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Iterators can be designed so that they are fail fast or fail safe. Depending on the underlying implementation of the Iterator, a ConcurrentModificationException is thrown if the Collection is modified while Iterating over the data structure. It pays to understand how an Iterator will behave under both conditions. Lets try to implement fail fast Vs fail safe iterators of our own.

Our data structure for this example is pretty simple. It defines an interface that abstracts set and get operations on a structure. How the underlying classes handle invalid set operations or the size of the structure is implementation dependent

A data structure interface:

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public interface Data<T> extends Iterable<T>
{
    int size();
    T getElement(int position);
    void setElement(int position, T t);
}

An underlying implementation of such an Interface, could be say an array of Integers whose size is fixed. Invalid indexes on bounds are not allowed and the internal structure will not grow or shrink. An implementation is given below