Interning principle
lTo intern objects
in a class, we need a way to get to all objects
in that class that were ever created.
lThe client does not
use the constructor, but rather uses a factory method
that returns objects.
lThe factory method
checks to see whether the prescribed
object has already been created
lIf so, it returns the
existing objectÕs reference.
lIf not, it creates a new
object (using the constructor) and returns the reference to that object.