Black-box Testing treats the system as a "black-box", so it doesn't use knowledge of the internal structure explicitly. It is usually described as focusing on testing functional requirements. Synonyms for black-box include: behavioral, functional, opaque-box, and closed-box. 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 has no idea how the program actually arrives at those outputs.
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.
The so-called ``functionality testing'' is central to most testing exercises. Its primary objective is to assess whether the program does what it is supposed to do, i.e. it should meet user specified requirements. There are different approaches to functionality testing. One is the testing of each program feature or function in sequence. The other is to test module by module, i.e. each function where it is called first.
Advantages:
- More effective on larger units of code than glass box testing
- Tester needs no knowledge of implementation, including specific programming languages
- Tester and programmer are independent of each other
- Tests are done from a user's point of view
- Test cases can be designed as soon as the specifications are complete
Disadvantages
- 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 design
- 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 testing
