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In [[Class-based programming|class-based languages]] the ''classes'' are defined beforehand and the ''objects'' are instantiated based on the classes. If two objects ''apple'' and ''orange'' are instantiated from the class ''Fruit'', they are inherently fruits and it is guaranteed that you may handle them in the same way; e.g. a programmer can expect the existence of the same attributes such as ''color'' or ''sugar_content'' or ''is_ripe''.
In [[Prototype-based programming|prototype-based languages]] the ''objects'' are the primary entities. No ''classes'' even exist. The ''prototype'' of an object is just another object to which the object is linked. Every object has one ''prototype'' link (and only one). New objects can be created based on already existing objects chosen as their prototype. You may call two different objects ''apple'' and ''orange'' a fruit, if the object ''fruit'' exists, and both ''apple'' and ''orange'' have ''fruit'' as their prototype. The idea of the ''fruit'' class
===Dynamic dispatch/message passing===
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Languages that support classes almost always support [[inheritance (object-oriented programming)|inheritance]]. This allows classes to be arranged in a hierarchy that represents "is-a-type-of" relationships. For example, class Employee might inherit from class Person. All the data and methods available to the parent class also appear in the child class with the same names. For example, class Person might define variables "first_name" and "last_name" with method "make_full_name()". These will also be available in class Employee, which might add the variables "position" and "salary". This technique allows easy re-use of the same procedures and data definitions, in addition to potentially mirroring real-world relationships in an intuitive way. Rather than utilizing database tables and programming subroutines, the developer utilizes objects the user may be more familiar with: objects from their application ___domain.<ref>{{cite book|last=Jacobsen|first=Ivar|title=Object Oriented Software Engineering|year=1992|publisher=Addison-Wesley ACM Press|isbn=978-0-201-54435-0|pages=[https://archive.org/details/objectorientedso00jaco/page/43 43–69]|author2=Magnus Christerson|author3=Patrik Jonsson|author4=Gunnar Overgaard|url=https://archive.org/details/objectorientedso00jaco/page/43}}</ref>
Subclasses can override the methods defined by superclasses. [[Multiple inheritance]] is allowed in some languages, though this can make resolving overrides complicated. Some languages have special support for [[mixin]]s, though in any language with multiple inheritance, a mixin is simply a class that does not represent an is-a-type-of relationship. Mixins are typically used to add the same methods to multiple classes. For example, class UnicodeConversionMixin might provide a method unicode_to_ascii() when included in class FileReader and class WebPageScraper, which
[[Abstract class]]es cannot be instantiated into objects; they exist only for the purpose of inheritance into other "concrete" classes that can be instantiated. In Java, the <code>[[final (Java)|final]]</code> keyword can be used to prevent a class from being subclassed.{{sfn|Bloch|2018|loc=Chapter §2 Item 4 Enforce noninstantiability with a private constructor|p=19}}
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===Object-orientation and databases===
{{Main|Object-relational impedance mismatch|Object-relational mapping|Object database}}
Both object-oriented programming and [[relational database management systems]] (RDBMSs) are extremely common in software {{As of|2006|alt=today}}. Since [[relational database]]s
There are also [[object database]]s that can be used to replace RDBMSs, but these have not been as technically and commercially successful as RDBMSs.
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