Home > Articles

  • Print
  • + Share This
This chapter is from the book

Refused Bequest

Subclasses get to inherit the methods and data of their parents. But what if they don’t want or need what they are given? They are given all these great gifts and pick just a few to play with.

The traditional story is that this means the hierarchy is wrong. You need to create a new sibling class and use Push Down Method (359) and Push Down Field (361) to push all the unused code to the sibling. That way the parent holds only what is common. Often, you’ll hear advice that all superclasses should be abstract.

You’ll guess from our snide use of “traditional” that we aren’t going to advise this—at least not all the time. We do subclassing to reuse a bit of behavior all the time, and we find it a perfectly good way of doing business. There is a smell—we can’t deny it—but usually it isn’t a strong smell. So, we say that if the refused bequest is causing confusion and problems, follow the traditional advice. However, don’t feel you have to do it all the time. Nine times out of ten this smell is too faint to be worth cleaning.

The smell of refused bequest is much stronger if the subclass is reusing behavior but does not want to support the interface of the superclass. We don’t mind refusing implementations—but refusing interface gets us on our high horses. In this case, however, don’t fiddle with the hierarchy; you want to gut it by applying Replace Subclass with Delegate (381) or Replace Superclass with Delegate (399).

  • + Share This
  • 🔖 Save To Your Account