CASE STUDY
412
A SIMPLE CALCULATOR
Problem:It’s useful to have a calculator that you can display on your computer screen
when you look around and can’t find your handheld one. Let’s write a Java application
that simulates a simple calculator. Once we have the basic design, we can easily extend
it to provide other functions.Brainstorming:Rather than look at the problem statement, you pick up your pocket cal-
culator. After all, this problem involves simulating the pocket calculator. What objects
do you see? A register on the top that shows what you are entering and displays the an-
swer, an On button, buttons for each of the 10 decimal digits, buttons for each of the
four basic operations (addition, subtraction, multiplication, and division), a Clear
button, and a button marked with an equals sign. There are buttons for percent and
square root, but you do not use them often enough to add them to the simulation. So
the first list of proposed classes is as follows:Filtering:We certainly need a register to show the output. Do we need an On button?
No, running the application is equivalent to the On button. However, we do need some
way of quitting the application. We could have a Quit button or we could let the
window-closing event end the application. Window closing... We need an object to rep-
resent the calculator itself; that is, we need a window to hold all the other objects. What
about the buttons for each digit? No, we can let the user input a number into a text field
rather than clicking individual buttons. This means that we need a label for the text
field. Do we need an Enter button? No, we can let the button for the operation signal
that the value is ready to be read. We do need a button for each operation. We certainly
need a Clear button to set the register value to zero. What about the Equals button? On
the calculator, it signals that we want to see the results so far in the register. Instead of
an Equals button, let’s just display the result of each operation in the register as we go
along. Here is our filtered list of classes:Register
On button
Buttons for the digits
Buttons for four operations
Clear button
Equals button