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Canonical Object Idiom
Defining a Baseline Set of Functionality for Objects
by Bill Venners
First Published in JavaWorld, September 1998

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Alternative ways to implement the idiom
If you wish to disallow cloning of an object, you can simply choose not to implement Cloneable, unless a superclass already implements Cloneable. In that case, you'll need to override clone() and throw CloneNotSupportedException. If a superclass implementation of clone() has removed CloneNotSupportedException from its throws clause, you should either change that superclass or allow cloning in the subclass.

If you wish to disallow serialization, you can simply choose not to implement Serializable.

Note that defining finalize() is not part of this idiom. A finalizer is not appropriate in general cases, although under certain circumstances you may want to write a finalizer. For advice on writing finalizers, follow the link from the Resources section to a previous Design Techniques article on that subject.

Note also that defining toString() is missing from the list above. I left it out because I believe Object has a reasonable default implementation for this method. Object's toString() method returns a string composed of the name of the class, an "at" sign ("@"), and the unsigned hexadecimal representation of the hash code of the object. If you do override toString, you should return a string that "textually represents" the object. The returned result should be concise, informative, and easy to read.

One other thing missing from the canonical object idiom is a no-arg constructor. Any class that has a no-arg constructor and implements Serializable is a JavaBean. See the Resources section for a link to a discussion of when it is appropriate to make a class into a bean.

Idiom issues
To help spark some discussion on the Flexible Java Forum, a discussion forum devoted to Java design topics, I will throw out some of the issues that may present themselves with this idiom (see the Resources section for a link to the forum):

Next month
In next month's Design Techniques I'll talk about composition and inheritance.

A request for reader participation
I encourage your comments, criticisms, suggestions, flames -- all kinds of feedback -- about the material presented in this column. If you disagree with something, or have something to add, please let me know.

You can either participate in a discussion forum devoted to this material or e-mail me directly at bv@artima.com.

Resources

About the author
Bill Venners has been writing software professionally for 12 years. Based in Silicon Valley, he provides software consulting and training services under the name Artima Software Company. Over the years he has developed software for the consumer electronics, education, semiconductor, and life insurance industries. He has programmed in many languages on many platforms: assembly language on various microprocessors, C on Unix, C++ on Windows, Java on the Web. He is author of the book: Inside the Java Virtual Machine, published by McGraw-Hill. Reach Bill at bv@artima.com.

This article was first published under the name The Canonical Object Idiom in JavaWorld, a division of Web Publishing, Inc., September 1998.

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