Understanding and Resolving Null Pointer Exception in Mockito with WebTestClient

Discover how to solve the `Null Pointer Exception` in Mockito tests when using the WebTestClient. This guide explains the problem and provides a step-by-step solution to enhance your testing process.
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Understanding and Resolving Null Pointer Exception in Mockito with WebTestClient

When working with Java’s Mockito framework, encountering a Null Pointer Exception during testing can be frustrating. This issue often arises when the arguments passed to mocked methods do not match what the tests anticipate. In this guide, we will address a specific case related to testing a PUT request using WebTestClient, and provide a clear solution to rectify the issue.

The Problem: What’s Causing the Null Pointer Exception?

In the provided test case, we are attempting to verify a method that deals with updating a user profile. Here’s a brief look at the code snippet causing the dilemma:

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

The Error Message

The error message we encounter states:

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

This indicates that although the expected and actual arguments look identical, they are being treated as different due to reference comparison.

The Solution: Implementing the Equals Method

The core of the issue lies in how Java handles object comparison. Since ProfileDto is a custom object, Java’s default behavior checks object references instead of the actual data. This can lead to unexpected Null Pointer Exceptions during testing if the objects are not considered equal, even when they contain the same data.

Step 1: Override the Equals Method

To resolve this, we need to override the equals method in the ProfileDto class. This method should compare relevant fields to determine if two instances are equivalent:

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

Step 2: Implement the HashCode Method

It’s also a good practice to override hashCode whenever we override equals:

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

This ensures the proper functioning of collections that rely on hash codes.

Step 3: Update the Test

After implementing the equals method, your test should now work as expected. The test case will correctly match the arguments passed in the when() and verify() methods, preventing Null Pointer Exceptions due to argument mismatches.

Conclusion

By implementing a proper equals method in your data transfer objects (DTOs), you can avoid Null Pointer Exceptions in Mockito tests. This allows Mockito to correctly compare object states, ensuring that your tests are both robust and reliable.

If you continue to encounter issues, consider reviewing other aspects of your tests or consulting documentation for deeper insights. Happy coding!