Question

When you Iterate a collection using Java 1.5 modern for loop and remove some element
concurrentmodifuicationexception is thrown.

But when I run follwoing code it does not throw any exception:

    public static void main(String a []){
          Set<String> strs = new HashSet<String>();
          strs.add("one");
          strs.add("two");
          strs.add("three);

          for(String str : strs){
                   if(str.equalsIgnoreCase("two"){
                          strs.remove(str);
                   }
          }  
    }   

Above code does not throw ConcurrentModificationException. But when I use any such for loop in my web application service method it always throws one. Why? I am sure no two threads are accessing the collection when it runs in service method So what causes the difference in two scenarios where it is thrown in one and not in the other?

Answer 1

I get a ConcurrentModificationException when running your code (after fixing the few typos).

The only scenarios where you would not get a ConcurrentModificationException are:

• if the item that you remove is not in the set, see example below:
• if you remove the last iterated item (which is not necessarily the last added item in the case of a HashSet)

public static void main(String[] args) {
    Set<String> strs = new HashSet<String>();
    strs.add("one");
    strs.add("two");
    strs.add("three");

    for (String str : strs) {
        //note the typo: twos is NOT in the set
        if (str.equalsIgnoreCase("twos")) {
            strs.remove(str);
        }
    }
}