factory method : Java Glossary

2006-02-21 by Roedy Green ©1996-2008 Canadian Mind Products
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factory method

A factory is a sort of generic constructor that might produce any of a number of different kinds of objects. You have to implement it as a static method, rather than a constructor. Inside you can implement in several ways:

Have a big switch and select the right constructor.
Use a Hashtable/HashMap and .clone() to reproduce pre-initialised exemplar objects.
use getInstanceOf to create arbitrary objects from just the class name provided as a String.

If you want to give others the ability to add new types to the factory, the static method pretty well has to fob the work off on a factory delegate object or objects that can be changed. You can can’t directly override a static factory method, and others would ignore a overridden instance factory method.

You can also use factories to build complicated objects of only a single class that must be built-in stages, when there are too many parameters to handle conveniently with a constructor. You want to avoid allowing any malformed or partly-formed objects loose out into the universe.

In Javadoc, click USE to discover factory methods that will produce objects of the current class. Often there is no public constructor and you have to dig around to find the corresponding factory method in some other class. It is not formally linked to the class it is the factory for.

Sample Factory Pattern Code

Here is the code for two simple-minded factories, that illustrate some of the things factories are commonly used for:

Birth control. Preventing the creation of invalid objects.
Automatically creating specialised versions of objects.
Automatic post processing of newly created objects.

design patterns
factory method
Javadoc
prototype

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