{"id":10455,"date":"2024-12-15T12:15:31","date_gmt":"2024-12-15T11:15:31","guid":{"rendered":"https:\/\/oopm.org\/?page_id=10455"},"modified":"2025-01-24T11:25:13","modified_gmt":"2025-01-24T10:25:13","slug":"14-1-visibility-mechanisms","status":"publish","type":"page","link":"https:\/\/oopm.org\/?page_id=10455","title":{"rendered":"14.1 Visibility mechanisms"},"content":{"rendered":"
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Here we introduce some language mechanisms that may be used to separate the representative elements from the non-representative elements. The language elements are called access modifiers<\/em> and they may specify the accessibility of attributes of an object. Access modifiers define the scope and visibility of these attributes in the code. This means that the programmer may specify to what extent a local data-item, method or class may be accessed in other parts of the code.<\/p>\n\n\n\n

The most common types of access modifiers are public<\/em>, private<\/em>, and protected<\/em>, but some languages support more as is the case for qBeta (see below). Each access modifier provides different levels of access. <\/p>\n\n\n\n

%private<\/h2>\n\n\n\n

As a first example, we will show how to use the access modifier %private <\/code>to protect balance<\/code> and interestRate<\/code> of Account<\/code> objects:<\/p>\n\n\n\n

class<\/strong> Account(owner: var<\/strong> String):\n   addInterest: -\"-\n   deposit(amount: var<\/strong> float): -\"-\n   withdraw(amount: var float) -> newB: var<\/strong> float: -\"-\n   getBalance -> bal: var<\/strong> float: -\"-\n   getInterestRate -> iRate: var<\/strong> float: ...\n   setInterestRate(iRate: var<\/strong> float): ...\n   %private\n   balance: var<\/strong> float\n   interestRate: var<\/strong> float<\/code><\/pre>\n\n\n\n
The access modifier %private<\/code> before the declarations of balance<\/code> and interestRate<\/code> specfies that balance<\/code> and interestRate<\/code> may only be accessed within class Account<\/code>.<\/p>\n\n\n\n
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Representative and non-representative parts of models in the bank domain<\/figcaption><\/figure>\n\n\n\n
%protected<\/h2>\n\n\n\n
When using %private<\/code> in class Account<\/code> as above, no other classes or objects may acces balance<\/code> and interestRate<\/code>. However, this may be inconvenient for subclasses of Account<\/code> like SavingsAccount<\/code>, where we may add methods that manipulate these attributes.<\/p>\n\n\n\n
The access modifier %protected <\/code>may be used to specify that some attributes may only be accessed within a class and its subclasses.<\/p>\n\n\n\n
We may thus revise class Account<\/code> as follows:<\/p>\n\n\n\n
class<\/strong> Account:\n   -\"-\n   %protected\n   balance: var<\/strong> float\n   interestRate: var<\/strong> float\nclass<\/strong> SavingsAccout: Account\n   -- balance and interestRate may now be accessed here\n   -\"-\naClerk: obj<\/strong>\n   -- balance and interestRate may not be accessed\n   -\"-<\/code><\/pre>\n\n\n\n
%public<\/h2>\n\n\n\n
For completeness, the access modifier %public<\/code> may be used to specify that some attributes are visible outside the object-descriptor. However, %public<\/code> is default<\/em> in qBeta – if no access modifier is specified for some attributes they are visible outside the object-descriptor. Sometimes it may be useful to use %public<\/code> if one prefer to have e.g. the private attributes appearing before the ones that are public. This is the case for the version of class Account<\/code> below:<\/p>\n\n\n\n
class<\/strong> Account:\n   %protected \n   balance: var<\/strong> float\n   interestRate: var<\/strong> float\n   %public\n   addInterest: -\"-\n   -\"-<\/code><\/pre>\n\n\n\n
Here balance<\/code> and interestRate<\/code> are declared in the beginning as %protected<\/code> followed by the methods of the class – these are thus explicitly declared as %public<\/code>.<\/p>\n\n\n\n
%package_boundary and %package<\/h2>\n\n\n\n
This section is a reference section, which may be skipped during a first reading. So far we have no examples using the package access modifiers, but one will be given in section