How to Effectively Test Methods in Java That Call Other Methods Using Mockito

Learn how to test Java methods that call other methods without encountering null pointer exceptions. Use Mockito to streamline your unit testing for service layers effectively.
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Introduction: The Testing Challenge

In the world of software development, particularly when working with Java and frameworks like Spring, unit testing can sometimes feel overwhelming. A common problem many face is how to effectively test a method that depends on another method, especially when the latter retrieves data from an external source like a database. This issue often leads to frustrating errors, such as null pointer exceptions.

In this post, we will explore a real-world scenario where a method in a service layer is attempting to sort barcodes based on price but runs into a null pointer exception when calling another service to retrieve the necessary data. We’ll learn how to set up an effective test using Mockito, enabling us to mock dependencies and isolate our tests.

The Method We Want to Test

The method in question is designed to sort a list of barcodes based on their total price. Here’s a simplified version of it:

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

Breakdown of the Method:

Input: Takes a set of barcodes by book type.

Processing:

Calls calculatePriceByBarcode() from priceOperations to get the price for each barcode.

Combines barcode and their corresponding price into a new string format (e.g., "ABC/10").

Sorting: Sorts the combined results in ascending order based on price.

The Issue

Testing the Method with Mockito

To effectively test the sortBarcodesByTotalPrice method without hitting a null pointer exception, we can leverage the Mockito framework to create mock objects of our dependencies, allowing us to control their behavior during tests.

Here’s how we can rewrite our test:

Step 1: Set Up Your Test

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

Key Points to Note:

Defining Mock Behavior: The when(...).thenReturn(...) pairs set up how the mock will behave. We specify what it should return for each specific input (barcode).

Dependency Injection: By passing the mocked priceOperations to the BookService constructor, we ensure that our method can call the mock without running into null pointer issues.

Assertions: Finally, we verify that the output is as expected based on our defined behavior.

Conclusion

Testing methods that rely on other methods can be tricky, but with the right tools like Mockito, it becomes manageable. By mocking dependencies and injecting them into the classes you are testing, you create a controlled environment for your tests. This process not only helps you avoid common pitfalls like null pointer exceptions but also ensures that your tests are precise and focused.

By understanding these techniques, you can enhance your unit testing skills in Java, leading to more robust and maintainable code. Happy coding!