Blackbox Testing

INTRODUCTION

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. 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,

Though centered around the knowledge of user requirements, black box tests do not necessarily involve the participation of users. Among the most important black box tests that do not involve users are functionality testing, volume tests, stress tests, recovery testing, and benchmarks . Additionally, there are two types of black box test that involve users, i.e. field and laboratory tests. In the following the most important aspects of these black box tests will be described briefly.

Testing Strategies/Techniques

• 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 output
• The number zero should be tested when numerical data is to be input
• Stress testing should be performed (try to overload the program with inputs to see where it reaches its maximum capacity), especially with real time systems
• 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 functional tests
• According to Beizer the following is a general order by which tests should be designed:
1. Clean tests against requirements.
2. Additional structural tests for branch coverage, as needed.
3. Additional tests for data-flow coverage as needed.
4. Domain tests not covered by the above.
5. Special techniques as appropriate–syntax, loop, state, etc.
6. Any dirty tests not covered by the above.

Black box testing Methods

Graph-based Testing Methods

• Black-box methods based on the nature of the relationships (links) among the program objects (nodes), test cases are designed to traverse the entire graph
• Transaction flow testing (nodes represent steps in some transaction and links represent logical connections between steps that need to be validated)
• Finite state modeling (nodes represent user observable states of the software and links represent transitions between states)
• Data flow modeling (nodes are data objects and links are transformations from one data object to another)
• Timing modeling (nodes are program objects and links are sequential connections between these objects, link weights are required execution times)

Equivalence Partitioning

• Black-box technique that divides the input domain into classes of data from which test cases can be derived
• An ideal test case uncovers a class of errors that might require many arbitrary test cases to be executed before a general error is observed
• Equivalence class guidelines:
1. If input condition specifies a range, one valid and two invalid equivalence classes are defined
2. If an input condition requires a specific value, one valid and two invalid equivalence classes are defined
3. If an input condition specifies a member of a set, one valid and one invalid equivalence class is defined
4. If an input condition is Boolean, one valid and one invalid equivalence class is defined

Boundary Value Analysis

• Black-box technique that focuses on the boundaries of the input domain rather than its center
• BVA guidelines:
1. If input condition specifies a range bounded by values a and b, test cases should include a and b, values just above and just below a and b
2. If an input condition specifies and number of values, test cases should be exercise the minimum and maximum numbers, as well as values just above and just below the minimum and maximum values
3. Apply guidelines 1 and 2 to output conditions, test cases should be designed to produce the minimum and maxim output reports
4. If internal program data structures have boundaries (e.g. size limitations), be certain to test the boundaries

Comparison Testing

• Black-box testing for safety critical systems in which independently developed implementations of redundant systems are tested for conformance to specifications
• Often equivalence class partitioning is used to develop a common set of test cases for each implementation

Orthogonal Array Testing

• Black-box technique that enables the design of a reasonably small set of test cases that provide maximum test coverage
• Focus is on categories of faulty logic likely to be present in the software component (without examining the code)
• Priorities for assessing tests using an orthogonal array
1. Detect and isolate all single mode faults
2. Detect all double mode faults
3. Multimode faults

Specialized Testing

• Graphical user interfaces
• Client/server architectures
• Documentation and help facilities
• Real-time systems
1. Task testing (test each time dependent task independently)
2. Behavioral testing (simulate system response to external events)
3. Intertask testing (check communications errors among tasks)
4. System testing (check interaction of integrated system software and hardware)

Advantages of Black Box Testing

• 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
• Will help to expose any ambiguities or inconsistencies in the specifications
• Test cases can be designed as soon as the specifications are complete

Disadvantages of Black Box Testing

• 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

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