Practice exercise -- Calculator

Task

First, build the GUI layout for a calculator, so that it looks like this:

• http://www.cs.fsu.edu/~myers/cop3252/rec/calc.png

• The "display" area should be an uneditable display of information, like on a real calculator. The display contents will change based on response to events, but it will not be a box that can get the cursor focus and be typed directly into. Suggestion: An uneditable text field would work nicely
• There should be button components for each of the digits, the decimal point, and the four basic arithmetic operations. (i.e. all the buttons in the picture for exercise 11.9 should be present).
• There should also be a Clear button, with the label 'C', (like on a real calculator).

Adding functionality to this layout

Events to handle

• Each button should trigger an action event, which will respond appropriately to the button that was pressed
• Key events are also to be active, such that pressing the letter or symbol corresponding to a button will trigger the same event that pressing the button would. (For example, pressing the '5' key is the same as clicking the '5' button)
• While pressing the '=' key should be the same as clicking the '=' button, also make the 'RETURN' key an event that activates this same feature (the '=' button, which finalizes a pending operation in the calculator).

Functionality

• When a number is typed, the digits will show up in the display as the user types them. (e.g. if '1' is pressed, '1' shows up in the display. Then, if '2' is pressed immediately after, "12" is now on the display. etc).
• Decimal numbers need to be taken into account. However, if a decimal is already part of the number being entered, pressing the '.' button again would have no effect
• The '=' button is not the only one that causes a result to be shown in the display area. For example, if the buttons are activated in this sequence: 12+34+ , then on the last button pressed so far (the '+'), the intermediate result 46 shows in the display area until new data is entered for this string of operations.
• Some calculators take order of operations into account in a string of continuous operations (cascaded operations with no = key). You do NOT need to do this. Assume that any operation key (including = ) will "enter" the previous operation and display the result.
  • Example: If I enter 2 + 4 * 5 = , this will do 2+4 when I click the * button, and display 6. Then, when the next operand is entered and = is pressed, it will do 6*5 and display 30.
  • Note: This matches how the calculator program built in to Windows XP works, but some calculators are smarter than this, and will handle order of operations on a cascaded entry like this.
• If an operator button is pressed multiple times in a row, this should have no effect beyond the first one. (The only valid entry after one of the 4 arithmetic operators will be another numerical operand).
• If the = button is pressed multiple times in a row, this should have no effect on the display, other than the first time
• If an operator button is pressed after the = button, then the current displayed result should become the first operand of the new operation
• Try out operations on the Windows calculator proram to see how the display on a calculator is commonly adjusted.
• Here's an example, with intermediate results shown, of how yours should update:

  Button pressed	Display showing
  1	1
  2	12
  +	12
  5	5
  .	5.
  3	5.3
  *	17.3
  2	2
  -	34.6
  2	2
  4	24
  .	24.
  9	24.9
  5	24.95
  =	9.65
  +	9.65
  8	8
  .	8.
  5	8.5
  =	18.15
  =	18.15