Lesson 7:

Program Design

The book has taken you through the design of an ATM program.  Let's look at three stages of this program a little more carefully.

1. After the specification stage came the time to decide what classes are needed and to lay everything out visually on the screen.  None of the widgets have any functionality yet.  We have created a class called Keypad to hold the button widgets of our numeric key pad.  
   
   a) Download ATM1.java and Keypad1.java from the web page and try them out at this point.  Feel free to make changes to the layouts or other properties so that you are sure you understand the code so far.
   
2. Now we are going to add some functionality.  The ATM needs to know whether we are just starting out, ready to make a deposit, or ready to make a withdrawal.  We have added an instance variable mode which will keep track of whether we are in START_MODE, DEPOSIT_MODE, or WITHDRAW_MODE.  We have added methods introStart(), introDeposit(), and introWithdraw().  Finally we have added the actionPerformed method to the applet to handle actions from the deposit, withdraw, clear, and enter buttons.  The methods handleDeposit  and handleWithdraw are left as "stubs" or in other words, their bodies are left blank for now.
   
   a) Download ATM2.java and Keypad2.java from the web page and try them out.  
   b) Look at the methods introStart, introDeposit,  and introWithdraw  carefully.  Make sure that you can answer the following questions:
           What is being done with the help.setText(...) and help.append(...) method calls?
           Why are we performing an assignment to the  mode instance variable?
           What is being done with the setEnabled method for the various buttons?
   c) Look at the actionPerformed method carefully.  Make sure you can answer the following questions:
           How are we checking what button caused the action event?
           Why are we using a switch statement for the modes?
           For each mode, why are we checking if some buttons are pushed and not others?
    
3. Now we finish up the ATM applet.  We add a listener for the Keypad.  We also change the display from a TextField to a new class NumField which extends TextField.  The NumField class only displays legal numeric values.  We now can fill in the stubs for handleDeposit and handleWithdraw.
   
   a) Download ATM.java, Keypad.java, and NumField.java from the web page and try them out.
   b) Look carefully at the class NumField and make sure that you can answer the following questions:
           Why does the method isLegal need to use an instance variable hasDecimalPoint?
           Where is the value of hasDecimalPoint changed?
           What does the command super.setText(...) mean?
            Do you understand what all the methods of NumField do?
   c) Look at the changes to Keypad and the actionPerformed method of ATM.  Make sure that you can answer the following questions:
           What class is the listener for the Keypad?
            How does actionPerformed know that a button from the pad was pressed?
            How does actionPerformed display the number off the pad button that was pressed?
   d) Look at the handleDeposit and handleWithdraw methods carefully and make sure that you understand what they are doing.