Hash Maps/AbstractHashMap

Notes

To recap, here is how these classes are organized:

Map is an interface class that defines the public interfaces that any Map data container must implement
AbstractMap is an abstract class that does the following:
- Define a map item (utility) class
- This is designed to take a K,V key value pair
- Also defines four more classes:
- Two Iterators and two Iterables - the key and value Iterator and Iterable classes.

itemSet() method:

The itemSet() method returns an iterator over the set of all MapItems
itemSet() returns an iterator over MapItem types, so keySet() and valueSet() both utilize this and unwrap only the key or value from the MapItem object.
We aren't defining itemSet anywhere in this class - just using it. Then we implement it as an abstract method, requiring concrete classes to implement it.
itemSet must be defined in our concrete class
This means that eventually our concrete class will have three total Iterators and three total Iterables - two Iterators and two Iterables defined in the abstract class, for the keys and values, and one Iterator and one Iterable defined in the concrete class, when we know more about how the actual hash map is being implemented under the hood.

More iterators vs iterables:

Iterable just says, this thing can be used with for-each syntax (it has a specific way to return an iterator). It is part of the core language.
Iterator has to be defined for each container.

Exceptions:

Ordering of class declaration and undeclared class declarations

I could not use MapItem class undefined in the Map interface (most generic of all)
MapItem class was actually protected and lived in AbstractMap class
Therefore, we had to hold off declaring an abstract MapItem iterator until the AbstractMap class, not the Map interface