Divergent Change

We structure our software to make change easier; after all, software is meant to be soft. When we make a change we want to be able to jump to a single clear point in the system and make the change. When you can't do this you are smelling one of two closely related pungencies.

Divergent change occurs when one class is commonly changed in different ways for different reasons. If you look at a class and say, "Well, I will have to change these three methods every time I get a new database; I have to change these four methods every time there is a new financial instrument," you likely have a situation in which two objects are better than one. That way each object is changed only as a result of one kind of change. Of course, you often discover this only after you've added a few databases or financial instruments. Any change to handle a variation should change a single class, and all the typing in the new class should express the variation. To clean this up you identify everything that changes for a particular cause and use Extract Class to put them all together.