A Philosophy of Software Design

by John Ousterhout

Up until this point I have been discussing specialization in the context of class and method design. Another form of specialization occurs in the code for method bodies, in the form of special cases. Special cases can result in code that is riddled with if statements, which make the code hard to understand and are prone to bugs. Thus, special cases should be eliminated wherever possible. The best way to do this is by designing the normal case in a way that automatically handles the edge conditions without any extra code. In the text editor project, students had to implement a mechanism for selecting text and copying or deleting the selection. Most students introduced a state variable in their selection implementation to indicate whether or not the selection exists. They probably chose this approach because there are times when no selection is visible on the screen, so it seemed natural to represent this notion in the implementation. However, this approach resulted in numerous checks to detect the “no selection” condition and handle it specially. The selection handling code can be simplified by eliminating the “no selection” special case, so that the selection always exists. When there is no selection visible on the screen, it can be represented internally with an empty selection, whose starting and ending positions are the same. With this approach, the selection management code can be written without any checks for “no selection”. When copying the selection, if the selecti...

Special cases