Telling Testing Methods Apart — Black-box, White-box, Grey-box

Testing software is essential in ensuring the quality of your product. Without it, you risk technical debt and an underwhelming response to your software’s release. Testers play a crucial role in software development by conducting thorough analyses to ensure that your software meets all requirements, is secure, responsive, complete, and easy to use. Their process of investigation exposes hidden errors, unpredictable behavior, and functional inconsistencies so that developers can fix all the issues before release.

There are different software testing approaches available today, but the most effective and popular methods are black-box, white-box, and grey-box testing. These methods take different approaches but are effective in helping developers keep their code clean and functionality in check. This article will help you understand these methods and identify their strengths and weaknesses, ensuring you can get the highest quality of your software and avoid technical debt that can harm your business in the long run.

Black-box testing

Black-box testing is a method in which testers perform tests without any knowledge of the internal structure or source code of the software being tested. They do not need to have programming knowledge or outstanding coding skills to perform these tests.

The main goal of this testing method is to interact with the software’s user interface, test its functionality, and ensure that every input and output of the system meets the specified requirements. This testing method is also known as functional testing or specification-based testing.

A testing team independently performs these tests from the perspective of end-users during the STLC process. Testers provide valid or invalid inputs and verify the outputs against the expected outcomes. Any unexpected results or deviations are documented and reported to the development team, which helps them find and eliminate functional errors and inconsistencies early on.

This testing method can be used at every level of software testing, including unit, integration, system, and acceptance testing. In unit testing, the black-box method is used to test the interface against the specifications given by the client. In integration testing, the goal is to find and eliminate errors in the interaction between integrated components of the interface. The black-box method can also be effectively applied in system testing to analyze the system’s compliance with the requirements, as well as in acceptance testing, where it can help validate the acceptability of a software product by testing it in various unexpected situations and circumstances.

Some of the most common black-box test design techniques include:

1. When debugging software that relies on decision tables with embedded if-then-else and switch-case statements, decision table testing is a useful way to identify which actions correspond to which conditions.
2. Error guessing is a testing technique where testers rely on their experience and intuition to design test cases that aim to identify potential causes of software failure or errors.
3. All-pairs testing is a technique used to test all the possible discrete combinations of each pair of input parameters. This really helps find common bugs that are usually hidden in interactions between pairs of parameters.
4. Equivalence partitioning is a testing technique that divides input data into smaller partitions or classes of equivalent data. This helps in deriving test cases that cover each partition at once, which reduces the time needed for testing.

Black-box testing pros and cons

Black-box testing is a useful technique for identifying any ambiguity, vagueness, and contradictions in functional specifications. It allows testers to evaluate and improve the implementation of functionality without directly interfering with the software’s code. One of the main advantages of black-box testing is that it is entirely impartial, as an independent team performs the testing, separating the perspective of end-users from that of developers. Black-box testing is also the quickest method for developing test cases, as it does not require programming knowledge and can be easily performed by non-technical testers.

However, this method is only effective for testing small software pieces. Testing large and complex software thoroughly using this method would be inefficient and time-consuming. Additionally, clear and comprehensive specifications are required for this method to be effective. Otherwise, it will be extremely challenging to design test cases and the testing scenarios will provide limited coverage.

White-box testing

White-box testing is a method used to analyze the internal structure and logic of software. It is also called structural or logic-driven testing.

Unlike black-box testing that focuses on functionality, white-box testing requires testers to have strong coding skills, full knowledge of the software being tested, and access to all source code and architecture documents. This method is time-consuming and is usually performed by professional developers.

Testers who use the white-box testing method study the code and other internal aspects of the software, determine all the valid and invalid inputs, and verify the outputs against the expected outcomes. They check statements, conditions, code paths, and dataflows to ensure that there are no hidden errors or defect-prone elements.

White-box testing can be applied at the unit testing level, but it is mostly used for integration testing and regression testing today. The method allows testers to check paths within units for code defects and other issues that would prevent the software from working as intended. This is done before any integration with previously tested code happens, which reduces the risk of getting errors later in development. In integration testing, the method helps analyze interactions between different interfaces and subsystems. During regression testing, the white-box method can be very effectively applied through the usage of the white-box test cases recycled at the unit and integration testing levels.

Some of the most common white-box test design techniques include:

1. Control flow testing is a structural testing strategy that uses the software’s control flows to test the logic of the code by executing input values and checking if they meet the required results.
2. Data flow testing identifies coding errors by detecting improper data values and data flow anomalies. The technique focuses on highlighting problematic areas of code, so that additional testing can be done to eliminate these errors.
3. The branch testing technique validates code branches and eliminates abnormalities. Not all code runs in a continuous flow; it branches out to perform specific functions based on different if/else conditions.

