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How to Mock a Loop in Unit Tests with Mockito

Learn how to efficiently mock a map in your Java unit tests with Mockito to resolve issues related to loops during testing. --- This video is based on the question https://stackoverflow.com/q/70000103/ asked by the user 'Fedor Nikolaev' ( https://stackoverflow.com/u/17262444/ ) and on the answer https://stackoverflow.com/a/70000299/ provided by the user 'Melchizedek' ( https://stackoverflow.com/u/5670909/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Try to make mock test, but there is a loop Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- How to Mock a Loop in Unit Tests with Mockito When writing unit tests, especially in a Java environment using frameworks like Spring Boot, you may encounter situations where your methods involve loops that interact with collections such as maps. This can lead to challenges when trying to mock certain behaviors. In this guide, we will tackle a common problem where a for loop within a service method needs proper mocking to ensure tests run correctly. Let's dive in! The Problem at Hand In the provided example, the method checkAvailable() in the CartRecordService class iterates through a map of records and checks if there is enough quantity of items available. If you're trying to test this method but facing issues due to the loop not triggering interactions with mocks, you're not alone. The log indicates that the expected method call was never invoked, leading to potential test failures. The problematic area looks like this: [[See Video to Reveal this Text or Code Snippet]] When running the test, you might observe an error message indicating that goodsService.enoughQuantity() was not called at all. This typically happens because the method getRecords() is not returning the mocked data as anticipated. The Solution: Mocking the Loop Effectively To address this issue, we need to ensure that the getRecords() method of the CartRecord mock is returning the intended data before the loop executes. Here’s how you can do it: Step-by-Step Implementation Setup the Mock Behavior: In your test method, explicitly mock the output of the getRecords() method to return the desired map. Use Mockito to Define Behavior: [[See Video to Reveal this Text or Code Snippet]] Run Your Tests: With the mocked data in place, execute your tests. This setup allows your checkAvailable() method to iterate over the mocked records successfully. Example Mock Test Implementation Here’s how your complete test might look after implementing the above solution: [[See Video to Reveal this Text or Code Snippet]] Key Points to Remember Mocking Dependencies: Always ensure that any dependencies you need in your tests are correctly mocked to return expected values. Verify Interactions: Use Mockito.verify() to check that the mocked methods were called the expected number of times. Conclusion Mocking in Java unit tests, especially with loops and collections, can be tricky. However, with the right approach using Mockito, you can handle these scenarios effectively. By ensuring that mock returns are set up correctly, as shown in the example, you can prevent issues arising from uninvoked methods during testing. This not only enhances the reliability of your tests but also streamlines the debugging process when things go awry. Happy coding!