Error: Concurrent Modification Exception

[Leticia Barbalho]

Hello guys,

I'm getting the following error:

java.util.ConcurrentModificationException
        at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:448)
        at java.util.AbstractList$Itr.next(AbstractList.java:419)
        at auge.action.GeraRelatorioMPNecessariasAction.execute(GeraRelatorioMPNecessariasAction.java:108)
        at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)
        at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
        at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
        at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)

The thing is that I'm not finding where the error is exactly. Here comes my code:

            List ordemItensList = OrdemItensService.getInstance()
                                                   .getOrdemItensOP(Integer.valueOf(numOrdem));
            
            ListIterator ordemItensIt = ordemItensList.listIterator();
            
            List listAcumulador = new ArrayList();
            
            while(ordemItensIt.hasNext()){
                OrdemItens oi = (OrdemItens) ordemItensIt.next();
                
                List mpList = MPFichaTecnicaService.getInstance()
                                                   .getMPFichaTecnicFT(oi.getProduto().getProduto());
                
                ListIterator mpIt = mpList.listIterator();
                
                while(mpIt.hasNext()){
                    MPFichaTecnica mpFT = (MPFichaTecnica) mpIt.next();
 
                    ListIterator acumuladorIt = listAcumulador.listIterator();
                    while (acumuladorIt.hasNext() || listAcumulador.size()==0){
                        
                        if (listAcumulador.size()==0){
                            listAcumulador.add(mpFT);
                        } else {
                            MPFichaTecnica mpFTAcumulador = (MPFichaTecnica) acumuladorIt.next();
                            if (mpFTAcumulador.getMateriaPrima().getMateria()
                            .compareTo(mpFT.getMateriaPrima().getMateria())==0) {
                                listAcumulador.remove(acumuladorIt.previousIndex());
                                int quantidade = mpFTAcumulador.getQuantidade().intValue()
                                + mpFT.getQuantidade().intValue();
                                mpFTAcumulador.setQuantidade(new Integer(quantidade));
                                listAcumulador.add(mpFTAcumulador);
                            } else {
                                listAcumulador.add(mpFT);
                            }
                        }
                    }
                }
            }

What am I doing that I should not do? What's so wrong? For as far as I know that's the only way of doing what I want. And I still don't see my error.

Anyone could help? Thanks

[Jeff Albertson]

Your error is clear. While you are iterating over listAcumulador with
ListIterator acumuladorIt, you are applying add and remove methods to
listAcumulador -- this is enough to cause the exception.

I don't understand your code so I can't suggest specific code, but
among your options is to (1) use *ListIterator's* add and remove methods,
or (2) make a copy of your list and iterator over the copy, but
change the original list (be careful if you choose to do this!).