Summary
Should DTOs have public variables? Or should they have private variables with getters and setters?

I'm teaching an XP Immersion(TM) class this week. We are writing FitNesse tests as the acceptance tests for the application we are writing. One of the programmers was using a RowFixture. RowFixture requires the use of a DTO (Data Transfer Object) and insists that the variables be public. The programmer was aghast. "DTOs are always made with private variables and getters and setters!" he said. "Why?" I asked.

Why indeed? Has the religion of OO carried us so far away from common sense that we can't recognize a data structure when we see one? Why do we need to bloat our code with a bunch of useless getters and setters just to fulfill some dogmatic rule that nobody can adequately explain?

In my view OO programs contain two kinds of entities: Objects and Data Structures. Objects have private data and public methods. Data Structures have public data and no methods (or sometimes trivial navigational methods). There are very good reasons for keeping the variables in an object private. We want to know which functions can manipulate them. We want to protect the invariants of the object. We don't want others to depend on our details (DIP). On the other hand there is no good reason to use getters and setters in a data structure. A data structure is simply a packet of data, nothing more.

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About the Blogger

Robert C. Martin (Uncle Bob) has been a software professional since 1970 and an international software consultant since 1990. He is founder and president of Object Mentor Inc., a team of experienced consultants who mentor their clients worldwide in the fields of C++, Java, OO, Patterns, UML, Agile Methodologies, and Extreme Programming. In 1995 Robert authored the best-selling book: Designing Object Oriented C++ Applications using the Booch Method, published by Prentice Hall. From 1996 to 1999 he was the editor-in-chief of the C++ Report. In 1997 he was chief editor of the book: Pattern Languages of Program Design 3, published by Addison Wesley. In 1999 he was the editor of "More C++ Gems" published by Cambridge Press. He is co-author, with James Newkirk, of "XP in Practice", Addision Wesley, 2001. In 2002 he wrote the long awaited "Agile Software Development: Principles, Patterns, and Practices", Prentice Hall, 2002. He has published many dozens of articles in various trade journals, and is a regular speaker at international conferences and trade shows.