Black-box testing and white-box testing offer two different means of testing software. While one is not inherently “better” than the other, both forms possess specific pros and cons to be weighed against the needs of the tester. These two forms are not mutually exclusive and “gray-box testing” adopts characteristics of both.
Black-box quality testing checks software purely for results in a manner that does not require programming knowledge or an understanding of the software’s code. In this form of results-based testing, the tester merely knows that a certain input should produce the corresponding output; the tester has no comprehension of how the software actually creates the output. Some typical test methods include state-transition tables, boundary value analysis, and error guessing.
The strengths and weaknesses of black-box testing both hinge upon the simplicity of results-based testing and lack of required technical knowledge. In short, it’s simpler to implement, but that simplicity limits the scope of the tests. Most programs have thousands, if not millions of inputs, so it is practically impossible to test every single input/output. Because specialized knowledge is not needed, it is easier and cheaper to build testing teams. Even so, regular changes to the software tend to break these tests because of their reliance on a steady interface, potentially prolonging the testing and raising costs. Finally, black-box quality testing has the potential of higher objectivity because all testing is done from the user’s point of view; the designer is usually not part of the process.
A white-box test of software checks the inner workings of whatever is being tested. It requires intimate knowledge of the software’s code as well as the requisite programming skills to repair errors. Some examples of white-box testing include mutation testing, code coverage, and fault injection.
White-box quality testing is much more efficient than black-box testing when debugging programming code. White-box testers will find bugs more quickly because of their working knowledge of the software, but this expertise and training increases the cost. It is possible to defray this cost with automated testing of isolated areas of the programming code. White-box testing also struggles to test the unpredictable behavior of the common user. It can only test for anticipated inputs; these typically do not include the experimental or misbegotten ideas of the average person.
With large teams, thorough testing of any software usually incorporates both types; this is called gray-box or layered testing. Gray-box quality testing frequently involves a black-box group submitting bug reports to a white-box team, or a white-box group validating bug-fixes through the other testers’ inputs.
The question of whether to use black-box, white-box, or gray-box testing can only be answered when the developers consider their needs, budget, and the complexity of their software. Even so, nearly all contemporary software development uses gray-box quality testing as a natural part of the alpha and beta release process.