public class ConcurrentModificationException4D
Exception that is thrown by the collections classes when it is detected that
a modification has been made to a data structure when this is not allowed,
such as when a collection is structurally modified while an Iterator is
operating over it. In cases where this can be detected, a
ConcurrentModificationException will be thrown. An Iterator that detects
this condition is referred to as fail-fast. Notice that this can occur
even in single-threaded designs, if you call methods out of order.