A Philosophy of Software Design

by John Ousterhout

As a final example, consider the Java String class and its substring method. Given two indexes into a string, substring returns the substring starting at the character given by the first index and ending with the character just before the second index. However, if either index is outside the range of the string, then substring throws IndexOutOfBoundsException. This exception is unnecessary and complicates the use of this method. I often find myself in a situation where one or both of the indices may be outside the range of the string, and I would like to extract all of the characters in the string that overlap the specified range. Unfortunately, this requires me to check each of the indices and round them up to zero or down to the end of the string; a one-line method call now becomes 5–10 lines of code. 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,...

Simpller software