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A Comprehensive Guide to Unit Testing

What is Unit Testing? - A Comprehensive Guide

November 10, 2022
Shinji KanaiShinji Kanai
Shinji Kanai


Software development involves different steps and processes, ranging from writing code and testing every function to debugging and deploying. Unit testing is an important test method used by QA teams to ensure that a software product is free of errors and meets all essential requirements. Nowadays, unit testing has become imperative in the software testing cycle to ensure that each unit of the software product is completely functional and operates as planned. Unit testing is generally considered the first step in the software development life cycle (SDLC). In unit testing, testers validate each testable part of an application to ensure that each unit of the app's code works as intended. Unit testing is normally executed in the early development stages of an application by developers or testers.

Check out: 10 Ways to Accelerate the Software Testing Cycle

This blog discusses unit testing's importance, advantages, and various testing techniques in detail.

What is Unit Testing?

In a software application, the smallest part of a code that can be tested is called a unit of code. Unit testing is a testing method used in the software development cycle to validate that a unit of code will behave as intended and that the tested code will produce the same result every time the test is performed. Unit testing comes under white-box testing, and it is generally done by writing test cases that exercise individual functions, methods, and classes. Unit tests are normally written in pseudocode. The accuracy of the code, code coverage, implementation and maintenance of coding standards, and verification of the functionality covered by the piece of code are the main components or functions verified by QA experts with unit testing.

Here are the four important steps used by QA teams to run unit tests.

  • Writing test cases: Developing multiple test cases of applications’ components.
  • Review and edit: Verifying the written test cases and editing them if there are any errors. 
  • Baseline: Ensuring whether each code line is in a manner or not.
  • Execution: Executing test cases to identify possible errors.

Test cases of unit testing are simple and clear so that they are easy to read and understand, even if they have complex logic and lots of variables. Unit testing is performed before integration testing, which saves a lot of time and costs. Moreover, unit tests can be carried out manually or with test automation tools.

How Do Developers Use Unit Tests?

Developers use unit tests to validate the functionality of individual components or units of code, ensuring they perform as expected. This process typically involves writing test cases for each function or method in a module, simulating inputs, and verifying that the outputs match the expected results. Unit tests are often automated and run frequently during development to catch defects early. By isolating each unit, developers can quickly identify and fix issues, leading to more reliable and maintainable code.

When Is Unit Testing Less Beneficial?

While unit testing is a powerful tool, it may be less beneficial in certain scenarios:

  • High-Level Integration: Unit tests focus on individual units and may not capture issues that arise from the interaction between different units.
  • Rapidly Changing Code: In projects with rapidly evolving codebases, maintaining unit tests can become cumbersome and time-consuming.
  • Simple or Short-Lived Projects: For small projects or prototypes not intended for long-term use, the overhead of writing and maintaining unit tests may not be justified.
  • Legacy Code: In cases where the existing codebase lacks modularity, writing unit tests might require significant refactoring, which can be impractical.

What Are Unit Testing Best Practices?

  • Write Independent Tests: When writing Unit tests, ensure each test you write is independent and does not rely on other tests or shared states to avoid false positives or negatives.
  • Test Single Responsibility: Focus on testing one functionality per test case to ensure clarity and simplicity.
  • Use Descriptive Names: Name your tests clearly to describe what they are testing, which makes it easier to understand test failures.
  • Automate Tests: Integrate unit tests into your CI/CD pipeline to run automatically on code changes, ensuring continuous validation.
  • Mock External Dependencies: Use mock objects to simulate external dependency behaviors to isolate the unit under test.
  • Write Tests Before Code: Adopting Test-Driven Development (TDD) can help ensure your code meets the requirements from the outset.
Read: Fundamentals of Test Harness

Difference Between Unit Tests and Other Types Of Tests?

  • Unit Testing vs. Integration Testing: Unit testing focuses on individual code units in isolation, while integration testing examines the interactions between different units or components to ensure they work together as expected.
  • Unit Testing vs. System Testing: System testing evaluates the system's compliance with specified requirements. Unlike unit testing, which is more granular, it covers end-to-end scenarios and user interactions.
  • Unit Testing vs. Functional Testing: Functional testing verifies that the software performs its intended functions from a user perspective without delving into the internal workings of the code. On the other hand, unit testing ensures that individual code units function correctly.
  • Unit Testing vs. Regression Testing: Regression testing ensures code changes do not affect existing functionality. Unit tests can be part of regression testing but focus more on individual units than the entire system.
  • Unit Testing vs. Acceptance Testing: Acceptance testing ensures the software meets the acceptance criteria. End-users or stakeholders typically do it, while developers do unit testing during development.

