Description: This class creates an user interface for

$*$ a four$-$function integer calculator. The keys are arranged

$*$ in a $4$ x $4$ grid, which is at the center of the Frame's

$*$ border layout. At the north of the border layout is the

$*$ calculator's accumulator$/$display$,$ implemented with a text

$*$ field. This version creates an instance of the Calculator

$*$ class which performs the operations associated with the

$*$ interface. See the actionPerformed$()$ method.