```
Stack<String> ST = new Stack<String>();
ST.add("one");
ST.add("two");
ST.add("three");
ST.add("four");

ST.addAll(ST.subList(0,2));
System.out.println(ST);

```

Following Simple Code give j`ava.util.ConcurrentModificationException`.I not able to figure it out what is the reason behind for this exception?