By understanding these differences, developers can use unit testing with other testing types to ensure comprehensive coverage and software quality.

Importance of Unit Testing

Unit testing is crucial for developers during software development, as proper unit testing in the early stage of development can help them save time and money. The programmers or developers generally execute this testing in the development environment before combining the code with other existing functional modules of code. There is a high probability that the code turns out to be faulty if the unit testing of a software application is not performed.

Some of the main reasons why testers consider unit testing an important part of the software testing cycle include:

  • Identify bugs or errors at the early stages: As unit testing is executed before integration testing, possible errors and bugs can be detected early in the development cycle before they become pervasive. 
  • Easily and quickly debug software applications: In unit testing, it would be easier to detect the bugs, as the units are tested, not the combined modules, thus, making the debugging process easier and faster.
  • Isolate a unit of code and verify that it works properly: Unit testing helps QA experts make code more maintainable by allowing them to modify it without changing other parts of the app.
  • Test software application’s functionality: With the help of unit testing, testers can validate every module unit before integrating it into a software application. 
  • Reduce cost and save time: In the software testing cycle, integration testing is considered relatively high maintenance and complex. With unit testing done before integration testing, QA teams can save a lot of time and drastically cut the overall cost. 
Please check: JUnit: A Complete Guide

Different Unit Testing Techniques

Unit testing of software applications can be performed with the help of three main testing techniques:

1. Structural Testing Technique

The structural testing method is a test that evaluates the structure of a software application. This test is often known as white box testing. As structural testing necessitates a thorough understanding of the code, the development team considers this testing mandatory.

2. Functional Testing Technique

The functional testing method is a test that aims to verify that each app feature works as per the requirements of the software. This testing is performed by putting sample inputs, recording the resulting outputs, and verifying that the actual outputs match the expected outputs.

3. Error-based Testing Technique

The error-based testing technique is an experience-based testing technique where testers use their experience to guess the problematic areas of the software application. Some of the error-based testing techniques are historical test data, mutation testing, and fault seeding techniques.

Check out: Testing with Puppeteer - A Complete Guide

Major Benefits of Unit Testing

Unit testing is essential for fast and agile software development. It can also help development and QA teams have various short- and long-term benefits, the biggest ones being improved overall software application quality and customer satisfaction. Let’s check some of the major benefits of unit testing.

1. Accelerated Software Development 

Writing a complete set of unit tests helps testers understand inputs, outputs, errors, and the overall architecture of the code very well. As a result, development teams can identify bugs and errors in the code very early in the development process. It is very easy to be in an endless and unproductive testing cycle. However, unit tests can save a lot of time and effort for QA teams, as they remarkably identify regressions.

2. Minimum Regressions

Regression bugs are errors that are introduced after an update is made to the software and did not exist before that update. Bug fixes and new features can cause regression bugs to break existing functionalities. A well-designed and comprehensive unit testing will help QA teams prevent breaking functionality that previously worked.

3. Test in Isolation

Unit testing allows testers to evaluate the method logic in isolation from its dependencies. In this type of testing, when an error is introduced, only a specific type of unit test will fail and show that there is an error. QA teams will be able to understand what the underlying problem is and fix it immediately. Isolation allows all unit tests to be performed in random order. This is crucial because tests are a dynamic system — adding new tests, changing or deleting existing tests, and keeping test cases independent may reduce dependencies between tests. Some tests may fail during some runs and pass in others if done incorrectly. Debugging these types of failures is difficult and time-consuming. The main takeaway of this testing is that the order in which the unit testing is performed will not affect the outcome of the unit test.

Read: Shifting to automated software testing amidst the economic downturn

4. Code Modification, Refactoring, and Maintenance

Once testers have covered the bulk of their software applications by unit tests, it is assumed that they have covered all the bases for delivering high-quality applications on time. Moreover, unit testing gives developers the confidence to update the software application, knowing it will work as expected and prevent bugs and regressions.

5. Cost Reduction 

The cost of identifying and fixing an error during the initial stages of software application development is lower than doing it once the software is in production. This is because testing is context-dependent. When testers are engaged in the process of implementing new functionality, their mind is faster and better at identifying an issue than doing it weeks later when it needs to re-create the code structure from scratch. Errors that go unresolved late in the process can have significant negative implications for a company’s business, such as damaged reputation, payment of contractual penalties, customer dissatisfaction, or even a lawsuit.

