How to Unit Test a Method that Calls a Void Method in Java

Learn how to effectively test methods calling void functions in Java using JUnit and Mockito. Follow our step-by-step guide to improve your unit testing skills.
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How to Unit Test a Method that Calls a Void Method in Java

Unit testing plays a crucial role in ensuring the reliability and integrity of your code. However, one common challenge developers face is how to effectively test methods that interact with void methods. In this guide, we will explore how to unit test a scenario where a method invokes a void method and ensure that the correct operations are performed.

Problem Overview

You have a class, MyService, that invokes a method from another service class, MyRemovalService, to set a flag in the database. The method we want to test is setFlag(), which makes use of a void method (removeData) inside a loop. A typical ask during unit testing is to verify whether the removeData method is called with the correct data, despite having a void return type.

Let’s look at our code snippet before delving into the solution:

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Setting Up Your Test

To test the setFlag method, we’ll need to prepare our test class using JUnit and Mockito. Here’s the basic structure of our test class:

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Verifying the Void Method Call

Now, onto the crux of the issue: verifying that the removeData method of MyRemovalService is invoked correctly with the intended parameters, despite its void nature. This is done using the verify method provided by Mockito.

Step-by-Step Verification

Use the verify method:
Add the following lines inside your test after calling setFlag():

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Complete Test Example

Here’s how your complete test method should look:

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Conclusion

Unit testing methods that call void functions doesn't have to be challenging. By following the structured guidelines above, you can effectively ensure that the necessary void methods are being called with the expected data. Using tools like Mockito further simplifies this process by allowing us to verify method interactions easily.

If you're looking to enhance your unit testing strategies, remember to keep practicing, explore more complex testing scenarios, and use the powerful features of libraries like JUnit and Mockito. Happy coding!