What is Unit Testing? Definition, Benefits & Process

Relia Software

Relia Software

Quang Duc

Relia Software


Unit testing is a software testing strategy that tests individual code parts. Developers can use this test to find issues early in the development process.

what is unit testing

Table of Contents

If you're a software engineer, you've probably heard the term "unit testing" at some time in your career. These tests help to ensure that the code is functioning properly and discover any mistakes or flaws. But what precisely is it, and how does it function? 

In this blog post, we'll go over the significance of this type of testing, how it works, the benefits it provides, and the basic steps for using it. Whether you're new to testing or want to improve your understanding of this vital topic, this piece has something for you. So, let's get started!

What is Unit Testing?

Unit testing is a core software development method that isolates and tests individual code pieces. But what constitutes a "unit" in this context? In software development, a unit can refer to various granular components that execute certain functions. This could be a function, a method within a class, a module, or any other well-defined code that can be tested logically and independently.

By isolating and testing these units independently, developers may validate their operation and uncover problems or unusual behavior early in the software development process. This proactive approach contributes to the overall quality and reliability of the program being created.

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Why is Unit Testing Important? 

Improved Code Quality and Fewer Bugs

Unit tests carefully check code units for mistakes and inconsistencies. Unit testing finds and fixes these flaws early on, preventing them from spreading between development phases. This proactive approach produces cleaner, more reliable code.

Saved Time and Resources

Instead of spending hours debugging a complex issue that could have been discovered early, apply unit testing to reduce time significantly. Early identification frees up time and resources for developers to add additional features and functions.

Catching Errors Early

Unit testing helps developers spot and fix bugs before they become bigger difficulties. Early intervention saves time and energy and avoids integration and deployment issues.

Increasing Confidence and Maintainability

Well-written unit tests operate as living code documentation, increasing developer confidence and maintainability. They clarify each unit's behavior and functions, enhancing developer confidence in the codebase. These tests provide a reference point for future code modifications, making them easier and less error-prone.

unit testing importance
Unit Testing Importance (Source: Internet)

Basic Process of Unit Testing

Unit testing may seem complicated, however, it may be simplified into a few steps:

Step 1: Unit Identification

Selecting the code unit to test is the first step. This could be a function, class method, module, or other well-defined code with unambiguous functionality. Remember to test these units separately from the codebase.

Step 2: Test Case Design

You must write test cases to examine the unit's functionality completely. These test cases should cover several scenarios, such as:

  • Positive test cases check normal unit behavior. A positive test case for a rectangle area function would provide proper dimensions and ensure the method delivers the correct area.

  • Negative test cases examine how the unit handles invalid or unexpected input. A negative test case for the area calculation function might use negative dimensions and check for an error message.

Step 3: Write Unit Tests

After creating test cases, write code. Unit testing frameworks in most programming languages include tools and functions for writing and running unit tests. These frameworks simplify and standardize testing.

Step 4: Test and Assess

Now it's time to conduct tests and analyze results. It will be good when the tested unit is working properly. Failure of any tests indicates a unit problem. Therefore, unit testing is great because you can find the problem and correct it before it affects other code. 

Step 5: Refactor and Repeat

Software development is iterative. As your code changes, you may need to refactor the tested unit. In such circumstances, you must review your unit tests to ensure they still reflect the unit's functionality.  


Now that we've reviewed the unit testing process, let's contextualize it with a real-world application. Consider a simple function that calculates the area of a rectangle using its length and width as inputs. Here's how we could do unit testing for this function:

Identify the unit: In this case, we use the [calculate_area] function to calculate the area of a rectangle depending on its length and breadth.

Develop test cases:

Positive Test Case 1:

  • Input: Length = 5; Width = 3 (valid dimensions).

  • Expected Output: Area = 15 (length x width)

Positive Test Case 2:

  • Input: Length = 10; Width = 0 (valid dimensions with zero width)

  • Expected outputE: Area = 0 (0 x Width).

Negative Test Case:

  • Input: Length = -2 and Width = 5 (negative length)

  • Expected output: Error message indicating invalid input (function should not take negative dimensions).

Write Unit Tests: 

# Assuming Python and the `unittest` framework

def test_calculate_area_positive_1(self):
  length = 5
  width = 3
  expected_area = length * width
  actual_area = calculate_area(length, width)
  self.assertEqual(actual_area, expected_area)

# Similarly, write test cases for other scenarios

Test and Assess: Verify that all unit tests pass, confirming that the calculate_area function works. If a test fails, examine the error message to find and fix the function issue.

Refactor and Repeat: Remember, unit testing is ongoing. If you add input value validations to the [calculate_area] function, check your unit tests to make sure they still work.

Techniques of Unit Tests 

Structural Unit Testing

Structural testing is a white-box testing approach in which a developer creates test cases based on the internal structure of the code. The strategy entails determining all feasible paths via the code. The tester selects test case inputs, runs them, and decides the correct output. 

Primary structural testing methods include:

  • Statement, branch, and path testing—every program statement, branch, or path is tested. 

  • Conditional testing lets developers run code based on value comparisons to decide a test's path.

  • Expression testing compares the application to different regular expression values.

unit testing structural
Structural Unit Testing (Source: Internet)

Functional Unit Testing

Functional unit testing is a black-box testing method for determining the functionality of an application unit. The primary functional approaches include:

  • The size and kind of input objects are tested and compared to equivalence classes.

  • Boundary value analysis—tests software's response to inputs beyond boundary values.

  • Syntax checking—tests that verify software syntax interpretation.

  • Equivalent partitioning divides software unit input data into partitions and applies test cases to each segment.

Error-based Techniques

Error-based unit tests should ideally be written by the software developers who created the code. They include:

  • Fault seeding—coding known problems and testing till found.

  • Mutation testing involves modifying source code statements to determine if the test code finds errors. Mutation testing is costly, especially in large applications.

  • Historical test data—calculates test case priority using previous executions.

Unit Testing Practices

Test-driven Development

Test Driven Development (TDD) requires programmers to build tests before production code and then write code to pass them. With a little confidence from that initial test, the programmer can rework and refactor to build the cleanest code possible. As with most basic ideas, execution is difficult. TDD needs a distinct mindset and the tenacity to persevere through a slow learning curve.

Checking Your Work

TDD isn't new, but it's largely for developers. The rest are testing and reviewing their work. Although writing unit tests after production code is more traditional, it is still useful. If you've taken maths in the past decade, you know it.

After checking your work and confirming that the code is working as expected, the unit tests change. Simple tests that may be performed with every product build detect unexpected code changes.

Documenting Code

Code documentation is a hassle, as shown by how little is written. Unit testing encourages better development techniques and leaves behind code that describes your product's behavior, making documentation easier. Instead of feeding the documentation with code changes, you'll update a working set of checks.

unit testing practices
Best Practices of Unit Testing (Source: Internet)

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So, in conclusion, what is unit testing? Unit testing is a core software development approach essential to software development and ensures code quality and dependability. You may identify defects early, enhance code maintainability, and gain confidence by writing and running small, isolated tests for each code unit.

To maximize this testing, build tests that cover a wide range of situations and edge cases and run and update them as you make code changes.

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