How to avoid ConcurrentModificationException while iterating through a list

Question

I have a Country class which keeps a list that consisted of Human objects. And in Country class I have following function;

public void processOneDay(int day, List<Country> countryList, int numberOfCountries ){
        ListIterator<Human> iter = people.listIterator();
        while(iter.hasNext()){
            Human h = iter.next();
            h.move(day, countryList, numberOfCountries);
        }
    }

The move() method of Human class is responsible for movement of a Human from one country to other but this method is removing that person from source country's person list and add it to destination country's people list. So, doing this operation while iterating causes me ConcurrentModificationException. I tried to use remove function of iterator itself but I messed up things more.So, how can I handle this situation?

NickL · Accepted Answer · 2017-01-15 00:08:27Z

2

Just make a copy of the list, you can use this one for iterating.

List<Human> copy = new ArrayList<Human>(people);

answered Jan 15, 2017 at 0:08

NickL

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bikeman868 · Accepted Answer · 2017-01-15 00:20:50Z

0

As you discovered, you can't modify a list that you are iterating, so there are two choices, either you need to iterate over a copy of the list, or you need to traverse the list using a for loop that and directly index the items in the list.

Copying the list is much easier to write, but if the list is very big you will waste lots of CPU cycles and memory. Using for (var i = 0; i < list.Count; i++) list[i] is a bit more complicated but much more efficient. Note that if you remove items from the list then you need to not increment i.

answered Jan 15, 2017 at 0:20

bikeman868

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