Software Testing Strategies

Unit Testing

It focuses on the smallest unit of software design - the software component or module. Using the component level design description as a guide, important control paths are tested to uncover errors within the boundary of the module. The relative complexity of tests and uncovered errors is limited by the constrained scope established for unit testing. The unit test is white-box oriented, and the step can be conducted in parallel for multiple components. The tests that occur as part of unit tests are listed below:

• Module Interface is tested to ensure that information properly flows into and out of the program unit under test. Tests of data flow across a module interface are required before any other test is initiated. If data does not enter and exit properly, all other tests are moot.
• Local Data Structures are examined to ensure that data stored temporarily maintains its integrity during all steps in an algorithm's execution.
• Boundary Conditions are tested to ensure that the module operates properly at boundaries established to limit or restrict processing.
• Independent Paths are exercised to ensure that all statements in a module have been executed at least once.
• Error Handling Paths are also tested.

Selective testing of execution paths is an essential task during the unit test. Basis path and loop testing are effective techniques for uncovering a broad array of path errors.Among the more common errors in computation are

• misunderstood or incorrect arithmetic precedence,
• mixed mode operations
• incorrect initialization
• precision inaccuracy
• incorrect symbolic representation of an expression.

Comparison and control flow are closely coupled to one another (i.e., change of flow frequently occurs after a comparison). Test cases should uncover errors such as

• comparison of different data types
• incorrect logical operators or precedence
• expectation of equality when precision error makes equality unlikely
• incorrect comparison of variables
• improper or nonexistent loop termination
• failure to exit when divergent iteration is encountered
• improperly modified loop variables

Among the potential errors that should be tested when error handling is evaluated

• Error description is unintelligible
• Error noted does not correspond to error encountered
• Error condition causes system intervention prior to error handling
• Exception-condition processing is incorrect
• Error description does not provide enough information to assist in the location of the cause of the error.

Boundary testing is the last (and probably most important) task of the unit test step. Software often fails at its boundaries. That is, errors often occur when the nth element of an n-dimensional array is processed, when the ith repetition of a loop with i passes is invoked, when the maximum or minimum allowable value is encountered.