Container exercise  *  * @author aguo
 * Exercise 13: "Note that this exception does not always indicate that an object has been concurrently modified by a different thread. If a single thread issues a sequence of method invocations that violates the contract of an object, the object may throw this exception. For example, if a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will throw this exception", this is a description of currentmodification
please put the Iterator in a method!!, this will avoid the currentmodification error
Container exercise  13, fix the exercise 13, now I follow the original design. This tell me you should make sense the author first, this is why test object work but it doesn't work when put it back.