Lab #6 Solution

Goal: This lab will introduce you to Java’s built-in facilities for exception handling, and show you why they might be a good idea.




Copy the Lab 6 directory by doing the following, starting from your home directory.




cp -r ˜cs61b/lab/lab6 . cd lab6




In this lab, you will modify a banking application. BankApp is a command-line driven program that allows you to create and manipulate customer bank accounts.




Part I: Compile It (1 point)

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Compile BankApp.java and all the classes it depends on. javac -g BankApp.java

Fix the compile-time errors by adding a try/catch statement in the appropriate place(s). (Don’t add a "throws" clause to the offending method; use a try/catch.)




Once you have the program running, experiment with it to see how it works.

What happens if you try to deposit a negative amount of money? What happens if you try to deposit to or withdraw from a nonexistent account?




There is a subtle bug in the withdraw operation. What is it?

(Hint: Under what _condition_ can you foil VirtualTeller.withdraw?)




Excepting the withdraw bug, the program handles errors correctly, but does so rather clumsily in the code. For instance, when you try to deposit to a nonexistent account, two error messages are printed. (Why?) Error-handling is not done well.




Part II: Throwing Exceptions (2 points)

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An exception class called BadAccountException has been created for your use; it should be thrown whenever an account cannot be found. Add the appropriate "throw" statement to VirtualTeller.java. (You should only need one.) Next, eliminate all the print statements generated when an account is not found.

Add an appropriate "throws" clause wherever necessary. Be sure to catch the

BadAccountException in main(); errors should not cause the application to halt.




Is your modified code for VirtualTeller easier to read? Have you eliminated the problem of double error messages? (If not, you put the "throw" statement in the wrong place.)




PART III: A New Exception (1 point)

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The withdraw method still has a bug, and when you try to deposit an invalid

(e.g. negative) amount, a nonsensical "New balance" statement is printed.

Fix these problems by creating a class "BadTransactionException", and adding the appropriate "throw" statements and "throws" clauses to VirtualTeller.java. Again, make sure the exception is caught in main(). Note that you will have to recompile AccountData explicitly.

EXTRA: Slightly different behavior (optional)

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Observe that when an exception occurs during a transaction, you are prompted for a new command. Suppose you want slightly different behavior: if an exception occurs during a command, then the command is automatically repeated

so you don’t have to retype it. How can you do this by moving ONE line of code in BankApp.java?




Check-off

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Demonstrate the application for your TA or Lab Assistant. Show him/her the changes in your code.




1 point: Run your program and show that it behaves well if an invalid command is entered.

2 points: Show that your program behaves well when you try to deposit to a nonexistent account. Explain which method you put the "throw" statement in.

1 point: Show what happens when you try to deposit or withdraw an invalid amount.