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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
Abstract Classes - Java

In Java you can write a class from scratch, or you can write one class based on another, using the 'extends' keyword and providing only the methods that change in the new class from the base class, plus any extras.

Let's say we're writing an accounts system, for example ... we have a base class that's a "line entry" and we have subclasses for "employee", "regularevent","capitalcost" and "course" ... then we can write four subclasses, and we'll never instantise (create) just a lineentry. The getcost method will differ for the employee and regularevent classes, as they are time dependent, whereas the capitalcost and course class methods return the same value irrespective of the time. So there needs to be a getcost method in each subclass.

But I want an array of lineentrys ... and I need to be able to call getcost on each of them and have it run a different (polymorphic) method. Solution - I define lineentry as an abstract class - i.e. one who's definition isn't complete and so the constructor cannot be called directly - and define the getcost method as abstract.

As an extra, if getcost is to be a different piece of code in various groups of subclasses, I can introduce extra intermediate abstract classes to ensure that each peice of code only has to be written once - this is shown in the diagram.

(written 2009-05-16)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
J710 - Java - Extending Classes and More
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [1066] Final, Finally and Finalize - three special words in Java - (2007-02-05)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1294] An example of Java Inheritance from scratch - (2007-08-00)
  [1501] Java - using super to call a method in the parent class - (2008-01-10)
  [1538] Teaching Object Oriented Java with Students and Ice Cream - (2008-02-12)
  [1556] Java - a demonstration of inheritance on just one page - (2008-02-26)
  [1819] Calling base class constructors - (2008-10-03)
  [2434] Abstract classes, Interfaces, PHP and Java - (2009-10-03)
  [2604] Tips for writing a test program (Ruby / Python / Java) - (2010-01-29)
  [2860] What methods are available on this Java object? - (2010-07-08)
  [3047] What is a universal superclass? Java / Perl / Python / Other OO languages - (2010-11-13)
  [4334] Splitting out code into name blocks for clarity and reusability - (2014-11-30)
  [4366] Changing what operators do on objects - a comparison across different programming languages - (2014-12-26)
  [4394] Philosophy behind object design - and how I applied in to a Java example - (2015-01-14)
  [4419] Java Inheritance example - group of classes - step by step - (2015-02-08)
  [4422] Objects - from physical to virtual or abstract - Java - (2015-02-10)


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An FAQ on the Apache httpd and Apache Tomcat web servers, and on using them together
Abstract Classes - Java
Choosing the right version of Java and Tomcat
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What Linux run level am I in?
You cannot please all of the people all of the time
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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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