Black box and white box testing: Our software testing terminology

Following our main article “Software testing: methods, pitfalls and tips” on testing, we would like to take a closer look at some terms and test methods.
Today, we will focus on the terms “black box testing” and “white box testing”.
Both approaches play an important role in software quality assurance, but they differ fundamentally in their methods and perspectives.
Let’s summarize briefly: What is black box testing? Black box testing, also known as functional testing, is a test method in which the internal structure or working mechanism of the application to be tested is unknown – in other words, a “black box” with unknown content.
In black box testing, the testers focus exclusively on the input and the expected output without knowing how the application processes these results internally.
The main aim is to find functional errors, user interface errors, security vulnerabilities and errors in data structures or external database access that could impair functionality. Advantages of black box testing:

• No need to know the code: Testers do not need to have any programming knowledge or understand the internal code.
• Testing from the user’s perspective: This approach simulates user behavior and experience, which helps to identify relevant errors.

Disadvantages of black box testing:

• Limited coverage: As testers do not know what is happening internally, some parts of the code may remain untested.
• Potentially inefficient: Without knowledge of the internal logic, tests can be redundant or insufficient for complex functions.

What is whitebox testing? In contrast, whitebox testing, also known as structural testing or glassbox testing, is based on knowledge of the internal structures of the code under test.
For example, testers use their knowledge of the code to design tests that cover specific parts of the program, including loops, branches and internal code.
The goal is to ensure that all paths through the code are tested to uncover hidden bugs. Advantages of whitebox testing:

• Thorough test coverage: Enables testing of all paths, decisions and loops in the code.
• Greater efficiency: In contrast to black box testing, testers can specifically identify and focus on weak points in the code.

Disadvantages of whitebox testing:

• Requires a high level of expertise: testers must have extensive knowledge of programming and application design in order to carry out whitebox testing correctly
• Time consuming: Creating and running tests that cover every possibility can be very time consuming, but this is also rewarded with extremely high quality test results.

When is which form of testing used? The choice between whitebox and blackbox testing depends on various factors, including the phase of the development process, the type of project, the specific goals of the testing and the available resources.
For example, black box testing is particularly suitable in the early stages of development when the aim is to check requirements and functionality from the end user’s perspective without delving into the complexity of the code.
It is ideal for acceptance testing and for validating the user interface and business logic.
On the other hand, whitebox testing is preferable when a deep technical review of the code is required, such as for security checking and performance optimization.
It is particularly valuable in later stages of development or maintenance when changes are made to the code and it is important to ensure that no new bugs have been introduced.
In addition, whitebox testing is essential when it comes to checking code coverage to ensure that all branches and paths through the code are executed during the testing process.
Nevertheless, blackbox testing may be less suitable in scenarios where a deep understanding of the internal workings of the software is necessary to identify safety-critical or complex technical issues.
Similarly, whitebox testing may not be the best choice for projects where there is a lack of technical resources or where testers do not have access to the source code.
In many cases, blackbox and whitebox testing complement each other by looking at different perspectives and levels of software quality.
The most effective testing strategy, which enables a comprehensive evaluation of the software and ensures both user experience and technical integrity, is therefore often a combination of both approaches.