What is Black Box Testing?

Software testing contributes to the overall quality of software by identifying bugs, then correcting the reported issues. Although all types of testing have the same objective in ensuring the system meets both the functional and non-functional requirements, testing techniques and focuses vary.

Black box testing is a widely used testing method. As the name suggests, the system to be tested is approached as though a closed or black box where testers cannot readily see what’s inside. Black box testing focuses on assessing what the built system does, rather than how it has been developed.

A black box testing approach can be used at any level of testing such as with Unit, Integration, System, or Acceptance. Black box testing is also called Behavioral Testing because it involves testing the software’s performance from outside. The technique does not require testers to know the internal structures of the system. With this type of testing, the test cases are designed by referring to the specifications. Therefore, it is sometimes also referred to as Specification Testing.

Black Box Testing Techniques

A well-designed test case is a strong indicator of the extent of testing coverage. The two primary components of any black-box testing are to identify the input and determine the expected output. While planning the tests, testers cover all positive and negative scenarios, but testing all input combinations is beyond the scope. Testers use several different techniques for black box testing to create efficient test cases and achieve accurate coverage.

Black Box testing techniques reduce redundancy and increase total coverage, but not every method is suitable for each project. Before implementing and designing test cases using a black box technique, testers need to understand which practices best apply to their project.

1. Equivalence Class Partition: In this method, the input is divided into groups. An illustration of this could be testing a mobile phone input field in an application form. The possibility of testing using every valid phone number is untenable. To test a phone field, creating three partitions would be an ideal approach. One would be to check a number of fewer than nine digits. Another would be to test to see if the error is thrown if a user enters 11 digits. The last input will be to verify if the system accepts any ten-digit phone number.
2. Boundary Value Analysis (BVA): Input ranges are tested using boundary values. For example, consider an eCommerce site that limits purchasing to only five units of a product. In this scenario, the test case must check that the system does not allow buying to be a negative or zero quantity, or any number greater than five. The test must run using the inputs as -1, 0, 5 and 6.
3. State Transition Testing: This technique is used to determine an application’s behavior based on the system’s different states. A useful example would be the way a website performs for a logged-in vs. a non-logged in user. If a logged-in user accesses the system, it generally offers more features such as access to their account, quick checkout, etc. So, creating a state transition checklist helps to reduce the overall execution time.
4. Decision Table Testing: When the application involves several business rules, creating a decision table ensures that it takes corrective action when it meets a specific condition. Let’s consider an eCommerce website that runs several promotions based on the amount of sale such as:
   a) Shop for x amount, and get 10% off
   b) Shop for y amount and get 20% off
   c) Shop for z amount and get 50% off
   In such cases, creating a decision table listing the rules and outputs helps validation.
5. Error Guessing: is a technique utilized by experienced testers. This method involves testing using input, which has a record of failures and could have a high probability of future failures. For testers aware of the application’s ins and outs, it is easy to guess and identify those areas.

Why Black Box Testing is so popular with testers

With the rising demand for software to have been tested before launch, every development project now includes QA for verifying the system’s quality. Formerly, software used to be launched without any formal QA testing taking place, but today, the value of testing is properly understood. Of all the different testing methods, black box testing is a popular choice because of the coverage it offers and the ease by which it is conducted.

In functional testing, black box testing is the most preferred method. Tests are designed based on requirement, and there is no dependency on code development so the test design phase can run in parallel to development.

The focus with black box testing, is mainly on user-based testing. If a project demands a high-level testing review, this type of testing will do the job because it helps in testing application usability.

Testers do not need to have a working knowledge of programming languages, because the method does not involve testing at the level of internal structures. Lacking language skills is no bar to conducting black box testing.

Conclusion

A significant advantage to using black box testing is that it is a cost-effective and result-driven software testing method suitable for large and complex projects.