{"id":7757,"date":"2024-09-27T11:23:19","date_gmt":"2024-09-27T09:23:19","guid":{"rendered":"https:\/\/oopm.org\/?page_id=7757"},"modified":"2024-12-13T13:38:22","modified_gmt":"2024-12-13T12:38:22","slug":"8-1-3-example","status":"publish","type":"page","link":"https:\/\/oopm.org\/?page_id=7757","title":{"rendered":"8.1.3 Example"},"content":{"rendered":"
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Here we show an example of writing general code<\/mark> using the extended type rule.<\/p>\n\n\n\n

As an example, a customer may have different kinds of accounts. In the example below we add a SavingsAccount<\/code> and a CreditAccount<\/code> to JohnSmithProfile<\/code>. We also add a method totalBalance<\/code> to Customer<\/code> – this method computes the sum of the balance of all accounts of a given customer.<\/p>\n\n\n\n

class<\/strong> Customer\n   -\"-\n   totalBalance -> bal: var<\/strong> float:\n      accounts.scan\n         bal := current.balance\nJohnSmithsProfile: obj<\/strong> Customer(\"John Smith\")\nanAccount: ref<\/strong> Account\nanAccount := SavingsAccount(JohnSmithProfile)\nJohnSmitProfile.addAccount(anAccount)\nanAccount.deposit(300)\n...\naCreditAccount: ref<\/strong> CreditAccount\naCreditAccount := CreditAccount(JohnSmithProfile)\nJohnSmitProfile.addAccount(aCreditAccount)\naCreditAccount.maxCredit := 999\n...\nconsole.print(\"Total balance sum: \" + JohnSmithProfile.totalBalance<\/code><\/pre>\n\n\n\n
Here we see examples of assigning a reference to an object to a variable of a more general type. One example is:<\/p>\n\n\n\n
anAccount := SavingsAccount(JohnSmithProfile)<\/code><\/pre>\n\n\n\n
The expression SavingsAccount(JohnSmithProfile)<\/code> generates an object of type SavingsAccount<\/code> – the reference to this object is then assigned to anAccount<\/code> which has the more general type Account<\/code>.<\/p>\n\n\n\n
Another example is:<\/p>\n\n\n\n
JohnSmitProfile.addAccount(aCreditAccount)<\/code><\/pre>\n\n\n\n
Here a reference variable of type CreditAccount<\/code> is assigned to the parameter of addAccount<\/code>, which is of type Account<\/code>.<\/p>\n\n\n\n
The example also shows that it is possible to write code that works for all kinds of Account<\/code> objects. This is the case for the method addAccount<\/code> that adds the Account<\/code> being passed as argument to the accounts<\/code> array of the Customer<\/code> object.<\/p>\n\n\n\n
Another example is the new method totalBalance<\/code>, which has the statement:<\/p>\n\n\n\n
accounts.scan\n   bal := current.balance<\/code><\/pre>\n\n\n\n
This statement scans through all objects in the accounts array – current<\/code> refers to the current Account<\/code> object, and the type of current<\/code> is Account<\/code>. The expression current.balance<\/code> access the balance<\/code> attribute of the account object bering referred to by current<\/code>. This works since all objects in accounts<\/code> have a balance<\/code> attribute independently of being an instance of Account<\/code>, SavingsAccount<\/code> or CreditAccount<\/code>.<\/p>\n\n\n\n
Consider the statement<\/p>\n\n\n\n
aCreditAccount.maxCredit := 999<\/code><\/pre>\n\n\n\n
Here the attribute maxCredit<\/code> is accessed. This is possible since aCreditAccount<\/code> is of type CreditAccount<\/code> and thus must refer to an object of type CreditAccount<\/code>.<\/p>\n\n\n\n
If we had used the variable anAccount<\/code> instead (as we did for generating the SavingsAccount<\/code>), we cannot access the attribute maxCredit<\/code> since only CreditAccounts<\/code> has this attribute.<\/p>\n\n\n\n
In section