The Differences Between Unit Testing, Integration Testing and Functional Testing

Unit testing, integration testing, and functional testing are all vital for software applications since each employs a separate method to evaluate the application.

Nevertheless, no matter the circumstance, functional testing can’t be supplanted by unit testing or integration testing .

Unit Testing Vs Integration Testing Vs Functional Testing

Unit testing involves evaluating individual components of an application independently, without any interactions with dependencies, to confirm that the code is operating correctly.

On the flip side, integration testing confirms if various components function as expected when unified as a group.

Functional testing emphasizes testing a particular functionality within the system, which might interact with dependencies, to ensure that the code is completing intended operations.

Functional tests are aligned with integration tests, however they specifically test the full application’s functionality with all the code running simultaneously, alike to a comprehensive integration test.

Unit testing inspects a single unit of the system, while functional testing evaluates the operation of an application against the projected functionality described in the system requirement specification. On the other hand, integration testing checks the intercommunication of combined modules in the system.

Obtaining the highest return on investment is crucial by having the maximum unit tests possible, fewer integration tests, and the least quantity of functional tests for your codebase.

Test Pyramid:

Unit tests are simpler and swifter to write and execute compared to integration and functional tests. However, the time and effort needed for implementing and maintaining the tests amplify from unit testing to functional testing, as visualized in the pyramid above.

Example:

In order to understand these three types of testing, let’s use a comprehendible example of a functional mobile phone that requires a “battery” and a “SIM card”.

In unit testing, the battery is inspected for its usage, capacity, and other specifications, while the SIM card is evaluated for its activation.

During integration testing, the battery and the SIM card are merged or assembled to initialize the mobile phone.

In functional testing, the operation of the mobile phone is examined in terms of its characteristics, battery consumption, and SIM card functionalities.

This exemplification provides a basic understanding of unit, integration, and functional testing. Now, let’s explore a technical example of a login page:

Each web application demands users/customers to log in. The “Login” page of an application possesses the following elements:

• Account/Username
• Password
• Login/Sign in Button

For unit testing, the test cases might include field length for username and password fields, valid input field values, and activation of the login button only after valid values are inserted into both fields.

For integration testing, the test cases might include confirmation if the user views a welcome note after inserting valid values and clicking the login button, and if the user is successfully redirected to the welcome page or home page upon logging in.

Functional testing includes additional test cases such as inspecting the projected behaviour of the login function, the presence of a welcome message after a successful login, error messages for invalid logins, stored site cookies for login fields, login for inactivated users, and the existence of a “forgot password” link for users who have lost their passwords.

There are many more scenarios that a functional tester considers while conducting functional testing, but a developer cannot include all of them in unit and integration test cases.

Now, let’s delve into unit testing, integration testing, and functional testing individually.

What is Unit Testing?

Unit testing involves testing a “unit,” which can be the smallest part of an application that is possible to test, for instance an individual function or method. Software developers curate unit test cases to ensure that the unit satisfies the requirements and operates as projected.

Important points about unit testing:

• Unit testing is carried out by software developers using white box testing techniques, prior to integration testing.
• Unit tests inspect both the positive behavior, i.e., accurate output for valid input, and errors that occur with invalid input.
• Identifying problems/bugs at an early stage diminishes project costs. Unit testing enables developers to rectify issues effortlessly and with insignificant impact.
• Unit tests concentrate on small parts of code or individual operations, so the problems/errors found are isolated and do not influence other test cases.
• Unit tests simplify testing and make it easier to resolve problems at a later stage, as only the most recent changes need to be tested.
• Unit tests save time, cost, and effort, and are reusable and easy to maintain.

Noted unit testing tools include JUnit (Java framework), PHPUnit (PHP framework), NUnit (.Net framework), etc.

What is Integration Testing?

Integration testing involves testing the combination of different parts/modules of a system. It examines how effectively the units/modules operate collectively when combined.

Integration testing is performed on individual units/modules that passed unit testing, and it determines whether the aggregation of units/modules produces the expected output.

Integration testing can be conducted by independent testers or developers.

There are three separate types of integration testing approaches:

a) Big Bang Integration Approach

In this approach, all the units or components are integrated and tested concurrently, typically when the complete system is ready for integration testing. Only the combination of units or components is tested, but not the entire system.

The advantage of the big bang approach is that everything incorporated is tested simultaneously, however it becomes challenging to identify malfunctions.

Example: In the figure below, Unit 1 to Unit 6 are integrated and tested using the Big bang approach.

b) Top-Down Approach

In this approach, integration is tested from the top levels to the bottom ones, step by step. Each unit is tested individually, and then the lower levels are integrated one by one until the final level is tested.

The top-down approach is a natural method of integrating modules, as it aligns with how things occur in real situations. However, the major concern with this approach is that major functionality is tested at the end.

c) Bottom-Up Approach

In the bottom-up approach, units/modules are tested from the bottom to the top level, step by step, until all levels of units/modules are fused and tested as a single entity. This approach uses stimulator programs referred to as drivers to assist integration testing. It is simpler to identify problems or errors at the lower levels.

The primary drawback of the bottom-up approach is that upper-level issues can only be identified when all units are integrated.

Unit Testing vs Integration Testing

Let’s examine the differences between unit testing and integration testing:

Functional Testing

Functional testing is a black box testing technique that tests the operation of an application to yield the desired output for particular inputs.

Test cases are grounded on requirements and scenarios, and can range from one to many for each operation.

Functional testing includes test summary, prerequisites (if any), test case input steps, test data (if necessary), predicted output, and notes (if necessary).

There are two categories of functional testing: requirement-based and business scenario-based. Requirement-based testing focuses on testing per the requirements, while business scenario-based testing focuses on testing from a business perspective.

The disadvantage of functional testing is the possible redundancy in testing and the potential to overlook logical mistakes.

Exact Difference

Here are some of the significant distinctions between unit testing, integration testing, and functional testing:

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Unit testing Integration testing Functional testing Definition and purpose Tests individual parts/modules one by one Tests amalgamation of two or more units/modules combined Tests application behavior per requirements Complexity Not complicated Somewhat more intricate than unit testing More advanced contrasted to unit and integration testing Testing techniques White box testing Combination of white box and black box testing Black box testing Main focus Individual modules/units Integration of modules/units Entire application operation Covered errors/issues Module-specific problems Integration-specific problems Operational issues and scenarios

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