Your complete guide to Black Box and White Box Testing

When it comes to software development, Quality Assurance can often be an unsung aspect of the process.  For software businesses, however, ensuring product quality and reliability is essential to their success. Black box and white box testing are two popular branches of software testing used by QA teams to ensure that software in development operates as intended and doesn’t behave in unpredictable ways.  

Whether providing custom software solutions for corporate clients or developing new functionalities and features as part of a SaaS platform, accurate performance, availability, and reliability are keys to software success. Without adequate QA testing, your software products could be accruing technical debt and putting client satisfaction at risk. Testing should cover both the input and expected outputs of software, as well as how these applications arrive at their outputs. That’s where black box and white box testing come in. 

While the end goal of software testing is to ensure a defect-free product that meets clients’ needs, there are various aspects of software applications that need to be assessed.  

To provide coverage for both the functional and code level aspects of applications, different testing paradigms are necessary. Holistic testing can ensure better coverage, and thus, better software quality. 

Understanding how black box and white box testing enable this starts with understanding what each testing methodology contributes to software QA. 

What is black box testing? 

Black box testing (also referred to as Behavior testing) typically ensures that applications under development do what they’re supposed to. Black box testing focuses primarily on the inputs and outputs and isn’t concerned with how the results or outputs are achieved.  

For all intents and purposes, internal workings like the operations and logic are less important than ensuring the application operates in accordance with customer requirements and specifications 

As it’s abstracted away from the details of how the software works, black box testing methods are most often used for high-level testing. These methods tend to focus on how end-users will actually interact with the finished product. As a result, it focuses on verifying that given the right inputs, the application under testing (AUT) produces the expected outputs. 

For example, imagine you’re developing a messaging application. End-users don’t need to know how the application works or what’s going on under the hood. For the application to be successful, however, users need it to send and receive messages, archive and delete messages, and manage contact lists. 

Black box testing would focus primarily on ensuring that those functionalities work as intended. Any outputs that deviate from tester/user expectations might therefore indicate defects in the application. 

Common types of black box testing include: 

• Functional testing (e.g., Unit Testing, Sanity Testing, Smoke Testing, User Acceptance Testing, etc.) 

• Non-functional testing 
• Regression testing
  

What is white box testing? 

White box testing (also known as open box, clear box, or glass box testing) is typically not concerned with ensuring that the outputs of applications are always correct. Rather, testers are concerned with how the application processes inputs and produces the expected outputs. As such, it seeks to ensure that the software it’s testing is fast, usable, reliable, and secure. 

White box testing requires the testing personnel to be knowledgeable about how the application works and what the underlying operations and logic do. One of the core goals of white box testing is verifying the logical flow of operations through the application. While this involves following the flow of data from input to output, it focuses more on ensuring that the internal structure and code in the application work as intended to produce the desired outcomes. 

White box testing is commonly used to: 

• understand how inputs are handled by the application 
• identify logical errors  
• uncover possible security risks or vulnerabilities 
• identify redundant conditional logic in visible code paths 
• verify the functionality of functions, statements, and objects defined in the application
  

Common types of white box testing include: 

• Unit testing 
• Integration testing 
• Penetration testing 

• Mutation testing 

Key differences between black box and white box testing 

Both testing methodologies are essential for improving software quality and ensuring customer success. However, they approach software functionality and reliability from very different angles. Some of the key differences between white box and black box testing include: 

Test Coverage vs Code Coverage 

When it comes to testing, “coverage” is a term that commonly crops up. Depending on the testing methodology used, however, it may mean very different things.  

Test Coverage in black box testing  

Test coverage is another QA metric. It’s often used in black box testing to quantify how much testing is actually performed during testing. It’s a good metric for tracking the quality and extent of the testing performed. It can also be used to help QA teams develop tests that provide better coverage.  

Test coverage can be calculated as: 

If the application under test has 1000 lines of code and 650 lines have executed in all test cases, the resulting test coverage would be 65%. 

Code Coverage is a white box testing convention 

Code coverage is a typical QA and software quality KPI that measures the percentage of the software’s code that has been “covered” by testing. One of white box testing’s main objectives is to cover as much of the code as possible. Overall code coverage is usually an aggregation of more granular test coverage metrics like: 

1. Function Coverage:
   The percentage of total defined functions that have been called and function as expected. 
2. Line Coverage:
   The percentage of total lines of code that have been tested and executed successfully. 
3. Statement Coverage:
   The percentage of executable statements that have been tested and run as expected. 
4. Branch Coverage:
   The percentage of total control structure branches that have been tested and executed successfully.
   
5. Condition Coverage:
   The percentage of expressions that require a true or false value, that have been tested and return either of the expected outputs.
   

Which is better, black box or white box testing? 

While it may seem like a straightforward question, it’s often not possible to say whether one form of testing is objectively better than the other. This is because white box and black box testing are both integral parts of QA in software development. Better software quality depends on sufficient code and test coverage and that means ensuring that all your bases are covered––from the user level down to the nuts and bolts of the application. 

It should come as no surprise that a combination of the two, the so-called grey testing method, exists. Grey box testing, as the name implies, finds a middle ground between white box and black box testing.  

It requires only partial knowledge of the underlying structure and logic of the application. Grey box testing is often used to expose and identify defects stemming either from the code level or the user level. While it approaches testing in more of a black box kind of way (i.e., focusing on the inputs and outputs), it applies the knowledge of the internal operations of the application to test design. 

Better software quality depends on adequate testing 

Ultimately, the goal of testing in software development is to improve the quality of the finished product. By combining white box and black box testing it’s possible to achieve higher coverage of your applications. This can help to ensure reliability, security, and functionality.  

Better testing in your software engineering can also provide insights into your development and QA teams’ performance. Learn more about engineering testing here.