6. Easy Upgradation of Third-Party Libraries

Software applications depend on third-party libraries. Unit tests allow testers to upgrade any third-party library without a defect outside their control. Updating third-party technology is particularly important when new features, significant improvements, and bug fixes are released that will benefit the software. The more often testers do it, the easier it will be to manage the updates.

7. Automated Unit Testing

Nowadays, unit testing has become a vital component of the automated continuous integration process. Continuous integration provides a consistent approach toward quality software and ensures that any adjustments to the code are tested, no matter what. As a result, software applications are getting more reliable and easier to maintain, and customers are delighted. There are many automated tools available in the market to support unit testing in the software development cycle, and some of them are Jtest, JUnit, PHPUnit, etc. JUnit testing is the most popular automated testing tool, which is based on the Java language. It provides assertions to identify various testing methods. JUnit automated testing also verifies the data before putting it into the code.

Please check: A complete guide to choosing a test automation tool

Double the Benefits of Unit Testing with HeadSpin

1. Identify UX and Performance Issues

HeadSpin's unit testing helps organizations gather insights into their software applications' performance and user experience. Some metrics, like packet-level network data and device KPIs, are measured and analyzed through the AI-based Platform to provide actionable insights for organizations. Enterprises can gather insights about their software applications, such as issue cards, time series, session videos, and packet-level details to improve their applications' performance.

2. Use Data Science and AI for Root Cause Analysis 

HeadSpin can help development and QA teams analyze degradation across new app builds, OS releases, feature additions, and locations. HeadSpin’s data-driven Platform will also help customers get insights into third-party API and SDK behavior and performance-impacting UX. Testing teams can get root cause analysis while performing unit tests to fix the errors in their software applications.

3. Execute End-to-End Automated Unit Testing on Real Devices 

Organizations can perform end-to-end automated unit testing of mobile apps and software applications with the HeadSpin Platform. HeadSpin offers end-to-end visibility and UX and functional assessments for companies. HeadSpin's end-to-end automated unit testing can help companies increase collaboration for product management and release efficiencies across teams and team members. With HeadSpin, companies can reduce the time-to-market of their products and stay competitive in today's crowded app space.

4. Perform Unit Testing Remotely with Global Teams

With the HeadSpin Platform, a company's QA experts and teams worldwide can remotely access and perform unit testing of their software applications on real devices on the real carrier and Wi-Fi networks in 90+ locations worldwide from their desktops or phones without SDK or local team support. Organizations can choose HeadSpin's multiple deployment models, namely on-prem, cloud (single tenant), cloud (multi-tenant), and create their own lab, as per their testing requirements

Test your apps on real devices and get accurate results. Learn more.


Unit testing is the cornerstone of software application development, as it involves the testing of individual components of a software application. The prime focus of this testing is to verify that all the individual units are working in the intended way. The unit testing method has become a vital step in the software development process, as if done correctly, it can help identify initial code errors that may be more difficult to find in recent test stages. Unit testing has a lot of advantages, and its absence can cause issues and thus can have a profound effect on the software application. It also ensures that all units are working the way they should in the development phase and ensure maximum software performance. It is difficult for testers to find a reliable testing provider to manage the software application's testing phase.

The main features of the HeadSpin Platform, the best testing solution, which align with the key components of unit testing during application/software development, are:

  • Automated unit testing: It supports all major unit testing automation frameworks, like JUnit, TestNG, and more.
  • Continuous monitoring: It provides 24*7 access to any device, running any application, on any network, anywhere in the world, for continuous quality assurance.
  • CI/CD integration to automate the testing of new builds: It automatically sniffs new builds, runs tests, and provides insights on performance regression.

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Q1. What are some of the popular unit testing frameworks?

Ans: NUnit, JUnit, TestNG, Mockito, and PHPUnit.

Q2. What is test-driven development?

Ans: Test-driven development is a software development method in which test cases are created to validate what the code will do. In this method, test cases for each functionality are tested, and if the test cases fail, then a new code is written to pass the test and make the code simple and bug-free.

Q3. What is grey box testing? 

Ans: Grey box testing is a QA method used to test a software application with partial knowledge of the internal structure of the application. The focus of grey box testing is to identify the defects due to improper code structure.

Q4. What is acceptance testing?

Ans: Acceptance testing is a method of testing where a software application is tested for acceptability. The major focus of this test is to validate the compliance of the software with the requirements and assess whether it is ready for delivery.

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What is Unit Testing? - A Comprehensive Guide

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