Exception in thread "main" java.util.ConcurrentModificationException

package test;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Test {

public static void main(String[] args) {

List list = new ArrayList();
list.add("1");
list.add("2");
list.add("3");

Iterator iterator = list.iterator();
while(iterator.hasNext()){

Object next = iterator.next();
if(next.equals("2")){
list.add("5");
}else{
System.out.println(next);
}
}
System.out.println(list);
}

}