CMPU-181, Spring 2013
Interactions like those seen in class
March 6, 2013


Welcome to DrJava.  Working directory is /Users/hunsberg/Desktop
> // The BankAcct class that we defined in class (!) contains four different kinds
> // of information:  (1) class-oriented data (i.e., "fields")
> //                  (2) class-oriented functions (i.e., "methods")
> //                  (3) object-oriented data (template for creating objects)
> //                  (4) object-oriented methods (functions that an object can be asked to "run").
> // 
> // Example of class-oriented data:  BankName.
> BankAcct.BankName
"CMPU-181 Bank of Vassar"
> // Example of class-oriented function/method:  cofunc.
> BankAcct.cofunc()
Hello from our bank!
> // Note that class-oriented data and functions/methods can be used even if no BankAcct
> // objects have been created.  That is because they are independent of any objects.
> // Below, we will create some BankAcct objects.  Each BankAcct object has three fields,
> // called:  BALANCE, OWNER and PASSWORD.  The NEW keyword is used to create new objects.
> new BankAcct()
Not doing any initialization!
BankAcct@71257687
> // Notice that the above expression created a new BankAcct object and then automatically
> // called the BankAcct "constructor" function that takes no inputs.  That constructor
> // function caused the "Not doing any initialization!" string to be displayed.
> // Below, we cause a different "constructor" to be called, one that takes some input
> // values that initialize the contents of the new BankAcct object.  To enable us to
> // view the contents of the object after we create it, we'll store the new object in
> // a variable called myAcct.
> BankAcct myAcct = new BankAcct("luke", "pword", 1000)
> myAcct.owner
"luke"
> myAcct.balance
Static Error: No field in BankAcct has name 'balance'
> // The above error was caused by the fact that the BALANCE field is declared to
> // be PRIVATE.  That is, it is not available for direct inspection.  Similar remarks
> // apply to the PASSWORD field.
> myAcct.password
Static Error: No field in BankAcct has name 'password'
> // Using PRIVATE fields enables the BankAcct class to control access to certain
> // information.  However, if a user knows the password, the getBalance method
> // can be used to access the balance of an account.
> myAcct.getBalance("pword")
1000.0
> // But if we don't give the right password:
> myAcct.getBalance("wrongWord")
Hey!  Password incorrect!
0.0
> // Similar remarks apply to making a withdrawal:
> myAcct.makeWithdrawal("pword", 25)
Withdrawal authorized!
New balance = 975.0
25.0
> myAcct.makeWithdrawal("wrongWord!", 25)
Withdrawal not authorized!
0.0
> // Also, we can't withdraw a negative amount, or an amount that would cause our
> // balance to go negative.
> myAcct.makeWithdrawal("pword", 20303030)
Withdrawal not authorized!
0.0
> myAcct.makeWithdrawal("pword", -35)
Withdrawal not authorized!
0.0
>