Your instructor will assign one or more of the following problems. Submit all appropriate files for grading, including code files, screen captures, supplemental files (e.g., image files), and text files.

1. For this assignment, extend the calculator you built in the lab to do the following:

   • The calculator should implement a form of memory in which it remembers the result of the most recent computation;
   • The controller should allow the user to type the string “memory” into either of the operand fields indicating that the value from memory should be used for that operand;
   • The calculator should set the initial memory value to 0.0;

   For example, if the user uses the calculator to compute “1” + “1”, and then computes “memory” + “3”, the answer should be 5 (i.e., the remembered value 2 plus 3). There are different ways to implement these features. Here is one way:

   • Calculator - Modify the Calculator class to: store the remembered value in an instance variable; initialize the instance variable to 0.0; provide an accessor method for the memory value; and include code to update the memory value whenever a new computation is made.
   • CalculatorController - Modify the actionPerformed() method in the CalculatorController class to detect the use of the string “memory” in either of the operand fields and replace that string with value accessed from the calculator object. Note that because the calculate() method receives double values, the controller cannot pass it the string value “memory”. Thus, your controller must replace the string “memory” with the appropriate remembered value from the calculator, a double, before calling calculate().
   • CalculatorTest - Include an appropriate unit test case for the calculator’s memory feature. Note that because your CalculatorController is handling the “memory” string, you cannot build a unit test exercising that part of your implementation; you only need to unit test the memory features in the Calculator class itself (i.e., that initial memory value is 0.0 and that the memory value changes on every new calculation).

   Extra-credit: Implement the calculator so that the memory value is undefined at first. If the user tries to use the memory feature before performing a calculation, the Calculator class should throw an appropriate exception and the CalculatorController should handle that exception. Be sure to unit test this feature.

   Note that while the lab version of the calculator could have been implemented as a static library, this memory extension requires the use of instance variables and methods.