How to Mock Default Methods in Interface Using JUnit and Mockito

Discover effective techniques to mock default methods in interfaces with JUnit and Mockito for seamless unit testing in Java.
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How to Mock Default Methods in Interface Using JUnit and Mockito

Unit testing can be a challenging aspect of software development, especially when dealing with default methods in interfaces. In Java, interfaces can contain default methods, which can lead to complications when you want to mock them for testing purposes. This guide aims to walk you through the problem and provide a comprehensive solution using JUnit and Mockito.

Understanding the Problem

Let's consider a scenario where you have an IMap interface with a default method fromSourceList. This interface is implemented by a class called Mapping. When you attempt to unit test another class, Class, that interacts directly with this interface, you may face challenges stubbing the default methods using Mockito.

Example Structure

Here's a simplified structure referencing the original question:

[[See Video to Reveal this Text or Code Snippet]]

When you attempt to stub or mock the fromSourceList method, you might try using:

However, these methods may not yield the expected results.

Solution Overview

To effectively mock default methods in interfaces, we can modify how we inject dependencies into the class we are testing.

Steps to Mock Default Methods

Change the Dependency Injection: Instead of creating an instance of Mapping in Class, pass it through the constructor.

Use Mockito to Mock: Leverage Mockito's mocking capabilities to inject a mock of IMap into the class under test.

Setup the Test Case: Write a test case that stubs the fromSourceList method appropriately.

Code Implementation

Here’s how you can implement this:

Step 1: Modify the Class

Change the Class definition to accept an IMap instance via the constructor:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Create the Test Class

Implement your test using Mockito to mock the IMap interface:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By following this method, you can effectively mock default methods in interfaces using JUnit and Mockito. This approach helps ensure that your unit tests are not only clean and maintainable but also effective in validating your business logic without dependencies interfering.

If you've struggled with mocking default methods before, give this method a try in your unit tests, and see how it simplifies your testing process!