Black Box and White Box Testing: Everything You Need to Know

Suppose you are a manufacturer looking to introduce a new electric kettle. Your product team develops it and releases it in the market without checking to make sure it is not faulty. A customer plugs it in and the kettle malfunctions. The complaints pile up. So what do you do? Before re-launching the kettle, you will make sure to “test” it.

In the same way, web and mobile applications need to be tested before deployment to make sure they are error-free and work as expected. Black box and white box testing are the two main types of software testing that prove helpful here.

Software testing does not have to be an isolated phase of the software development life cycle. It needs to be done at various stages of SDLC. An app can be tested for accuracy and correctness right from the design and prototyping stage up until the moment right before launch.

What Is Black Box Testing?

Black box testing refers to a type of software testing process that requires the tester to test an application without possessing any knowledge of the application’s internal structure. The application that is being tested can be thought of as a black box that they cannot see inside. However, they do not go in completely blind.

Black box testers are informed about the expected behaviour of the software. Instead of worrying about the technologies that have been used to put together and develop the app, the tester focuses on whether or not the app meets all the requirements listed in the Software Requirements Specification (SRS). With the help of a user interface, they can check the various features in the app and see whether each of those features is performing its designated function(s).

Testers are supposed to make sure that the app is carrying out all its functions not just correctly but also efficiently. There may be certain non-functional requirements that need to be met, such as data security or the interface’s ease of use.

It seems pretty simple, doesn’t it? Well, it is not! The tester needs to know how to carefully determine what test cases to run based on the specified requirements, and then analyse the outcomes obtained.

Black box testing is also known as:

• Specification-based testing
• Behavioural testing
• Data-driven testing
• Input-output testing

No matter what stage of testing you are at (unit testing, acceptance testing, or anything in between), you can perform black box testing at any point.

Example of black box testing

An example of black box testing is login functionality testing for a website.

Scenario:

A tester verifies the login feature by entering different sets of valid and invalid credentials without knowing the internal code.

Test cases:

1. Valid Credentials: Enter correct username and password → Expect successful login.
2. Invalid Password: Enter correct username but incorrect password → Expect error message.
3. Empty Fields: Leave username and password blank → Expect validation error.
4. SQL Injection: Enter SQL commands instead of a password → Expect rejection.

The tester focuses only on the input and expected output without any knowledge of how authentication is implemented internally.

Advantages of black box testing

1. No need for code knowledge – testers do not need to understand the internal code structure, making it accessible to non-developers.
2. Unbiased testing – since testers focus only on inputs and outputs, they can find unexpected issues without being influenced by the code implementation.
3. Effective for large systems – it is useful for validating large and complex applications where understanding every piece of code is impractical.
4. Simulates real user experience – helps ensure that the application behaves as expected for end-users.
5. Easier test case creation – based on requirements and specifications, making it straightforward to design test scenarios.

Disadvantages of Black Box Testing

1. Limited coverage – cannot verify the internal logic, code efficiency, or security vulnerabilities.
2. Difficult to identify edge cases – some intricate errors related to code structure or logic might go unnoticed.
3. Repetitive testing – since it relies on inputs and outputs, redundant test cases may be created without optimising for minimal test execution.
4. Hard to pinpoint the root cause – if a test fails, it is challenging to determine what caused the issue without looking at the code.
5. Time-consuming for complex systems – since it relies on trial and error, testing large applications can take longer compared to white box testing.

What Is White Box Testing?

White box testing refers to a type of software testing process that requires the tester to possess in-depth knowledge of the internal code structure of the application that they are testing. The tester needs to not only ensure that the app is functioning correctly but also know how the app is performing its functions.

Compared to black box testing, white box testing is done at a much lower level. Often, only those who have worked on the code for a particular application are able to test it thoroughly. This includes the developers and technical engineers who are aware of the internal structure and technologies used in the development process. This is because they should be able to thoroughly understand what goes on behind each feature or functionality present in the app. The inputs that they choose for conducting the tests need to be based on their knowledge of the app’s internal design and structure.

White box testing helps to determine and confirm the correctness of each and every path that the app takes when fed different kinds of inputs.

Due to the transparent nature of the testing process where the tester needs to be able to see beyond the app’s surface-level operation, white box testing is sometimes also referred to as clear-, glass-, transparent-, or open-box testing. Other names include:

• Code-based testing
• Logic-coverage or logic-driven testing
• Structure-based or structural testing

While white box testing is usually performed on the lowest level i.e. unit testing, it can also be applied to integration or system testing.

