CIS 180: Object-Oriented Programming
Homework #7

Objectives

• To practice designing graphical user interfaces
• To practice event handling

Problem Statement

For this project you are asked to complete the design and implementation of the AddingMachine class. You will be adding code to the homework from last week to make the adding machine's buttons functional. The adding machine is like a calculator application, but only does addition. The adding machine should function as follows:

Initially the number 0 is displayed. The display is updated as the user presses digit buttons to enter numbers. When the enter button is pressed, the number on the display is added to the previous sum, and the result is displayed. If the enter button is pressed two or more times in a row, only the first press will have any effect. Pressing the clear button resets the adding machine to its initial state.

Click the button below to run a sample solution that demonstrates how the adding machine should work.

Design

• Think about what additonal fields will be needed. For example, what information does the AddingMachine need to have in order to update the display when the enter button is clicked? How will that information be stored?
• In order to handle button clicks you will have to implement the ActionListener interface. What method is required by that interface? Can you use additional methods to further decompose and simplify the problem? Hint: There are three types of buttons that can be clicked -- digits, enter, and clear. You might write a method to handle each of these types of button clicks.
• Update your UML class diagram of the AddingMachine class to include all of the fields and methods.

Implementation

1. Add declarations of any additional fields required.
2. Add the declaration that AddingMachine implements the ActionListener interface, and add a stub (method with no code between the curly brackets) for the method required by the interface.
3. Add stubs for any addtional methods of your design.
4. Add code to the constructor method to register as an action listener with each of the buttons.
5. Add code to change the display when digit buttons are pressed. First, just update the display to show the last digit button pressed (one digit). When that is working, modify your code so that the user can enter multiple digit numbers on the display.
6. Implement the Clear function of the adding machine.
7. Implement the Enter function of the adding machine.

Hints

• The adding machine needs to keep track of two numbers: the value that is currently displayed, and the value in the adding machine's memory.
• It will be easier if you keep track of the value displayed on the adding machine as an Integer rather than as a String of digits, so that the value can be used in arithmetic.
• You can convert an Integer value to a String when necessary by sending a toString message.
• When a button is clicked (or menu item selected, etc.), you can get the String that labels the button by sending the ActionEvent a getActionCommand message.
• You can use the Integer class parseInt method to convert a String of digits to an integer.
• Putting together the two previous hints, you can find the int value corresponding to a digit button that was pressed from the ActionEvent, e:
  

     int digit = Integer.parseInt(e.getActionCommand()); 

  This will cause an error if the button that was clicked is not labeled with an integer.

When your assignment is complete, add some comments to the comment block of the java source code explaining any problems you encountered in completing the assignment and describing any bugs in your solution.

Use the subject line CIS-180 HW#7 in your email.

There will be a 10% penalty for assignments received after the due date. Assignments will not be accepted more than one week past the due date.