A Philosophy of Software Design

by John Ousterhout

The Java substring method would be easier to use if it performed this adjustment automatically, so that it implemented the following API: “returns the characters of the string (if any) with index greater than or equal to beginIndex and less than endIndex.” This is a simple and natural API, and it defines the IndexOutOfBoundsException exception out of existence. The method’s behavior is now well-defined even if one or both of the indexes are negative, or if beginIndex is greater than endIndex. This approach simplifies the API for the method while increasing its functionality, so it makes the method deeper. Many other languages have taken the error-free approach; for example, Python returns an empty result for out-of-range list slices. When I argue for defining errors out of existence, people sometimes counter that throwing errors will catch bugs; if errors are defined out of existence, won’t that result in buggier software? Perhaps this is why the Java developers decided that substring should throw exceptions. The error-ful approach may catch some bugs, but it also increases complexity, which results in other bugs. In the error-ful approach, developers must write additional code to avoid or ignore the errors, and this increases the likelihood of bugs; or, they may forget to write the additional code, in which case unexpected errors may be thrown at runtime. In contrast, defining errors out of existence simplifies APIs and it reduces the amount of code that must be written. ...