Unit Testing – Software Testing

Last Updated : 21 Apr, 2025

Unit Testing is a software testing technique in which individual units or components of a software application are tested in isolation. These units are the smallest pieces of code, typically functions or methods, ensuring they perform as expected.

Unit testing helps identify bugs early in the development cycle, enhance code quality, and reduce the cost of fixing issues later. It is an essential part of Test-Driven Development (TDD), promoting reliable code.

Table of Content

What is unit testing?

Unit testing is the process of testing the smallest parts of your code, like it is a method in which we verify the code’s correctness by running one by one. It’s a key part of software development that improves code quality by testing each unit in isolation.

You write unit tests for these code units and run them automatically every time you make changes. If a test fails, it helps you quickly find and fix the issue. Unit testing promotes modular code, ensures better test coverage, and saves time by allowing developers to focus more on coding than manual testing.

Prerequisite of Unit Testing

Understanding of the Software Development Process.
Knowledge ofProgramming languagesand Development tools.
Familiarity with the code base.
Ability to write test cases and understand expected outcomes.
Proficiency in usingunit testing frameworks and tools.
Awareness of best practices and guidelines for writing effective unit tests.
Clear understanding of the purpose and goals of unit testing in the software development lifecycle

Read More: Types of Software Testing

What is a Unit test?

A unit test is a small piece of code that checks if a specific function or method in is an application works correctly. It will work as the function inputs and verifying the outputs. These tests check that the code work as expected based on the logic the developer intended.

Unit Test

In these multiple tests are written for a single function to cover different possible scenarios and these are called test cases. While it is ideal to cover all expected behaviors, it is not always necessary to test every scenario.

Unit tests should run one by one, it means that they do not depend on other system parts like databases or networks. Instead, data stubs can be used to simulate these dependencies. Writing unit tests is easiest for simple, self-contained code blocks.

Unit testing strategies

To create effective unit tests, follow these basic techniques to ensure all scenarios are covered:

Logic checks: Verify if the system performs correct calculations and follows the expected path with valid inputs. Check all possible paths through the code are tested.
Boundary checks: Test how the system handles typical, edge case, and invalid inputs. For example, if an integer between 3 and 7 is expected, check how the system reacts to a 5 (normal), a 3 (edge case), and a 9 (invalid input).
Error handling: Check the system properly handles errors. Does it prompt for a new input, or does it crash when something goes wrong?
Object-oriented checks: If the code modifies objects, confirm that the object’s state is correctly updated after running the code.

Unit test example

Here is the example of the java with unit test cases with the proper unit test code

Create the Calculator Class

Java

public class Calculator {      // Method to add two numbers     public int add(int a, int b) {         return a + b;     }      // Method to subtract two numbers     public int subtract(int a, int b) {         return a - b;     } }

Create the TestNG Test Class

Java

package com.example.tests;  import org.testng.Assert; import org.testng.annotations.BeforeMethod; import org.testng.annotations.Test;  public class CalculatorTest {      private Calculator calculator;      // This method runs before each test method     @BeforeMethod     public void setUp() {         calculator = new Calculator();     }      // Test for the 'add' method     @Test     public void testAdd() {         int result = calculator.add(5, 3);         // Assert that the result of 5 + 3 is 8         Assert.assertEquals(result, 8, "Addition result is incorrect");     }      // Test for the 'subtract' method     @Test     public void testSubtract() {         int result = calculator.subtract(5, 3);         // Assert that the result of 5 - 3 is 2         Assert.assertEquals(result, 2, "Subtraction result is incorrect");     } }

Output:

Unit Testing Example Output

What are the benefits of unit testing?

Here are the Unit testing benefits which used in the software development with many ways:

Early Detection of Issues: Unit testing allows developers to detect and fix issues early in the development process before they become larger and more difficult to fix.
Improved Code Quality: Unit testing helps to ensure that each unit of code works as intended and meets the requirements, improving the overall quality of the software.
Increased Confidence: Unit testing provides developers with confidence in their code, as they can validate that each unit of the software is functioning as expected.

