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Containment, Associative Objects, Inheritance, packages and modules

Containment is where one object contains other objects - and it happens all over the place. A "town" object may, in a program, contain a number of "hotel" objects, a "location" object, zero or more "leisure" objects, "public transport hub" objects and so on.

Containment is different to inheritance - it's not the same thing at all. Inheritance is where one object has its code and behaviors based on the code of another - for example, the "leisure" object I mentioned in my example above could in fact be subtyped into "swimming pool" objects, "cinema" objects, "fitness centre" objects, and so on.

It's often sensible to bundle together a number of classes - where they'll be maintained by the same person or team, and used in conjunction with each other in the same applications. Depending on the programming language that you're using, you'll use the term Packages or Modules for such grouping of classes. And a package or module type relationship is different to a containment relationship, which is different to an inheritance relationship. It might be that our "leisure" and "public transport hub" objects are in the same module, but that does not imply inheritance, nor containment.

Design MATTERS - if you're going to make a new journey, you'll start by checking on the transport options and route rather than just getting in your car and driving randomly - and so it should be with your object / application design. By developing in a modular way, with frequent testing at each level, and thinking about the data, you should help yourself produce a robust, tailorable system - and diagrams that show which classes inherit from which, and what objects are contained in which, will help you think clearly from the start. Whether you go the whole way to using UML and / or extreme programming techniques will depend on you - and on the size and complexity of the project.

Containment and Inheritance are very much OO concepts, and for newcomers to OO, we cover them on our Python, Java, C++ and Ruby courses. For delegates learning Perl or PHP (which can be used in an OO or non-OO way), we cover the OO principles on specific courses - see [OO PHP] and [Perl for larger projects].
(written 2010-04-30, updated 2010-05-06)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Q907 - Object Orientation and General technical topics - Object Orientation: Design Techniques
  [4098] Using object orientation for non-physical objects - (2013-05-22)
  [3978] Teaching OO - how to avoid lots of window switching early on - (2013-01-17)
  [3928] Storing your intermediate data - what format should you you choose? - (2012-11-20)
  [3887] Inheritance, Composition and Associated objects - when to use which - Python example - (2012-10-10)
  [3878] From Structured to Object Oriented Programming. - (2012-10-02)
  [3844] Rooms ready for guests - each time, every time, thanks to good system design - (2012-08-20)
  [3798] When you should use Object Orientation even in a short program - Python example - (2012-07-06)
  [3763] Spike solutions and refactoring - a Python example - (2012-06-13)
  [3760] Why you should use objects even for short data manipulation programs in Ruby - (2012-06-10)
  [3607] Designing your application - using UML techniques - (2012-02-11)
  [3454] Your PHP website - how to factor and refactor to reduce growing pains - (2011-09-24)
  [3260] Ruby - a training example that puts many language elements together to demonstrate the whole - (2011-04-23)
  [3085] Object Oriented Programming for Structured Programmers - conversion training - (2010-12-14)
  [3063] Comments in and on Perl - a case for extreme OO programming - (2010-11-21)
  [2977] What is a factory method and why use one? - Example in Ruby - (2010-09-30)
  [2953] Turning an exercise into the real thing with extreme programming - (2010-09-11)
  [2889] Should Python classes each be in their own file? - (2010-07-27)
  [2878] Program for reliability and efficiency - do not duplicate, but rather share and re-use - (2010-07-19)
  [2865] Relationships between Java classes - inheritance, packaging and others - (2010-07-10)
  [2785] The Light bulb moment when people see how Object Orientation works in real use - (2010-05-28)
  [2741] What is a factory? - (2010-04-26)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2523] Plan your application before you start - (2009-12-02)
  [2501] Simples - (2009-11-12)
  [2380] Object Oriented programming - a practical design example - (2009-08-27)
  [2327] Planning! - (2009-08-08)
  [2170] Designing a heirarcy of classes - getting inheritance right - (2009-05-11)
  [2169] When should I use OO techniques? - (2009-05-11)
  [1538] Teaching Object Oriented Java with Students and Ice Cream - (2008-02-12)
  [1528] Object Oriented Tcl - (2008-02-02)
  [1435] Object Oriented Programming in Perl - Course - (2007-11-18)
  [1224] Object Relation Mapping (ORM) - (2007-06-09)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1047] Maintainable code - some positive advice - (2007-01-21)
  [836] Build on what you already have with OO - (2006-08-17)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [747] The Fag Packet Design Methodology - (2006-06-06)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [534] Design - one name, one action - (2005-12-19)
  [507] Introduction to Object Oriented Programming - (2005-11-27)
  [236] Tapping in on resources - (2005-03-05)
  [80] OO - real benefits - (2004-10-09)


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