Black Box Testing
Black Box Testing is testing without knowledge of the
internal workings of the item being tested. For example, when black box testing
is applied to software engineering, the tester would only know the "legal"
inputs and what the expected outputs should be, but not how the program actually
arrives at those outputs.
A D V E R T I S E M E N T
It is because of this that black box testing can be
considered testing with respect to the specifications, no other knowledge of the
program is necessary. For this reason, the tester and the programmer can be
independent of one another, avoiding programmer bias toward his own work. For
this testing, test groups are often used, "Test groups are sometimes called
professional idiots...people who are good at designing incorrect data." 1 Also,
do to the nature of black box testing, the test planning can begin as soon as
the specifications are written. The opposite of this would be glass box testing,
where test data are derived from direct examination of the code to be tested.
For glass box testing, the test cases cannot be determined until the code has
actually been written. Both of these testing techniques have advantages and
disadvantages, but when combined, they help to ensure thorough testing of the
Synonyms for black-box include: behavioral, functional, opaque-box, and
Advantages of Black Box
- more effective on larger units of code than glass box testing
- tester needs no knowledge of implementation, including specific
- tester and programmer are independent of each other
- tests are done from a user's point of view
- will help to expose any ambiguities or inconsistencies in the
- test cases can be designed as soon as the specifications are
Disadvantages of Black
- only a small number of possible inputs can actually be tested, to
test every possible input stream would take nearly forever
- without clear and concise specifications, test cases are hard to
- there may be unnecessary repetition of test inputs if the tester
is not informed of test cases the programmer has already tried
- may leave many program paths untested
- cannot be directed toward specific segments of code which may be
very complex (and therefore more error prone)
- most testing related research has been directed toward glass box
- black box testing should make use of randomly generated inputs
(only a test range should be specified by the tester), to eliminate any guess
work by the tester as to the methods of the function
- data outside of the specified input range should be tested to
check the robustness of the program
- boundary cases should be tested (top and bottom of specified
range) to make sure the highest and lowest allowable inputs produce proper
- the number zero should be tested when numerical data is to be
- stress testing should be performed (try to overload the program
with inputs to see where it reaches its maximum capacity), especially with real
- crash testing should be performed to see what it takes to bring
the system down
- test monitoring tools should be used whenever possible to track
which tests have already been performed and the outputs of these tests to avoid
repetition and to aid in the software maintenance
- other functional testing techniques include: transaction testing,
syntax testing, domain testing, logic testing, and state testing.
- finite state machine models can be used as a guide to design
According to Beizer 2 the following is a general order by which
tests should be designed:
Clean tests against requirements.
- Additional structural tests for branch coverage, as needed.
- Additional tests for data-flow coverage as needed.
- Domain tests not covered by the above.
- Special techniques as appropriate--syntax, loop, state, etc.
- Any dirty tests not covered by the above.