Why am I getting java.util.ConcurrentModificationException?

When you iterate through a list, you can't remove items from it. Doing so causes the exception.

Do:

int size = list.size();
for (int i = 0 ; i< size ; i++) {
   list.add(0,"art");
   list.remove(6);
   System.out.println(list);
}

The problem is that you're directly modifying the List while an Iterator is trying to run over it. The next time you tell the Iterator to iterate (implicitly, in the for loop), it notices the List has changed out from under it and throws the exception.

Instead, if you need to modify the list while traversing it, grab the Iterator explicitly and use it:

List<String> list = ....
Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
    String s = iterator.next(); // must be called before you can call iterator.remove()
    iterator.remove();
}

You still can't insert into the List while this is going on, and this won't let you remove arbitrary elements, just the current one.

The problem is making a change to the list while using an implicit Iterator over it. For the quoted code, the simplest solution is to not use the Iterator at all:

for(int i=0; i<list.size(); i++) {
  list.add(0,"art");
  list.remove(6);
  System.out.println(list);
}

You may need to post more realistic code to get advice for the best solution. If you want to possibly remove the current item while iterating over the list, use an explicit Iterator loop, and use the Iterator's remove() method. In other cases, the best solution is to form a plan for the changes inside the loop, but execute it afterwards. For example, during a loop over list build a removeList containing a list of indexes of elements you want to remove. In a separate loop over removeList, remove those elements from list.

Because you are concurrently (simultaneously) modifying and iterating through a collection. Java doesn't like this

Since you don't actually use s, you can use a standard for loop

for(int i=0; i< list.size(); i++) {
      list.add(0,"art");
      list.remove(6);
      System.out.println(list);
}

The ConcurrentModificationException is thrown when iterating through the list and at the same time when you are trying to modify (add/remove) the contents of the list either through another thread or in a loop.

You can try using ConcurrentLinkedQueue or as John stated do a copy and modify the original in iteration.

Queue<String> list = new ConcurrentLinkedQueue<String>();

list.add("suhail");
list.add("gupta");
list.add("ghazal");
list.add("poetry");
list.add("music");
list.add("art");

int size = list.size();

for(int i = 0; i < size; i++){
    list.add("art");
    list.remove("art");
    System.out.println(list);
}

You would need to use iterator explicitly for that to work. Example:

 Iterator<String> iter = li.iterator();
   while(iter.hasNext()){
    if(iter.next().equalsIgnoreCase("some value"))
      iter.remove();
    }
   }

More info here: http://www.coderanch.com/t/233932/threads/java/deal-Concurrent-Modification-Exception but just google the exception and you will find lots of examples.