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Why getter and setter methods are evil

Make your code more maintainable by avoiding accessors

By Allen Holub, JavaWorld.com, 09/05/03

Page 6 of 6

The class then literally acted out the use case following these rules:

• Perform the activities that comprise the use case by talking to one another.
• You can only talk to your collaborators. If you must talk to someone else, you should talk to a collaborator who can talk to the other person. If that isn't possible, add a collaborator to your CRC card.
• You may not ask for the information you need to do something. Rather, you must ask the collaborator who has the information to do the work. It's okay to pass to that collaborator information he needs to do the work, but keep this interaction to a minimum.
• If something needs to be done and nobody can do it, create a new class (and CRC card) or add a responsibility to an existing class (and CRC card).
• If a CRC card gets too full, you must create another class (CRC card) to handle some of the responsibilities. Complexity is limited by what you can fit on a 4x6 index card.

A recording made of the entire conversation is the program's dynamic model. The finished set of CRC cards is the program's static model. With many fits and starts, you can solve just about any problem this way.

The process I just described is the OO design process, albeit simplified for a classroom environment. Some people design real programs this way using CRC cards. More often than not, however, designers develop the dynamic and static models in Unified Modeling Language (UML). The point is that an OO system is a conversation between objects. If you think about it for a moment, get/set methods just don't come up when you have a conversation. By the same token, get/set methods won't appear in your code if you design in this manner before you start coding.

Summing up

Let's pull everything together: You shouldn't use accessor methods (getters and setters) unless absolutely necessary because these methods expose information about how a class is implemented and as a consequence make your code harder to maintain. Sometimes get/set methods are unavoidable, but an experienced OO designer could probably eliminate 99 percent of the accessors currently in your code without much difficulty.

Getter/setter methods often make their way in code because the coder was thinking procedurally. The best way to break out of that procedural mindset is to think in terms of a conversation between objects that have well-defined responsibilities. Cunningham's CRC card approach is a great way to get started.

Parts of this article are adapted from my forthcoming book, tentatively titled Holub on Patterns: Learning Design Patterns by Looking at Code, to be published by Apress (www.apress.com) this fall.

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