exception - Why am I getting java.util.ConcurrentModificationException?

Question

As I run the following code :

    import java.util.LinkedList;

    class Tester {
      public static void main(String args[]) {
        LinkedList<String> list = new LinkedList<String>();
        list.add(new String("suhail"));
        list.add(new String("gupta"));
        list.add(new String("ghazal"));
        list.add(new String("poetry"));
        list.add(new String("music"));
        list.add(new String("art"));

        try {
            for(String s : list) {
            list.add(0,"art");
            list.remove(6);
            System.out.println(list);
        }
        }catch(Exception exc) {
            exc.printStackTrace();
        }

    }
}

I get an exception that says :

java.util.ConcurrentModificationException
    at java.util.LinkedList$ListItr.checkForComodification(Unknown Source)
    at java.util.LinkedList$ListItr.next(Unknown Source)
    at Tester.main(Tester.java:14)

Why am I getting this exception ?

Edit : tmpList is a LinkedList whose each node contains an object of type DepConfAttr. I am sorting the tmpList on the basis of memory (Highest memory first), which is one of the attribute of the object of DepConfAttr.

The above code reflected what I am trying to achieve through the following code

int size = tmpList.size();
        int elementBC = 0; // element being checked
        int startIndex = 1;
        for (DepConfAttr dca : tmpList) {
            long maxMem = dca.getMemory(); // Let this be the maximum memory
            for(int i = startIndex ; i < size ; i++) {
                DepConfAttr dcaTmp = tmpList.get(i);
                if(maxMem < dcaTmp.getMemory()) {
                    tmpList.add(elementBC, dcaTmp);
                    tmpList.remove(i+1);
                    maxMem = tmpList.get(elementBC).getMemory();                        
                }
            }
            elementBC++;
            startIndex++;
            size--;
        }

Answer 1

Why am I getting this exception ?

You're removing an item from the list other than via the iterator, while iterating over it. You're also adding to the list while you're iterating over it.

It's not really clear what you're trying to achieve here, but other than with the concurrent collections, you'll always get an exception when you try to do that.

One common fix to this is to create a copy of the list first and iterate over that, modifying the original as you go.