Iterator INTERFACE ANALYSIS

A terator is a set iterator. You do not need to care about the implementation of the set through the iterator access interface.
The iterator interface defines three methods:
1. hasnext ()
Whether there is another element.
2. Next ()
Returns the next element.
3. Remove ()
Delete the current element.

Only three simple methods are defined. Pay attention to the following points:
1) differences between iterator and enumeration
Iterator is used to replace enumeration. Enumeration only defines two methods and does not have the delete function.
2) The remove () method can be called only after the next () method is called, and the remove () method can only be called once at most once after each next () method is called. Otherwise, an illegalstateexception is thrown.