Problem
The invoice-printing program has an unfortunate flaw-it mixes "application logic" (the computation of total charges) and "presentation" (the visual appearance of the invoice). To appreciate this flaw, imagine the changes that would be necessary to draw the invoice in HTML for presentation on the Web. Reimplement the program, using a separate Invoice Formatter class to format the invoice. That is, the Invoice and Line Item methods are no longer responsible for formatting. However, they will acquire other responsibilities, because the Invoice Formatter class needs to query them for the values that it requires.