Equivalence class partitioning - Part 1

Wow...where does the time go? I was remiss last week in posting, and it has been a month since I posted about equivalence class partitioning. So, let's get back to it shall we?

Equivalence class partitioning (ECP) is a functional testing technique useful in either black box or white box test design. A technique is a systematic approach to help solve a complex problem. Techniques are not silver-bullets, but they are a logical and analytical approach to problem solving that heavily draws upon the tester's cognitive abilities (having a basis in or reducible to empirical factual knowledge) as opposed to random guessing (or little men running about inside one's head triggering turbid thoughts). Contrary to popular misconceptions the application of ECP is not a rote, brain-dead activity. The ECP technique requires in-depth knowledge of the data set (data type, encoding method, etc), the programming language used in the implementation, the algorithm structure, the operating environment, protocols, and even the hardware platform may impact how the data for a particular parameter might be decomposed. The effectiveness of the application of this technique solely lies in the testers ability to adequately decompose the data set for a given parameter into subsets in which any element from a specific subset would produce the same result as any other element from that subset.

Essentially the ECP technique analyzes input or output variable data for each specific parameter and decomposes the data into discrete valid and invalid class subsets. See my article in Software Testing & Performance magazine for a more in-depth understanding of the equivalence class partitioning technique. The article uses a simple next date program as an example because working with integer type data within a limited range is usually a good introduction to this complex technique.

A frequent reader of this blog suggested he often sees this technique applied to simple parameters limited to number type inputs, but would like to understand how this is applied to a more complex parameter that takes a string input. Interestingly enough, one of the exercises I designed in our internal training (and also used in the workshop I present regularly at the Software Testing and Performance conferences) deals specifically with string input. Let me outline the exercise.

Your goal is to adequately decompose the set of data (the ANSI Latin 1 character set) into valid and invalid equivalent class subsets to evaluate the behavior of the base filename parameter passed to COMDLG32.DLL's file save functionality from a user's perspective (GUI) on the Windows Xp operating system with an NTFS file system on a PC/AT platform.

NOTES:

• On the Windows environment the base filename parameter is separate and not interdependent on the extension parameter. (Some frequent readers may remember I explained this point previously to an individual who mistakenly assumed the base filename parameter and extension parameter were interdependent on the Windows environment.) So, remember, the purpose of this exercise is to focus initially on the base filename parameter (don't worry about the extension parameter...yet).
• To expose the user perspective of the file save functionality of COMDLG32 we will use Notepad.exe, and select the File -> Save As... menu item to instantiate the Save As... dialog.
• Characters greater than 0x7F can be entered using the numeric input method by holding down the ALT key and pressing number keypad keys equivalent to the characters decimal value. For example to enter the € character press and hold the ALT key and press the 0 1 2 8 keys on the numeric keypad. Release the ALT key and the character is displayed. (You can also use my Babel tool to generate specific characters by inputting the Unicode values for the characters in the Custom range groupbox and copying them into the filename control.)
• Not all characters can be entered using the numeric input method. For example, control sequences cannot be directly entered from the keyboard using this method (or by holding down the control key).
• In order to succeed, one must understand
  • Common programming concepts (especially for the C family of languages)
  • Windows File naming conventions
  • Windows Xp, file I/O APIs
  • Windows Xp NTFS file system
  • Knowledge of basic character encoding
  • Historical knowledge of FAT file systems
• A ECP table similar to the one outlined below may help you organize your subsets

Give it a try...next week I will offer a solution that you can compare your own results against.