White-box testing pros and cons

Compared to black-box testing, the white-box method is like a precision strike that reveals errors in hidden code by removing extra lines. This method provides in-depth knowledge of the source code, making it easier to deal with side effects. One of the important advantages of white box testing is that it enables traceability of every test on the source-code level, where every future change can be easily captured in the newly added or modified tests. White-box testing exposes every inconspicuous bottleneck in the code and provides the development team with maximum coverage and clear, concise feedback. It makes it easier for the development team to reduce technical debt by optimizing and maintaining the quality of their code. Additionally, white-box testing can also be automated, which further increases efficiency.

However, white-box testing is usually very time-consuming and complex. The approach requires testers to have top-notch programming skills and a thorough, code-level understanding of the software they are testing. This implies hiring top-notch engineers for the tests to be efficient, which makes the method more expensive. In terms of time spent on testing and the required skills, white-box testing is more demanding than black-box testing.

Moreover, the test results are strictly tied to the way the code was written. If the code tied to the same functionality is changed, it invalidates the previous assumptions, which can result in a failed test case with false positives. In addition, while the black-box method is excellent for functional testing, white-box testing will not be able to deliver, as it only focuses on the existing state of the software. This means it won’t be able to provide any feedback on the missing functionality, leaving many paths untested.

Grey-box testing

Black-box and white-box testing have different focuses and show significant advantages in one area while being inefficient or having serious flaws in the other. Grey-box testing combines the benefits of both methods while neutralizing most of the flaws through effective, balanced combining of the two. This method increases the coverage of testing techniques by focusing on all layers of the software, regardless of its complexity.

While black-box testers check that interfaces and functionality are working correctly, and white-box testers dig into the internal structure and fix the source code of the software, grey-box testing deals with both simultaneously in a non-intrusive manner. The grey-box method is particularly useful for complex systems as it uses a straightforward black-box approach, which enables anyone from developers to testers to end-users to perform the tests.

To design test cases, an engineer requires partial knowledge of the internal structure, including documentation on data structures, architecture, and functional specifications of the software. The generated test cases aim to find and eliminate defects in the structure and close any gaps that might enable improper usage of the software.

Grey-box testing is best suited for integration testing and can be applied to web applications, which don’t have source code or binaries and cannot be tested using the white-box method. It can also be used to test business domain testing to ensure that the software meets the requirements.

Some of the most common grey-box test design techniques include:

1. Matrix testing traces and maps user requirements to ensure complete test coverage and identify any missing functionality.
2. Regression testing is a technique used to check if the software works correctly after modifications and ensure that no new bugs arise and nothing obstructs the existing functionality. It is basically a software change impact analysis.
3. The pattern testing technique is used to analyze any defects that have been encountered in the build, design, and architecture of the software being tested. By analyzing these defects, the goal is to determine the root cause of the problem and take steps to prevent it from happening again in the future.

Grey-box testing pros and cons

The grey-box testing method is a hybrid of black-box and white-box testing, providing the benefits of both while minimizing their drawbacks. It is a non-intrusive method based on functional specifications, interfaces, and documentation that allows testers to inspect the software’s architecture without requiring full access to the source code or binaries. This results in an unbiased testing process with a clear boundary between testers and developers. Grey-box testing also allows for intelligent test authoring, enabling exceptional test scenarios to analyze data types, communication protocols, exceptions, and more.

However, grey-box testing requires excellent project management as it can be redundant if the developer has already run related test cases. Since grey-box testing has limited knowledge of the software’s internal structure and no access to its source code, it only provides partial test coverage, leaving many code paths untested. As a result, it is not suitable for algorithm testing. Furthermore, identifying defects in distributed systems using the grey-box method can be challenging.

Conclusion

This article compared and described the three main software testing methods used by companies to ensure the quality of their code and compliance with software requirements specifications. These methods help companies avoid technical debt in the long run. The main difference between black-box, white-box, and grey-box testing is their relationship with the software code.

The question of which software testing method is the best is often asked. However, it is wrong to say that one method is significantly better than the others since each method serves different goals best. Black-box testing uses requirements to derive external expectations and eliminate functional errors and inconsistencies. White-box testing investigates the source code to ensure there are no hidden errors or defect-prone elements. Lastly, grey-box testing utilizes high-level data and functional specifications to catch defects and ensure the software meets the requirements.