Write unit tests with JUnit and create mock objects with Mockito.
Automated testing is a critical practice for ensuring software quality and maintainability. Unit testing involves testing the smallest, isolated parts of your application (the 'units', typically individual methods or classes) to verify they behave as expected. JUnit is the most widely used testing framework for Java. It provides a test runner and a rich set of annotations and assertion methods to help you write and organize your tests. You create a separate test class for the class you want to test. Test methods are annotated with `@Test`. Inside a test method, you set up a scenario, call the method you are testing, and then use assertion methods (like `assertEquals()`, `assertTrue()`, `assertNotNull()`) to check if the actual outcome matches the expected outcome. Often, the class you want to test has dependencies on other classes (e.g., a service class that depends on a repository class). To test the service class in isolation, you don't want to use a real repository that connects to a database. This is where mocking comes in. Mockito is a popular mocking framework that allows you to create 'mock' or 'fake' objects that simulate the behavior of real dependencies. You can tell your mock repository what to return when a specific method is called on it. This allows you to test your service class's logic without any external dependencies, making your tests faster, more reliable, and truly focused on the unit under test.