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)

 Q907 - Object Orientation and General technical topics - Object Orientation: Design Techniques
  [80] OO - real benefits - (2004-10-09)
  [236] Tapping in on resources - (2005-03-05)
  [507] Introduction to Object Oriented Programming - (2005-11-27)
  [534] Design - one name, one action - (2005-12-19)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [747] The Fag Packet Design Methodology - (2006-06-06)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [836] Build on what you already have with OO - (2006-08-17)
  [1047] Maintainable code - some positive advice - (2007-01-21)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1224] Object Relation Mapping (ORM) - (2007-06-09)
  [1435] Object Oriented Programming in Perl - Course - (2007-11-18)
  [1528] Object Oriented Tcl - (2008-02-02)
  [1538] Teaching Object Oriented Java with Students and Ice Cream - (2008-02-12)
  [2169] When should I use OO techniques? - (2009-05-11)
  [2170] Designing a heirarcy of classes - getting inheritance right - (2009-05-11)
  [2327] Planning! - (2009-08-08)
  [2380] Object Oriented programming - a practical design example - (2009-08-27)
  [2501] Simples - (2009-11-12)
  [2523] Plan your application before you start - (2009-12-02)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2741] What is a factory? - (2010-04-26)
  [2785] The Light bulb moment when people see how Object Orientation works in real use - (2010-05-28)
  [2865] Relationships between Java classes - inheritance, packaging and others - (2010-07-10)
  [2878] Program for reliability and efficiency - do not duplicate, but rather share and re-use - (2010-07-19)
  [2889] Should Python classes each be in their own file? - (2010-07-27)
  [2953] Turning an exercise into the real thing with extreme programming - (2010-09-11)
  [2977] What is a factory method and why use one? - Example in Ruby - (2010-09-30)
  [3063] Comments in and on Perl - a case for extreme OO programming - (2010-11-21)
  [3085] Object Oriented Programming for Structured Programmers - conversion training - (2010-12-14)
  [3260] Ruby - a training example that puts many language elements together to demonstrate the whole - (2011-04-23)
  [3454] Your PHP website - how to factor and refactor to reduce growing pains - (2011-09-24)
  [3607] Designing your application - using UML techniques - (2012-02-11)
  [3760] Why you should use objects even for short data manipulation programs in Ruby - (2012-06-10)
  [3763] Spike solutions and refactoring - a Python example - (2012-06-13)
  [3798] When you should use Object Orientation even in a short program - Python example - (2012-07-06)
  [3844] Rooms ready for guests - each time, every time, thanks to good system design - (2012-08-20)
  [3878] From Structured to Object Oriented Programming. - (2012-10-02)
  [3887] Inheritance, Composition and Associated objects - when to use which - Python example - (2012-10-10)
  [3928] Storing your intermediate data - what format should you you choose? - (2012-11-20)
  [3978] Teaching OO - how to avoid lots of window switching early on - (2013-01-17)
  [4098] Using object orientation for non-physical objects - (2013-05-22)
  [4374] Test driven development, and class design, from first principles (using C++) - (2014-12-30)
  [4430] The spirit of Java - delegating to classes - (2015-02-18)
  [4449] Spike solution, refactoring into encapsulated object methods - good design practise - (2015-03-05)
  [4628] Associative objects - one object within another. - (2016-01-20)

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Going off at a tangent, for a ramble
Views of Wessex
Delegate Question - defining MySQL table relationships as you create the tables
Monitoring the success and traffic of your web site
Containment, Associative Objects, Inheritance, packages and modules
Model - View - Controller demo, Sqlite - Python 3 - Qt4
Connecting Python to sqlite and MySQL databases
PyQt (Python and Qt) and wxPython - GUI comparison
Public Open Source Training Courses running this summer and autumn in Melksham
A simple server benchmark script