java - Iterator but still ConcurrentModificationException -

all,

running issue of concurrentmodificationexception , struggling find resolution partly because can't see modifiying list while iterating it... ideas?? i've highlighted line causing issue (it3.remove()). @ standstill one..

edit: stacktrace:

exception in thread "thread-4" java.util.concurrentmodificationexception     @ java.util.arraylist$itr.checkforcomodification(unknown source)     @ java.util.arraylist$itr.next(unknown source)     @ com.shimmerresearch.advancepc.internalframeandplotmanager.subtractmaps(internalframeandplotmanager.java:1621) 

line 1621 corresponds it3.remove() in referenced code above.

private void subtractmaps(concurrentskiplistmap<string, plotdevicedetails> firstmap, concurrentskiplistmap<string, plotdevicedetails> secondmap) {      // iterate through secondmap     iterator<entry<string, plotdevicedetails>> it1 = secondmap.entryset().iterator();     while (it1.hasnext()) {         entry<string, plotdevicedetails> itentry = (entry) it1.next();         string mapkey = (string) it1entry.getkey();         plotdevicedetails plotdevicedetails = (plotdevicedetails)it1entry.getvalue();          // if find entry exists in second map , not in first map continue         if(!firstmap.containskey(mapkey)){             continue;         }           // iterate through list of channels belonging secondmap         iterator <plotchanneldetails> it2 = plotdevicedetails.mlistofplotchanneldetails.iterator();          while (it2.hasnext()) {             plotchanneldetails it2entry = it2.next();              // iterate through list of channels belonging firstmap             iterator <plotchanneldetails> it3 = firstmap.get(mapkey).mlistofplotchanneldetails.iterator();             innerloop:             while(it3.hasnext()){                 // if channel common both first map , second map, remove firstmap                 plotchanneldetails it3entry = it3.next();                 if(it3entry.mchanneldetails.mobjectclustername.equals(it2entry.mchanneldetails.mobjectclustername)){                     it3.remove(); // line causing concurrentmodificationexception                     break innerloop;                 }             }         }     } } 

plotdevicedetails.mlistofplotchanneldetails , firstmap.get(mapkey).mlistofplotchanneldetails reference same list.

whether plotdevicedetails , firstmap.get(mapkey) reference same object unknown without more information, share channel list.