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Relationships between Java classes - inheritance, packaging and others

When you're designing classes in Java, you need to consider their relationship with one another in various ways.

• You need to consider class INHERITANCE - which classes are based on which other classes. For example, a Pet could be based on (i.e. extended from) an Animal.

• You need to consider which INTERFACES your class implements - for that lets you define commonly named methods across a series of classes, and call those classes with the same code, even if they don't use inheritance.

• You need to consider which other classes objects in you class CONTAIN. For example, a Pet object will contain an address object, to hold information about where the pet lives.

• And you may wish to PACKAGE your classes - group them together under a name / named directory so that you can distibute them and handle them as a unit, and so that you can give them more permissions within the group than they would have outside.

Although there are similarities between these relationships, I wouldn't go so far as to sat they're the same, and indeed you need to consider all of them in your design.

On our recent Java Bootcamp Course, I wrote a new complete example of package. There's a "control" example - all in the one class - at Geraldine.java. I then took the object code and put it into a class called Isobel in a package called gowdie. See Isobel.java for that code, and if you want to see how it's called, there are examples in Hattie.java where we're specifying gowdie.Isobel in full every time we refer to it, and in Hilda.java. These new examples all include much more extensive comments - please follow the links if you want to read more programming detail, and to see sample output

Older examples (listed here for completeness, and to provide a link back to this article for delegates with old note sets) are axford.java - calling a class in the current package, sopworth.java - calling a class in a subpackage, sherston.java - importing a class from a package and finally pack.java - the class that's being used by the other examples. The older examples were all about a pack of cards - thus the class name "pack". In training use, the pack-of-cards examples are usually good ones, but in this case I'll admit it was a dumb idea as delegates used to get awfully confuser betwen "pack" - not a reserved word and "package" which has a specific Java meaning.




Footnote ... in case you wonder about the "gowdie.Isobel" class name ... on last week's course, we used names of ladies for classes, then we added famous surnames for associated data. A bit 'silly' / a bit of fun, if you like, and not recommended for production code. But by going through the alphabet, delegates can look back in the future and see the order of the examples just by going up the alphabet.

It turns out that "Isobel" with an "O" is a slightly unusual Scottish spelling of "Isabel" with an "A", and the most famous Isobel was Isobel Gowdie, a Scottish Witch. News to me - I'm not into that sort of thing, but seeing as the class was all about animals, it looks appropriate. Apparently, our Isobel Gowdie claimed that she could take on animal form ...
(written 2010-07-10, updated 2010-07-11)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
J708 - Java - Packages
  [2419] Where is my Java class? - (2009-09-24)
  [2114] Which Version of Java am I running? - (2009-04-02)
  [754] tar, jar, war, ear, sar files - (2006-06-10)

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)
  [2785] The Light bulb moment when people see how Object Orientation works in real use - (2010-05-28)
  [2747] Containment, Associative Objects, Inheritance, packages and modules - (2010-04-30)
  [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)

Q909 - Object Orientation and General technical topics - Object Orientation: Composite Objects
  [3979] Extended and Associated objects - what is the difference - C++ example - (2013-01-18)
  [3609] How do classes relate to each other? Associated Classes - (2012-02-12)
  [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)
  [2641] Object Oriented Programming in PHP - (2010-02-19)
  [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)


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Contact Information for Melksham Oak Community School
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Ruby - how does it compare and where is it the right language?
Relationships between Java classes - inheritance, packaging and others
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Fail Safe Error Handling in Java via Exceptions
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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