Project: calculator-gui Authors: Michael Kölling and David J. Barnes This project is part of the material for the book Objects First with Java - A Practical Introduction using BlueJ Sixth edition David J. Barnes and Michael Kölling Pearson Education, 2016 This program is meant to implement a simple pocket calculator. In its initial version, it is incomplete. The Calculator and UserInterface classes can be used as they are, but the CalcEngine, the bit that does the real calculations, has not been implemented. It is your task to finish the job. Have fun! To start this application (once you have implemented the missing bits) just create an object of class Calculator.