Finish innerclass exercise 23
Exercise 23: (4) Create an interface U with three methods. Create a class A with a
method that produces a reference to a U by building an anonymous inner class. Create a
second class B that contains an array of U. B should have one method that accepts and stores
a reference to a U in the array, a second method that sets a reference in the array (specified
by the method argument) to null, and a third method that moves through the array and calls
the methods in U. In main( ), create a group of A objects and a single B. Fill the B with U
references produced by the A objects. Use the B to call back into all the A objects. Remove
some of the U references from the B.
Notice there are difference for return a reference of interface and return a reference of subclass of interface, difference is init number of interface, first one will init only once second will init as many as references number