Benifits of Unit testing

Faster Development: Unit testing enables developers to work faster and more efficiently, as they can validate changes to the code without having to wait for the full system to be tested.
Better Documentation: Unit testing provides clear and concise documentation of the code and its behavior, making it easier for other developers to understand and maintain the software.
Facilitation of Refactoring: Unit testing enables developers to safely make changes to the code, as they can validate that their changes do not break existing functionality.
Reduced Time and Cost: Unit testing can reduce the time and cost required for later testing, as it helps to identify and fix issues early in the development process.

Disadvantages of Unit Testing

Time and Effort: Unit testing requires a significant investment of time and effort to create and maintain the test cases, especially for complex systems.
Dependence on Developers: The success of unit testing depends on the developers, who must write clear, concise, and comprehensive test cases to validate the code.
Difficulty in Testing Complex Units: Unit testing can be challenging when dealing with complex units, as it can be difficult to isolate and test individual units in isolation from the rest of the system.
Difficulty in Testing Interactions: Unit testing may not be sufficient for testing interactions between units, as it only focuses on individual units.
Difficulty in Testing User Interfaces: Unit testing may not be suitable for testing user interfaces, as it typically focuses on the functionality of individual units.
Over-reliance on Automation: Over-reliance on automated unit tests can lead to a false sense of security, as automated tests may not uncover all possible issues or bugs.
Maintenance Overhead: Unit testing requires ongoing maintenance and updates, as the code and test cases must be kept up-to-date with changes to the software.

How do developers use unit tests

Unit testing plays an important role throughout the software development process:

Test-Driven Development (TDD): In TDD, developers write tests before writing the actual code. This ensures that once the code is completed, it instantly meets the functional requirements when tested, saving time on debugging.
After Completing Code Blocks: After a section of code is finished, unit tests are created (if not already done through TDD). These tests are then run to verify that the code works as expected. Unit tests are rarely the first set of tests run during broader system testing.
DevOps and CI/CD: In DevOps environments, Continuous Integration/Continuous Delivery (CI/CD) automatically runs unit tests whenever new code is added. This ensures that changes are integrated smoothly, tested thoroughly, and deployed efficiently, maintaining overall code quality.

How do developers use unit tests

When is unit testing less beneficial?

Unit testing is not always necessary for every piece of code in every project. Here are some situations where it might be okay to skip unit tests:

Time Constraints: Writing unit tests can take a lot of time, even with automated tools. If your team is under pressure, they might get distract while writing tests and miss deadlines, leading to budget issues.
UI/UX Focus: If the main focus of the application is on design and user experience rather than complex logic, manual testing might be more effective than unit testing.
Legacy Code: Testing old or legacy code can be very challenging. If the existing code is not written well then it may be hard to add tests without reference writing, which can be time-consuming.
Changing Requirements: In projects where requirements are frequently changing, it may not be practical to write tests for all code, as the code may change before the tests are even run.

What are unit testing best practices?

Use a Unit Test Framework: Avoid writing custom various tests for every line of code. Instead, use testing frameworks like pytest or unittest in Python. These frameworks help automate and standardize testing across projects of all sizes.
Automate Unit Testing: Automate your unit tests to run during key development events, like pushing code to a branch or deploying updates. Scheduling tests ensures regular testing throughout the project lifecycle, catching issues early.
Assert Once: Each unit test should have only one assert statement. This provides a clear pass/fail result, making it easier to identify and fix issues without confusion.
Implement Unit Testing Early: Applying unit testing from the start of a project. This makes testing a natural part of the development process, saving time and reducing errors as the project grows.

