Any Program Has an Unlimited Number of Bugs

Let's take this simple Java method that calculates a sum of two integers as an example:

int sum(int a, int b) {
  return a + b;
}

This simple program has an unlimited number of bugs.

To prove this claim, we just need to put two thoughts together:

• First, a bug is something that compromises the quality of software, which, according to IEEE 610.12-1990, is "the degree to which a system meets specified requirements or user expectations."
• Second, requirements and expectations may be functional and non-functional. The latter include performance, resilience, robustness, maintainability, and a few dozen other NFRs.

It is obvious that there are at least two variables in this equation that are ambiguous: user expectations and maintainability. We can't be precise about them and that's why the number of bugs they will produce has no limit.

Of course, only a very limited subset of the entire set of bugs has any real business impact. Most of the bugs that exist in a program may stay there even after it is shipped to its users — nobody will ever find them or else the damage they cause to the user experience will be insignificant.

Finally, take a look at the method sum() one more time. How about these bugs:

• It doesn't handle overflows.
• It doesn't have any user documentation.
• Its design is not object-oriented.
• It doesn't sum three or more numbers.
• It doesn't sum double numbers.
• It doesn't cast long to int automatically.
• It doesn't skip execution if one argument is zero.
• It doesn't cache results of previous calculations.
• There is no logging.
• Checkstyle would complain since arguments are not final.
• I'm sure you can find many more.

BTW, Glenford J. Myers said something very similar in his book The Art of Software Testing, which I reviewed earlier.