How do classes relate to each other? Associated Classes ... or ... define: Associated classes, parent classes, superclasses, subclasses, base classes, abstract classes, interfaces and mixins.

Associated classes are the odd ones out here. They're classes that are used within other classes.

Let me give you an example, from the Object Oriented PHP course I gave on Friday. We defined a class called visit, in which each instance contained an array of access objects (as well as some other data). One or more accesses make up a visit, and so every visit object must contain access objects, but that's not in the same way that properties and methods are inherited - a term which applies to all the other terms that I used above.

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Let's look at all the other terms, and let's do so by example.

I define a class called pubtrans which contains information about a public transport offering. However, whilst public transport by one more or another is more or less the same, there are differences in the code logic needed. So I call my pubtrans class a parent class and I then provide extra (extended) logic to define a subclass for a train, and I also provide extra logic to define another subclass for a bus.

The parent class is sometimes referred to as the superclass, and you'll also find the term base class used to refer to the class on which another is based.

A class is referred to as an abstract class if you won't be creating (constructing) members directly. Taking our pubtrans as an example, that's likely to be an abstract class - every piece of public transport is some subtype of public transport be it a train, a bus, a ferry or a pogo stick. In an abstart class definition, you can usually include a requirement that each subclass must implement a particular method, thus ensuring that all the objects in this object tree are polymorphic.

An interface is a definition of a number of methods that must be defined in a class which implements it. It's rather like an abstract class which is so abstract that it contains no code at all. The advantage of an interface is that classes can implement multiple interfaces, then giving you something which approaches multiple inheritance in a language which only supports single inheritance such as Java.

Finally, a mixin is code / methods which are shared between a number of classes by being "mixed in" or added to them. They're currently used in Ruby and Python (of the languages that we train on) and will be present in PHP 5.4 onwards where they will be part of traits and offer a wider selection of operations such as aliasing and exclusion and will as mixingin.
(written 2012-02-12, updated 2012-02-18)

 Q909 - Object Orientation and General technical topics - Object Orientation: Composite Objects
  [3251] C++ - objects that are based on other objects, saving coding and adding robustness - (2011-04-17)
  [3152] Jargon busting - (2011-01-30)
  [3142] Private and Public - and things between - (2011-01-22)
  [2922] Getting the OO design write - with PHP a example - (2010-08-14)
  [2865] Relationships between Java classes - inheritance, packaging and others - (2010-07-10)
  [2641] Object Oriented Programming in PHP - (2010-02-19)
  [2170] Designing a heirarcy of classes - getting inheritance right - (2009-05-11)
  [1348] Screw it or Glue it? Access to Object variables - a warning - (2007-09-12)
  [1345] Perl and Shell coding standards / costs of an IT project - (2007-09-11)
  [592] NOT Gone phishing - (2006-02-05)
  [477] Class, static and unbound variables - (2005-10-25)

H108 - Objects in PHP
  [3608] Design Patterns - what are they? Why use them? - (2012-02-12)
  [3607] Designing your application - using UML techniques - (2012-02-11)
  [3211] Computer Graphics in PHP - World (incoming data) to Pixel (screen) conversion - (2011-03-24)
  [3210] Catchable fatal error in PHP ... How to catch, and alternative solutions such as JSON - (2011-03-22)
  [2921] Does copying a variable duplicate the contents? - (2010-08-14)
  [2774] PHP - Object Oriented Design in use - (2010-05-21)
  [2741] What is a factory? - (2010-04-26)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2680] Static class members in PHP - a documented example - (2010-03-16)
  [2632] Shipping a test harness with your class in PHP - (2010-02-12)
  [2435] Serialization - storing and reloading objects - (2009-10-04)
  [2434] Abstract classes, Interfaces, PHP and Java - (2009-10-03)
  [2380] Object Oriented programming - a practical design example - (2009-08-27)
  [2172] PHP4 v PHP5 - Object Model Difference - (2009-05-11)
  [2171] Cleaning up redundant objects - (2009-05-11)
  [2169] When should I use OO techniques? - (2009-05-11)
  [2160] PHP - getclass v instanceof - (2009-05-07)
  [1925] Introduction to Object Oriented Programming - (2008-12-06)
  [1820] Sorting objects in PHP - (2008-10-04)
  [1819] Calling base class constructors - (2008-10-03)
  [1682] Accounts in PHP - an OO demo - (2008-06-19)
  [1535] OO PHP demonstration - comparing objects and more - (2008-02-08)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1153] Object Oriented Model - a summary of changes from PHP4 to PHP5 - (2007-04-18)
  [1027] Cue the music, I'm happy. - (2007-01-09)
  [836] Build on what you already have with OO - (2006-08-17)
  [720] Planning a hotel refurb - an example of a Gant chart in PHP - (2006-05-14)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [485] North, Norther and Northest - PHP 5 Objects - (2005-11-04)
  [421] Don't repeat code - use loops or functions - (2005-08-21)
  [343] Should I use structured or object oriented? - (2005-06-10)
  [205] PHP5 lets you say no - (2005-02-07)
  [124] PHP v Java - (2004-11-20)
  [67] Object Oriented Programming in PHP - (2004-09-29)

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