Getting concurrent modification exception even after using iterator

Question

Consider the following code:

 if(!unmatchedNewTruckRequirementSet.isEmpty()){
        for(Iterator<TruckRequirement> iterator = unmatchedNewTruckRequirementSet.iterator(); iterator.hasNext();){
            TruckRequirement newTruckRequirement = iterator.next();
            for(Iterator<TruckRequirement> iterator1 = oldTruckRequirementSet.iterator(); iterator1.hasNext();){
                TruckRequirement oldTruckRequirement1 = iterator1.next();
                if(oldTruckRequirement1.getTruckClassName().equals(newTruckRequirement.getTruckClassName())) {
                    LogUtil.logInfo("Truck Requirement Changed ", "Truck Requirement Changed : From : {0} to {1} where Truck Class is: {2}",
                            new Object[]{oldTruckRequirement1.getNumberOfTrucks(), newTruckRequirement.getNumberOfTrucks(), newTruckRequirement.getTruckClassName()});
                    unmatchedNewTruckRequirementSet.remove(newTruckRequirement);
                }
            }LogUtil.logInfo("Truck Requirement Added ", "New Truck Requirement Added with Truck Class : {0} and number of Trucks : {1}",
                    new Object[]{newTruckRequirement.getTruckClassName(), newTruckRequirement.getNumberOfTrucks()});

    }else {
        if(oldTruckRequirementSet.size() > unmatchedNewTruckRequirementSet.size()){
            for(TruckRequirement truckRequirement : oldTruckRequirementSet) {
                LogUtil.logInfo("Truck Requirement Removed ", "Truck Requirement Removed with Truck Class : {0}",
                        new Object[]{truckRequirement.getTruckClassName()});
            }
        }
    }

I have used iterator to iterate over the two hashset: unmatchedNewTruckRequiremnetSet and the other one is oldTruckRequirementSet. When I try to remove a set from unmatchedNewTruckRequiremnetSet I get Concurrent modification exception. Please help.

Answer 1

You will get ConcurrentModificationException if you use Collection.remove or add method while looping.

You must use Iterator.remove() to remove and avoid the exception.

for(Iterator<TruckRequirement> iterator1 = oldTruckRequirementSet.iterator(); iterator1.hasNext();){
    TruckRequirement oldTruckRequirement1 = iterator1.next();
    if(oldTruckRequirement1.getTruckClassName().equals(newTruckRequirement.getTruckClassName())) {
        LogUtil.logInfo("Truck Requirement Changed ", 
                "Truck Requirement Changed : From : {0} to {1} where Truck Class is: {2}",
                new Object[]{oldTruckRequirement1.getNumberOfTrucks(), 
                newTruckRequirement.getNumberOfTrucks(), newTruckRequirement.getTruckClassName()});

        // Use the iterator to remove
        iterator1.remove();
    }
}