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Q:Inheritance seems error-prone. How can I guard against these errors?

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Q:Inheritance seems error-prone. How can I guard against these errors?

Answer: The previous two questions show that a programmer neeeds to be very careful when extending a class, and sometimes just in using a class that extends another class. Problems like these two lead John Ousterhout to say "Implementation inheritance causes the same intertwining and brittleness that have been observed when goto statements are overused. As a result, OO systems often suffer from complexity and lack of reuse." (Scripting, IEEE Computer, March 1998) and Edsger Dijkstra to allegedly say "Object-oriented programming is an exceptionally bad idea which could only have originated in California." (from a collection of signature files). I don't think there's a general way to insure being safe, but there are a few things to be aware of:
  • Extending a class that you don't have source code for is always risky; the documentation may be incomplete in ways you can't foresee.
  • Calling super tends to make these unforeseen problems jump out.
  • You need to pay as much attention to the methods that you don't over-ride as the methods that you do. This is one of the big fallacies of Object-Oriented design using inheritance. It is true that inheritance lets you write less code. But you still have to think about the code you don't write.
  • You're especially looking for trouble if the subclass changes the contract of any of the methods, or of the class as a whole. It is difficult to tell when a contract is changed, since contracts are informal (there is a formal part in the type signature, but the rest appears only in comments). In the Properties example, it is not clear if a contract is being broken, because it is not clear if the defaults are to be considered "entries" in the table or not.


This tip is reprinted on JavaFAQ.nu by by courtesy of Peter Norvig I am thankful for his important contributions to my site - 21 Infrequently Answered Java Questions. Alexandre Patchine


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