8.1.1 Common superclass

As mentioned, both types of accounts have a withdraw method, but the statement part of of these two methods are different.

We may define the common attributes in a general class Account like the one we have defined in the previous chapters:

class Account(owner: ref Customer):
   balance: var float
   interest: var float
   addInterest:
      balance := balance + (balance * interestRate) / 100
   deposit(amount: var float):
      balance := balance + amount
   withdraw(amount: var float) -> newB: var float:
      -"-
      newB := balance

We have included a partial description of a withdraw method to indicate that accounts in general have a withdraw method that returns the value of balance. In section below, we introduce a language mechanism called virtual method that makes it possible to describe such partial methods.

We may the use class Account in the description of the more specific accounts. For class SavingsAccount this looks as follows:

class SavingsAccount: Account
   releaseDate: var Date
   withdraw(amount: var float) -> newB: var float: 
      if (today > releaseDate) :then
         balance := balance - amount
      :else
         console.print("It is not possible to withdraw")
      newB := balance
    newReleaseDate(newDate: var Date, newInterest: var float):
      releaseDate := newDate
      interest := newInterest

Class SavingsAccount is described as a subclass of class Account — specified by Account following the ':' (colon) in class SavingsAccount, like: class SavingsAccount: Account -"-.

Making SavingsAccount a subclass of Account implies that all the attributes defined in class Account are also attributes of SavingsAccount. A SavingsAccount objects thus have attributes owner, balance, interest and deposit. Only the attributes special for a savings account need to be described in class SavingsAccount.

We note again, that a withdraw method is defined in both class Account and class SavingsAccount and we will show how to specify this using a virtual method in the next section.

We may make a similar description of class CreditAccount:

class CreditAccount: Account
   maxCredit: var float
   withdraw(amount: var float):
      if (-balance < maxCredit) :then
         balance := balance - amount
      :else 
         console.print("Not possible to withdraw beyond max credit") 
   changeCredit(newMax: var float, newInterest: var float):
       maxCredit := newCredit
       interest := newInterest 

Class CreditAccount is also described as a subclass of Account and only the attributes that are special for a credit account are specified.

As mentioned, SavingsAccount and CreditAccount are subclasses of Account. Account is a superclass of SavingsAccount and CreditAccount.

The figure below illustrates the sub/super-class relationships between the classes Account, SavingsAccount and CreditAccount. As used before, a rectangle with a yellow background represents a class and an arrow is used to point from a subclass to its superclass. We do not list the attributes from a superclass in a subclass – only attributes added in the subclass are shown.

• In general the term superclass refers to a class from which other classes are derived.
• The direct superclass is the class from which the class/singular object is explicitly derived as specified in the object descriptor for the class/singular object – see the extended definition of object descriptor in section .
• The term direct subclass is an immediate subclass of a given class.

Account is a direct superclass of SavingsAccount and CreditAccount, and these are both direct subclasses of Account.

It is in general possible to create as many subclasses as needed to represent a given concept classification hierarchy. In the above examples we may have several subclass of class Account and we may also have subclasses of SavingsAccount and CreditAccount. We give several examples of this in the following.