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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 articlesJ710 - Java - Extending Classes and More 
Objects - from physical to virtual or abstract - Java - (2015-02-10) 
Java Inheritance example - group of classes - step by step - (2015-02-08) 
Philosophy behind object design - and how I applied in to a Java example - (2015-01-14) 
Changing what operators do on objects - a comparison across different programming languages - (2014-12-26) 
Splitting out code into name blocks for clarity and reusability - (2014-11-30) 
What is a universal superclass? Java / Perl / Python / Other OO languages - (2010-11-13) 
What methods are available on this Java object? - (2010-07-08) 
Tips for writing a test program (Ruby / Python / Java) - (2010-01-29) 
Abstract classes, Interfaces, PHP and Java - (2009-10-03) 
Calling base class constructors - (2008-10-03) 
Java - a demonstration of inheritance on just one page - (2008-02-26) 
Teaching Object Oriented Java with Students and Ice Cream - (2008-02-12) 
Java - using super to call a method in the parent class - (2008-01-10) 
An example of Java Inheritance from scratch - (2007-08-00) 
What are factory and singleton classes? - (2007-06-04) 
Final, Finally and Finalize - three special words in Java - (2007-02-05) 
Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13) 
Think about your design even if you don't use full UML - (2006-03-24)
Some other Articles
Matching disparate referencing systems (MediaWiki, PHP, also Tcl)Camera to record where a picture was takenAre we IITT (Institute of IT Training) members?An FAQ on the Apache httpd and Apache Tomcat web servers, and on using them togetherAbstract Classes - JavaChoosing the right version of Java and TomcatServlet life cycle, and Java Servlet variable scopeWhat Linux run level am I in?You cannot please all of the people all of the timePatterns in numbers - room occupancy
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