Writing good code is not just about making it work – it is about making sure it keeps working. Unit testing helps you catch bugs early by testing each piece of your code separately.
In this blog, we will break down unit testing in plain narratives: what it is, how it works, and why it matters for your projects.
What is unit testing?
Unit testing involves evaluating the smallest functional parts of code to ensure their quality. It is a crucial practice in software development, where the code is broken down into small, functional units, each accompanied by a corresponding unit test.
These tests are written as code and can be run automatically whenever the software is updated. This approach allows developers to quickly identify and isolate bugs when a test fails. Unit testing promotes modular design and increases the overall quality and test coverage of the code. Automated unit testing also frees up more time for developers to focus on writing new code.

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What is a unit test?
A unit test is a segment of code that checks the correctness of a smaller, isolated portion of an application, typically a function or method. It aims to confirm that the code executes as expected based on the developer’s intended logic. Unit tests interact with the code only through inputs and evaluate the output using assertions (true or false conditions).
A code block can have multiple unit tests, referred to as test cases, which together aim to cover the full range of expected behavior. However, defining a complete set of test cases is not always required. Unit tests must operate independently, meaning they should not rely on other parts of the system, such as databases, objects, or network communication. If the code depends on external data, data stubs should be used. Writing unit tests is more straightforward for smaller, logically simple code segments.
What is data sub?
A stub acts as a small piece of code that replaces another component during testing. A key benefit of using stubs is the ability to obtain consistent results to make test writing easier. Even if the other components aren’t yet fully functional, you can still execute tests by using stubs.
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Types of unit tests
Different types of unit testing are designed to address these challenges individually. Each type offers unique advantages for identifying and resolving specific kinds of issues.
White-Box Testing
White-box testing, also known as structural or clear-box testing, is a testing method that examines the internal structure and workings of the code.
Testers here are not just scratching the surface. Instead, they analyze code structure, control flows, and specific logic paths with a thorough understanding of the programming language and system architecture. Because of this level of detail, white-box testing becomes especially effective for syntax errors and boundary issues.

Who Benefits
White-box testing offers precise insights into code paths, highlights coverage gaps, and catches bugs early in development. Primarily developers and quality assurance (QA) engineers with strong technical skills benefit from white-box testing. Since this testing requires an understanding of code structure and logic, it is ideal for those familiar with the codebase.
When in Development
White-box testing works best in the early stages of development and continues to be helpful throughout coding. Developers start using it as soon as they build individual components or modules. This timing helps them catch syntax errors, logic flaws, and performance issues before combining the code with other parts. White-box testing is also useful during code reviews, and it plays an important role in the continuous integration process.
Black-Box Testing
Black-box testing, sometimes called functional or specification-based testing, is the method of evaluating the software’s external behavior without regard to its internal code structure. Instead of focusing on how the code is built, black-box testing examines whether the code performs its intended functions.
Here, testers evaluate whether the software meets its functional requirements without looking beneath the surface. This makes black-box testing particularly useful for ensuring the software performs as intended under various conditions, while also uncovering usability and accessibility issues along the way.

Who Benefits
Black-box testing does not require in-depth technical knowledge, is accessible to testers with varying expertise levels, and often reveals defects caused by component interactions. QA testers, end-users, and stakeholders who may not have deep technical expertise benefit most from black-box testing. Black-box testing is accessible to anyone familiar with the software’s requirements or specifications.
When in Development
Black-box testing is particularly useful later in the development cycle, such as during system testing, acceptance testing, and post-release. This testing method is ideal for verifying that the application meets functional and usability requirements without needing to examine the underlying code.
Gray-Box Testing
Gray-box testing, meanwhile, combines elements of both white-box and black-box methods, offering a balanced approach by using partial knowledge of the system’s internals to guide testing.
Testers have limited knowledge of the code’s internal structure, allowing them to identify issues in data flows and application behavior while still maintaining a degree of objectivity and focusing on the external functionality. Additionally, this approach fosters communication between testers and developers, enhancing the overall effectiveness of the testing process.

Who Benefits
Gray-box testing allows for more thorough testing than black-box testing alone, detects bugs that might otherwise go unnoticed, and promotes better collaboration between testers and developers. Both developers and testers benefit from gray-box testing, as it requires a mix of technical knowledge and functional understanding. It is especially helpful for QA testers working closely with developers.
When in Development
Gray-box testing is typically used during integration and system testing stages, once individual components are built but before full system release. This approach allows testers to identify issues in data flows and functionality, especially at integration points between modules, and supports a collaborative testing process. It’s also valuable for regression testing when changes are made to the codebase.
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What does a unit test look like?
A unit can be almost anything you want it to be — a line of code, a method, or a class. Generally though, smaller is better. Smaller tests give you a much more granular view of how your code is performing. There is also the practical aspect that when you test very small units, your tests can be run fast; like a thousand tests in a second fast.
Below example is really simple, but it would give you a basic idea of what it means by small. Let’s consider this sample code written in Java.
public int addNumbers(int a, int b) {
return a + b
}
JUnit provides a straightforward way to test individual methods or components in Java, like the simple code example below. With JUnit, you can create small, isolated tests to verify that each piece of code behaves as expected, which is the essence of unit testing.
package agileTest.junit
import static org.junit.Assert.*;
import org.junit.Test;
public class SimpleTest {
@Test
public void addNumbersTest(){
int a = 3;
int b = 6;
assertEquals (addNumbers(a, b), 9);
}
}
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Unit testing strategy
To effectively create unit tests, follow these fundamental techniques to ensure comprehensive coverage:
1. Logic validation: Verify that the system executes the correct calculations and follows the expected paths through the code with the provided valid inputs. Ensure that all possible code paths are covered.
2. Boundary testing: Examine how the system responds to various input scenarios, including standard values, edge cases, and invalid data. For instance, if the expected input range is between 2 and 7, observe the behavior with typical values like 5, edge cases such as 2, and out-of-range inputs like 10.
3. Error handling: Assess the system’s response to incorrect inputs. Does it prompt the user to re-enter data, or does it result in a crash?
4. Object state verification: When code execution alters the state of persistent objects, confirm that these objects are updated as expected.
How Developers Utilize Unit Tests
Test-Driven Development (TDD)
In TDD, tests are written to meet functional requirements before the actual code is developed. Once coding is complete, the code is tested against these predefined tests to confirm it meets the specified requirements.
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Post-Coding Testing
When a block of code is completed (if not already covered by TDD), unit tests should be written to verify its functionality. Unit tests are the first set of tests run during system testing and form part of the comprehensive test suite.
DevOps Integration
In DevOps practices, unit tests play a key role in continuous integration and continuous delivery (CI/CD). They automatically run alongside other tests in the CI/CD pipeline to ensure code quality during ongoing development and deployment.
AgileTest – Enterprise QA and Test Management in Jira elevates test management and quality assurance by connecting your Git workflow with automated test management. When tests run in Bitbucket CI/CD pipelines, AgileTest automatically imports and analyzes the results, linking them directly to the Jira issues in real-time. This integration updates test execution statuses and can trigger automated actions in Jira based on test outcomes, eliminating manual reporting steps. By seamlessly connecting Git, Bitbucket, and Jira, AgileTest ensures your development and testing processes stay synchronized while maintaining clear visibility of your test coverage.

Conclusion
We have covered how to break down and test your code effectively, which tools can help, and how to make testing part of your daily development work. The goal? Building better, more reliable software with fewer headaches.