Difference between unit testing and other types of testing

There are several types of software testing methods, each having its specific role:

Integration Testing will be check that different parts of the system work together as expected.
Functional Testing checks if the software meets the pre-planned requirements.
Performance Testingwill make if the software runs smoothly, such as in the terms of speed and memory usage.
Acceptance Testingis performed manually by stakeholders to verify the software behaves as expected as we wants.
Security Testingexamines the software for vulnerabilities, including third-party risks in the Software development process.

These tests will require specialized tools and are usually performed after the main functionality is developed. Unlike these, unit tests are run every time the code is built, it can be written as soon as the code exists, and do not need special tools for the same, making unit testing one of the most fundamental testing types.

Types of Unit Testing

There are two types of Unit testing methods:

1. Manual unit testing

Manual Testing is like checking each part of a project by hand, without using any special tools. People, like developers, do each step of the testing themselves. But manual unit testing isn’t used much because there are better ways to do it and it has some problems:

It costs more because workers have to be paid for the time they spend testing, especially if they’re not permanent staff.
It takes a lot of time because tests have to be done every time the code changes.
It is hard to find and fix problems because it is tricky to test each part separately.
Developers often do manual testing themselves to see if their code works correctly.

Types of Unit Testing

2. Automated unit testing

Automation Unit Testing is a way of checking if software works correctly without needing lots of human effort. We use special tools made by people to run these tests automatically. These are part of the process of building the software. Here’s how it works:

Developers write a small piece of code to test a function in the software. This code is like a little experiment to see if everything works as it should.
Before the software is finished and sent out to users, these test codes are taken out. They’re only there to make sure everything is working properly during development.
Automated testing can help us check each part of the software on its own. This helps us find out if one part depends too much on another. It’s like putting each piece of a puzzle under a magnifying glass to see if they fit together just right.
We usually use special tools or frameworks to do this testing automatically. These tools can tell us if any of our code doesn’t pass the tests we set up.
The tests we write should be really small and focus on one specific thing at a time. They should also run on the computer’s memory and not need internet connection.

So, automated unit testing is like having a helper that checks our work as we build software, making sure everything is in its right place and works as expected.

Workflow of Unit Testing

Here is a workflow of unit testing in detail:

Workflow of Unit Testing

Unit Testing Techniques

There are 3 types of Unit Testing Techniques. They are follows:

Black Box Testing: This testing technique is used in covering the unit tests for input, user interface, and output parts.
White Box Testing: This technique is used in testing the functional behavior of the system by giving the input and checking the functionality output including the internal design structure and code of the modules.
Gray Box Testing: This technique is used in executing the relevant test cases, test methods, and test functions, and analyzing the code performance for the modules.

Unit Testing Tools

Choosing the correct unit testing tool is important for the following reasons:

Ensures that every system component is benefitted from achieving better product quality.
The debugging process is simplified.
Code Refactoring and debugging of the code becomes simpler.
If bugs are present, they would be defined earlier in the SDLC process.
Bug fixing costs would be reduced significantly

Following are some of the unit testing tools:

LambdaTest
JUnit
NUnit
TestNG
PHPUnit
Mockito
Cantata
Karma
Mocha
TypeMock.

Conclusion

Unit testing will validate individual units of software in a proper manner, checking if the function works correctly and meets the requirements of the project. While it may offer benefits such as early issue detection and improved code quality, it requires significant time and effort, which depends on the developers’ skills.

Checking the challenges, such as the difficulty in testing complex units and UI elements, unit testing is crucial for ensuring software quality and the longevity of the software.

Integration Testing - Software Engineering

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Article Tags :

Unit Testing – Software Testing

What is unit testing?

Prerequisite of Unit Testing

What is a Unit test?

Unit testing strategies

Unit test example

What are the benefits of unit testing?

Disadvantages of Unit Testing

How do developers use unit tests

When is unit testing less beneficial?

What are unit testing best practices?

Difference between unit testing and other types of testing

Types of Unit Testing

1. Manual unit testing

2. Automated unit testing

Workflow of Unit Testing

Unit Testing Techniques

Unit Testing Tools

Conclusion

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