Concurrent Modification exception [duplicate]

Question

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.

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();

From the JavaDoc: for ConcurrentModificatoinException: "it is not generally permssible for one thread to modify a Collection while another thread is iterating over it".

It simply means that if you still have an open iterator, you aren't allowed to modify the list because the iterator loop will break. Try moving ListIterator<String> it = s.listIterator(); till after the for loop.

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

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));
        }


    }
}