Mockito is a popular open-source Java library used for creating and managing mock objects in unit testing. Mock objects are simulated objects that mimic the behavior of real objects, allowing you to isolate the code you’re testing and verify interactions with dependencies. Mockito simplifies the process of creating and working with mock objects in Java unit tests.

Here are the basic concepts and usage of Mockito in Java:

1. Installation and Setup:

   • To use Mockito in your Java project, you need to include the Mockito library in your project’s dependencies. You can do this using build tools like Maven or Gradle.

2. Creating Mocks:

   • Mockito allows you to create mock objects for classes and interfaces. You can create a mock object using the Mockito.mock() method. For example:
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     import org.mockito.Mockito; // Create a mock for an interface MyInterface mockObject = Mockito.mock(MyInterface.class);

3. Stubbing Behavior:

   • You can specify the behavior of mock objects using the when(...).thenReturn(...) syntax. This allows you to define what should happen when specific methods of the mock object are called. For example:
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     Mockito.when(mockObject.methodToStub()).thenReturn(expectedResult);

4. Verifying Interactions:

   • Mockito provides methods to verify that specific interactions occurred with your mock objects. You can use verify(...) to ensure that certain methods were called with specific arguments and a specific number of times. For example:
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     Mockito.verify(mockObject).methodToVerify(argument);

5. Matchers:

   • Mockito provides matchers like any(), eq(), isNull(), and others to make it easier to define behavior and verify interactions with your mock objects.

6. Annotations:

   • Mockito can be used with annotations like @Mock, @InjectMocks, and @Spy to simplify the creation and management of mock objects in your test classes.

Here’s a simple example of using Mockito in a JUnit test class:

In this example, we use Mockito to create a mock object for MyDependency, stub its behavior, and verify that the interaction occurred as expected.

Mockito is a powerful tool for writing clean and effective unit tests in Java. It helps you isolate the code you’re testing from its dependencies, making your tests more focused and reliable. It’s widely used in Java development, especially in combination with testing frameworks like JUnit.