Answer:

Yes. You have tried to access private data from "outside" the object. The compiler lets you know what it thinks about that:

compiling: CheckingAccountTester.java
CheckingAccountTester.java:46: 
Variable balance in class CheckingAccount not accessible from class CheckingAccountTester.
         System.out.println( bobsAccount.balance );
                                        ^
CheckingAccountTester.java:47: 
Variable balance in class CheckingAccount not accessible from class CheckingAccountTester.
         bobsAccount.balance = bobsAccount.balance + 200;
                    ^
CheckingAccountTester.java:47: 
Variable balance in class CheckingAccount not accessible from class CheckingAccountTester.
         bobsAccount.balance = bobsAccount.balance + 200;
                                          ^
CheckingAccountTester.java:48: 
Variable balance in class CheckingAccount not accessible from class CheckingAccountTester.
         System.out.println( bobsAccount.balance );
                                        ^
4 errors

Careful Access Control

It may seem a bit silly that the CheckingAccount class uses private to prevent main() from seeing its variables, but then provides methods so that main() can access them anyway. The idea of this is that the access methods have control over each access to the private data. For example, a programmer can't increase the balance of a checking account by writing:

bobsAccount.balance = bobsAccount.balance + 200;

To increase the balance, the processDeposit() method must be used. A more elaborate method might check that it was OK to proceed before adding the deposit to the balance. It might check that the account has not been closed, might ask for a password before it allows access, and might log every change in a history file.

When data is private the only changes to it are made through a small number of access methods. This helps keep objects consistent and bug-free. If a bug is detected, there are only a few places to look for it.

QUESTION 5:

(Test of your memory: ) In the checking account example, what is the minimum balance before the 15 cents charged per check is dropped?