Example of white box testing

An example of white box testing is unit testing a function in a login module.

Scenario:

A developer tests an authentication function in the backend code.

Code example (Python):

def authenticate(user, password):

    stored_password = get_password_from_db(user)

    if stored_password and stored_password == password:

        return “Login Successful”

    else:

        return “Login Failed”

Test cases:

1. Valid Login: Call authenticate(“user123”, “correctPass”) → Expect “Login Successful”.
2. Invalid Password: Call authenticate(“user123”, “wrongPass”) → Expect “Login Failed”.
3. Non-Existent User: Call authenticate(“unknownUser”, “anyPass”) → Expect “Login Failed”.
4. Bypassing Authentication: Check if a direct database call is needed or if hardcoded values exist.

Here, the tester has access to the source code and verifies the internal logic, code paths, and possible vulnerabilities.

Advantages of White Box Testing

1. Thorough code coverage – tests internal structures, ensuring all code paths, loops, and conditions are executed.
2. Early bug detection – bugs are found during development, reducing costly fixes in later stages.
3. Optimised code performance – helps identify redundant code, inefficiencies, and security vulnerabilities.
4. Better security testing – exposes hidden vulnerabilities like logic flaws, hardcoded credentials, or backdoors.
5. Automation friendly – white box tests, like unit tests, can be automated, saving time in continuous integration (ci/cd) pipelines.

Disadvantages of White Box Testing

1. Requires programming knowledge – testers need coding expertise, making it unsuitable for non-developers.
2. Time-consuming – since it involves analysing the code structure, writing detailed test cases, and debugging, it takes longer than black box testing.
3. High maintenance – code changes require test updates, increasing maintenance efforts.
4. Not user-centric – focuses on internal logic rather than real-world user interactions, potentially missing UX issues.
5. Difficult for large systems – complex applications with millions of lines of code make full white box testing impractical.

What Is the Difference Between Black Box and White Box Testing?

A brief comparison of white box and black box testing can be drawn with the help of the following example:

Suppose your fridge is causing you problems. Everything that you store in it becomes soggy due to condensation. Now, you know this is unexpected behaviour, so something is definitely wrong. To test the fridge, you try adjusting temperature and other settings, but you continue to face the same issue: condensation. You decide to call someone from the fridge company so they can come over and see what is causing the issue. The expert that the company sends picks up the fridge, takes it apart for inspection and finds out where the fault was. In this case, you could say that you performed black box testing and the specialist acted as a white box tester.

There are many other differences between black box and white box testing. They are summarised in the table below.

Relevance of Black Box and White Box Testing

Both black box and white box testing are crucial in the software testing life cycle (STLC) to ensure a high-quality, secure, and reliable application. They complement each other by covering different aspects of software quality.

Relevance of Black Box Testing

• Ensures functional accuracy: Validates if the system meets user expectations without focusing on internal code.
• User-centric testing: Mimics real-world usage to detect usability and functional issues.
• Works for large-scale systems: Can be applied to complex software without requiring code access.
• Efficient for acceptance testing: Helps in user acceptance testing (UAT) before deployment.
• Helps in regression testing: Verifies that new changes do not break existing functionality.

Relevance of White Box Testing

• Ensures code quality: Detects logical errors, inefficient code, and security vulnerabilities.
• Improves performance: Identifies bottlenecks and redundant code.
• Essential for security testing: Uncovers hidden flaws like SQL injection vulnerabilities, buffer overflows, or hardcoded credentials.
• Facilitates unit testing: Allows developers to validate individual functions and modules.
• Enhances maintainability: Encourages better coding practices through test-driven development (TDD).

Final Thoughts

If software applications are released to the end-users without being tested properly, it can cause serious harm. It might leave the users utterly helpless and frustrated if they run into unexpected errors or faults, causing the developers to end up with negative reviews.

Thus, software quality assurance engineers and testers must perform due diligence before implementing new apps. Whether it is a web-based application or a native or hybrid mobile app development project, black box and white box testing must be part of every software application’s testing phase. The better their testing design and plan, the easier it will be to catch bugs early on, the better the application will perform, and the less likely it will be to cause problems for end-users.