The Differences Between Unit Testing, Integration Testing and Functional Testing

In this article, we have provided an extensive comparison of Unit, Integration and Functional Testing

For any software application, both Unit testing and Integration testing are very important as each of them employs a unique process to test a software application.

However, either one or even both cannot replace Functional testing at any point. Let’s look at the comparison in detail.

Unit Testing Vs Integration Testing Vs Functional Testing

Unit testing means testing individual modules of an application in isolation (without any interaction with dependencies) to confirm that the code is doing things right.

Integration testing means checking if different modules are working fine when combined together as a group.

Functional testing means testing a slice of functionality in the system (may interact with dependencies) to confirm that the code is doing the right thing.

Functional tests are related to integration tests, however, they signify to the tests that check the entire application’s functionality with all the code running together, nearly a super integration test.

Unit testing considers checking a single component of the system whereas functionality testing considers checking the working of an application against the intended functionality described in the system requirement specification. On the other hand, integration testing considers checking integrated modules in the system.

And, most importantly, to optimize the return on investment (ROI), your code base should have as many unit tests as possible, fewer integration tests and the least number of functional tests.

This is illustrated best in the following test pyramid:

Unit tests are easier to write and quicker to execute. The time and effort to implement and maintain the tests increases from unit testing to functional testing as shown in the above pyramid.

Example: 

Let us understand these three types of testing with an oversimplified example.

E.g. For a functional mobile phone, the main parts required are “battery” and “sim card”.

Unit testing Example – The battery is checked for its life, capacity and other parameters. The sim card is checked for its activation.

Integration Testing Example – Battery and sim cards are integrated i.e. assembled in order to start the mobile phone.

Functional Testing Example – The functionality of a mobile phone is checked in terms of its features and battery usage as well as sim card facilities.

We have seen an example in layman’s terms.

Now, let us now take a technical example of a login page:

Almost every web application requires its users/customers to log in. For that, every application has to have a “Login” page which has these elements:

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

For Unit Testing, the following may be the test cases:

• Field length – username and password fields.
• Input field values should be valid.
• The login button is enabled only after valid values (Format and lengthwise) are entered in both the fields.

For Integration Testing, the following may be the test cases:

• The user sees the welcome message after entering valid values and pushing the login button.
• The user should navigate to the welcome page or home page after a valid entry and click the Login button.

Now, after unit and integration testing is done, let us see the additional test cases that are considered for functional testing:

1. The expected behavior is checked, i.e. is the user able to log in by clicking the login button after entering a valid username and password values.
2. Is there a welcome message that is to appear after a successful login?
3. Is there an error message that should appear on an invalid login?
4. Are there any stored site cookies for the login fields?
5. Can a deactivated user log in?
6. Is there a “forgot password” link for users who have forgotten their passwords?

There are a number of such cases which come to the mind of a functional tester while performing functional testing. However, the developer cannot take up all cases while building the Unit and Integration test cases.

Thus, there are plenty of scenarios that are yet to be tested even after unit and integration testing.

It is now time to examine Unit, Integration and Functional testing one by one.

What is Unit Testing

As the name suggests, this level involves testing a unit.

Here the unit can be the smallest part of an application that is testable, be it the smallest individual function, method, etc. Software developers are the ones who write the unit test cases. The goal here is to match the requirements and the unit’s expected behavior.

Given below are a few key points about unit testing and its benefits:

• Unit testing is done before Integration testing by software developers using white box testing techniques.
• Unit testing not only checks positive behavior i.e. the correct output in case of valid input, but also the failures that occur with invalid input.
• Finding issues/bugs at an early stage is very useful and it reduces the overall project costs. As Unit testing is done before integration of code, issues found at this stage can be resolved very easily and their impact is also very low.
• A unit test tests small pieces of code or individual functions so the issues/errors found in these test cases are independent and do not impact other test cases.
• Another important advantage is that the unit test cases simplify and make testing of code easier. So, it becomes easier to resolve the issues at a later stage too as only the latest change in the code is to be tested.
• Unit testing saves time and cost, and is reusable and easy to maintain.

JUnit (Java framework), PHPUnit (PHP framework), NUnit (.Net framework) etc. are popular unit testing tools that are used in different languages.

What is Integration Testing

Integration testing involves testing the integration of different parts of the system together. Two different parts or modules of the system are first integrated and then integration testing is performed.

The aim of integration testing is to check the functionality, reliability, and performance of the system when integrated.

Integration testing is performed on the modules that are unit tested first and then integration testing defines whether the combination of the modules gives the desired output or not.

Integration testing can either be done by independent testers or by developers too.

There are 3 different types of Integration testing approaches. Let’s discuss each one of them briefly:

a) Big Bang Integration Approach

In this approach, all the modules or units are integrated and tested as a whole at one time. This is usually done when the entire system is ready for integration testing at a single point in time.

Please do not confuse this approach of integration testing with system testing, only the integration of modules or units is tested and not the whole system as it is done in system testing.

The Big Bang integration approach’s major advantage is that everything integrated is tested at one time.

One major disadvantage is that it becomes difficult to identify the failures.

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

b) Top-Down Approach

Integration of units/modules is tested from the top to bottom levels step by step.

The first unit is tested individually by writing test STUBS. After this, the lower levels are integrated one by one until the last level is put together and tested.

The top-down approach is a very organic way of integrating as it is consistent with how things happen in the real environment.

The only concern with this approach is that the major functionality is tested at the end.

c) Bottom-Up Approach

Units/modules are tested from bottom to top level, step by step, until all levels of units/modules are integrated and tested as one unit. Stimulator programs called DRIVERS are used in this approach. It is easier to detect issues or errors at the lower levels.

The major disadvantage of this approach is that the higher-level issues can only be identified at the end when all the units have been integrated.

Unit Testing vs Integration Testing

Having had enough discussion about unit testing and integration testing, let us quickly go through the differences between the two in the following table:

Functional Testing

A black box testing technique, where the functionality of the application is tested to generate the desired output on providing a certain input is called “Functional testing”.

In our software testing process, we do this by writing test cases as per the requirements and scenarios. For any functionality, the number of test cases written can vary from one to many.

Test cases are basically comprised of the following parts:

• Test Summary
• Prerequisites (if any)
• Test case input steps
• Test data (if any)
• Expected output
• Notes (if any)

“Requirement-Based” and “Business scenario-based” are the two forms of functional testing that are carried out.

For Requirement based testing, test cases are created as per the requirement and tested accordingly. In a business scenario based functional testing, testing is performed by keeping in mind all the scenarios from a business perspective.

However, the major disadvantage of functional testing is the probable redundancy in testing and the possibility of missing some logical errors.

Exact Difference

Let’s look at their differences.

Here are some of the major ones:

Also read => What is Feature Testing?

Conclusion

All three testing types are correlated. To attain full coverage, it is required to have unit tests for the paths/lines of code, functional and Integration tests for assurance that the ‘units’ work together cohesively.

Hope this article has given you a clear idea of Unit, Integration and Functional testing along with their differences, though there is much more to these forms of testing!! Don’t forget to provide your feedback/queries in the comments section below. We would love to hear from you.