Concurrent Modification exception

Question

I have this little piece of code and it gives me the concurrent modification exception. I cannot understand why I keep getting it, even though I do not see any concurrent mo

Answer 1

I cannot understand why I keep getting it, even though I do not see any concurrent modifications being carried out.

Between creating the iterator and starting to use the iterator, you added arguments to the list that is to be iterated. This is a concurrent modification.

    ListIterator<String> it = s.listIterator();  

    for (String a : args)
        s.add(a);                    // concurrent modification here

    if (it.hasNext())
        String item = it.next();     // exception thrown here

Create the iterator AFTER you've finished adding elements to the list:

    for (String a : args)
        s.add(a); 

    ListIterator<String> it = s.listIterator();  
    if (it.hasNext())
        String item = it.next();

Answer 2

If the above solutions doesn't work properly. You can use old for-loop for iterating a List at the same time adding new items. See the example below:

import java.util.*;

public class SomeClass {
    public static void main(String[] args) {
        ArrayList<AClass> aList = new ArrayList<AClass>(); // we will iterate this


        // this will cause ConcurrentModificationException. 
        // Since we are iterating the list, at the same time modifying it.
        /*for(AClass a: aList){
           aList.add(someMethod(a));
        }*/

        // old fashion for-loop will help
        int limit = aList.size();
        for(int i=0; ctr<limit; ++i){
           AClass a = aList.get(i);
           aList.add(someMethod(a));
        }


    }
}

Answer 3

to understand this lets look at source of HashMap implementation:

public class HashMap<K, V> extends AbstractMap<K, V> implements Cloneable, Serializable{

which contains HashIterator as below:

private abstract class HashIterator {
    ...
    int expectedModCount = modCount;
    ...

    HashMapEntry<K, V> nextEntry() {
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
        .... 
        }

every time you create a iterator:

• a counter expectedModCount is created and is set to value of modCount as entry checkpoint
• modCount is incremented in cases of use put/get (add/remove)
• nextEntry method of iterator is checking this value with current modCount if they are different concurrent modification exception is throw

to avoid this u can:

• convert map to an array (not recommended for large maps)
• use concurrency map or list classes (CopyOnWriteArrayList / ConcurrentMap)
• lock map (this approach removes benefits of multithreading)

this will allow you to iterate and add or remove elements at the same time without rising an exception

Concurrency map/list iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

More info on CopyOnWriteArrayList

Answer 4

This didn't work:

LinkedList<String> linkedList = new LinkedList<String>();
ListIterator listIterator = linkedList.listIterator();
linkedList.add("aa");
linkedList.add("bb");

This worked:

LinkedList<String> linkedList = new LinkedList<String>();
linkedList.add("aa");
linkedList.add("bb");
ListIterator listIterator = linkedList.listIterator();

Answer 5

You are not allowed to continue iterating over an iterator after the underlying list is modified. Here you create the iterator before adding a few items to s, and then proceed to do a hasNext() and a next() on it after the additions, leading to the ConcurrentModificationException

Answer 6

To avoid the ConcurrentModificationException, you should write your code like this:

import java.util.*;

public class SomeClass {

    public static void main(String[] args) {
        List<String> s = new ArrayList<String>();

        for(String a : args)
            s.add(a);

        ListIterator<String> it = s.listIterator();    
        if(it.hasNext()) {  
            String item = it.next();   
        }  

        System.out.println(s);

    }
}

A java.util.ListIterator allows you to modify a list during iteration, but not between creating it and using it.