's exact code that's failing:

for (int = 0; < eventlogs.size(); i++) {     messageeventlogbean msgeventlogbean = (messageeventlogbean) eventlogs.get(i);     eventlogs.remove(i);      try {         logwriter.write(msgeventlogbean, context);     } catch (fusionexception e) {         logger.error("error while writing event log bean ", e);     } } 

instead of standard list implementation, use anonymous class overrides add method , adds special code check if added object null, this:

    list<t> list = new arraylist<t>() {         public boolean add(t e) {             if (e == null) {                 throw new nullpointerexception("attempt add null list");             }             return super.add(e);         }     }; 

you should override "add" methods sure. when code adds null, explode , see exception in log , able see did examining stacktrace.

edited

to clear, impossible object in list "become null". can add null, or can remove object list, object already in list stay there until removed.

to clear, only way null in list putting there - ie adding via 1 of add() methods or addall() method. concurrent modification issues can not cause issue (you may concurrentmodificationexception, still won't put null in there).

code such as

object o = list.get(1); o = null; 

has no effect because you're nulling copy of reference object - list still reference object.

however, depending on design of bean objects, might mutable. while reference bean object remain intact, might possible of fields within bean become null. catch this, need code setters explode when given null argument, either re-writing class, or overloading setters via anonymous class (similar initial suggestion above).