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How to implement state-dependent behavior
Doing the State pattern in Java
Page 2 of 5
The following "precerpt," from The JBuilder Java Bible (due to be published this fall), describes how to implement the State pattern in Java.
Understanding the State pattern
The State pattern is useful when you have an object that can be in one of several states, with somewhat different behavior in each state. For example, when a car's RPMs are in the "power band," then the car's acceleration
is much quicker than when the revolutions per minute are outside that range. The Car object might have a state variable that indicates "in the power band," "below the power band," or "above the power band." The acceleration characteristics of
the car would then depend on its current state.
There are many cases in which an object's behavior depends on its current state. For example, when you're sleepy, it's hard to think. But when you're jazzed on coffee and chocolate, it can be easy! The examples go on and on. So it's worthwhile to have a readily usable State pattern to draw on.
The way the State pattern works is shown in the following diagram:
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More from JavaWorld
Resources
- To read Philip Bishop's article on polymorphism, see
http://www.javaworld.com/javaworld/javatips/jw-javatip27.html - The procedure for producing an enumeration of objects that are members of a class is described in this article, which tells
how to create a set of enumerated constants
http://www.javaworld.com/javaworld/jw-07-1997/jw-07-enumerated.html - To find out more about the Design Patterns book see
http://st-www.cs.uiuc.edu/users/patterns/Books.html#Gamma - Other books about patterns
http://st-www.cs.uiuc.edu/users/patterns/Books.html - More about design patterns in general, see
http://www.azuma.mgmt.waseda.ac.jp/nonaka/pattern.html - Another good source of information on design patterns
http://st-www.cs.uiuc.edu/users/patterns/patterns.html - An online version of Martin Fowler's book, UML Distilled summarizing UML notations, at
http://heg-school.awl.com/cseng/authors/fowler/umldist/umldist.html - Another great piece of work by Martin Fowler that includes an introduction to state modeling techniques is his Survey of Object-Oriented Analyis and Object-Oriented Design Techniques. The online verison is available at
http://www.awl.com/cp/fowlerindex.htm - For more information on the Unified Modeling Language (UML), see
http://www.rational.com/uml/ - The Java API for Inner Classes covers many interesting and useful extensions to the language including inner interfaces, anonymous
classes, instance initializers, blank finals, and anonymous arrays. This API can be found at
http://www.javasoft.com/products/jdk/1.1/docs/guide/innerclasses/spec/innerclasses.doc.html
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