{"id":1043,"date":"2023-09-08T14:43:16","date_gmt":"2023-09-08T12:43:16","guid":{"rendered":"https:\/\/oopm.org\/?page_id=1043"},"modified":"2024-12-15T09:33:49","modified_gmt":"2024-12-15T08:33:49","slug":"5-1-virtual-methods","status":"publish","type":"page","link":"https:\/\/oopm.org\/?page_id=1043","title":{"rendered":"8.3 Virtual methods"},"content":{"rendered":"
\"image_pdf\"<\/a>\"image_print\"<\/a><\/div>\n

In this section, we will introduce virtual methods. Virtual methods may be used to specify partial methods that may be further extended in subclasses.<\/p>\n\n\n\n

In the next example, we define withdraw<\/code> of Account<\/code> as a virtual method:<\/p>\n\n\n\n

class<\/strong> Account(owner: ref<\/strong> Customer):\n   ...\n   witdraw(amount: var<\/strong> float) -> newB: var<\/strong> float:<\n      inner(witdraw) <\/mark>\n      newB := balance<\/code><\/pre>\n\n\n\n
The symbol ':<'<\/code> specifies that withdraw<\/code> is a virtual method.<\/p>\n\n\n\n
The statement part of withdraw<\/code> consists of inner<\/code> followed by newB := balance<\/code>.<\/p>\n\n\n\n
The statement inner<\/code> specifies that when executed, possible statements in a further extension of withdraw in a subclass of Account<\/code> will be executed.<\/p>\n\n\n\n
In the next example, we show how to extend withdraw<\/code> in a class SavingsAccount<\/code>:<\/p>\n\n\n\n
class<\/strong> SavingsAccount: Account\n   ...\n   withdraw ::\n      if (today > releaseDate) :then\n         balance:= balance - amount\n      :else\n         console.print(\"It is not possible to withdraw\")<\/code><\/pre>\n\n\n\n
The symbol '::'<\/code> specifies that withdraw<\/code> is an extension<\/em> of withdraw<\/code> in the superclass Account<\/code> of SavingsAccount<\/code>.<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
In the following example, we invoke withdraw<\/code> on a SavingsAccount<\/code>:<\/p>\n\n\n\n
aSavingsAccount: obj<\/strong> SavingsAccount(JonhSmith)\nnewBalance: var<\/strong> float\nnewBalance := aSavingsAccount.withdraw(340)<\/code><\/pre>\n\n\n\n
Execution of aSavingsAccount.withdraw(340)<\/code> takes place as follows:<\/p>\n\n\n\n
    \n
  1. Execution starts by execution of the statements in withdraw<\/code> as defined in Account<\/code>.<\/li>\n\n\n\n
  2. The first statement to execute is inner<\/code>.<\/li>\n\n\n\n
  3. Execution of inner<\/code> implies that the statements of withdraw in SavingsAccount<\/code> are executed.<\/li>\n\n\n\n
  4. When the statements in SavingsAccount<\/code> have been executed, control returns to the statements after inner<\/code> in Account<\/code>.<\/li>\n\n\n\n
  5. This implies that newB := balance<\/code> is executed<\/li>\n\n\n\n
  6. Finally execution of withdraw<\/code> is completed.<\/li>\n<\/ol>\n\n\n\n
    The figure below illustrates the execution of aSavingsAccount.withdraw(340)<\/code>. The numbers show the order of execution of the involved statements:<\/p>\n\n\n\n
    \"\"
    Sequence of actions for the call SavingsAccount.withdraw(340)<\/figcaption><\/figure>\n\n\n\n
    We may extend withdraw<\/code> in a similar way in CreditAccount<\/code>:<\/p>\n\n\n\n
    class<\/strong> CreditAccount: Account\n   ...\n   withdraw ::\n      if (-balance < maxCredit) :then\n         balance := balance - amount\n      :else \n         console.print(\"Not possible to withdraw beyond max credit\") <\/code><\/pre>\n\n\n\n
    Invocation of withdraw<\/code> on a CreditAccount<\/code> object implies that execution of inner<\/code> will execute the statements in withdraw<\/code> in SavingsAccount<\/code>.<\/p>\n\n\n\n
    A virtual print method<\/h2>\n\n\n\n
    We now add one more virtual method to Account<\/code> and its subclasses in the form of a print method that prints the status of a given account on the console.<\/p>\n\n\n\n
    Part of the print of status is the same for all accounts whereas other parts are special for the subclasses.<\/p>\n\n\n\n
    class<\/strong> Account(owner: ref<\/strong> Customer):\n   ...\n   createStatement:<\n      stmt: var<\/strong> String\n      stmt := \"Account statement for \" + owner.name + '\\n'\n              + \"The account is a \"\n      inner(Account)\n      stmt := stmt + \"The balance is: \" + balance + '\\n'\n   print:\n      console.print(createStatement)\n   sendEmail:\n      owner.email(createStatement)\n\nclass<\/strong> SavingsAccount: Account\n   ...\n   createStatement::\n       stmt := stmt + \"savings account.\" + \n               \"\\nRelease date is \" + releaseDate.asText<\/code><\/pre>\n
    \n\t\t<\/a>\n\t\t<\/div>","protected":false},"excerpt":{"rendered":"
    In this section, we will introduce virtual methods. Virtual methods may be used to specify partial methods that may be further extended in subclasses. In the next example, we define withdraw of Account as a virtual method: The symbol ‘:<‘ specifies that withdraw is a virtual method. The statement part of withdraw consists of inner […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":1008,"menu_order":3,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-1043","page","type-page","status-publish","hentry"],"mb":[],"mfb_rest_fields":["title","gutenberg_elementor_mode"],"_links":{"self":[{"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages\/1043","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/oopm.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1043"}],"version-history":[{"count":37,"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages\/1043\/revisions"}],"predecessor-version":[{"id":10435,"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages\/1043\/revisions\/10435"}],"up":[{"embeddable":true,"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages\/1008"}],"wp:attachment":[{"href":"https:\/\/oopm.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1043"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}