What are the differences between @GenerateMocks and @GenerateNiceMocks

@GenerateMocks

- Behavior for Unstubbed Methods: When a method is called on a mock object generated with `@GenerateMocks` and no stub has been defined for that method, Mockito will throw an exception. This behavior ensures that you explicitly define stubs for all methods that are expected to be called during a test, helping to catch potential oversights in test setup.

- Usage: This annotation is typically used when you want strict control over the behavior of your mocks, ensuring that any unexpected method calls are immediately flagged as errors.

- Deprecation Status: `@GenerateMocks` is considered less recommended compared to `@GenerateNiceMocks` due to its strict behavior, which can lead to more verbose test code if not managed carefully.

@GenerateNiceMocks

- Behavior for Unstubbed Methods: In contrast, mocks generated with `@GenerateNiceMocks` will return a default, legal value for the method's return type when an unstubbed method is called. This default value is not intended for use in the test logic but serves to prevent runtime errors due to null or missing values.

- Usage: This annotation is recommended for most use cases because it simplifies test writing by reducing the need to stub every possible method call. It makes tests more robust by avoiding failures due to forgotten stubs.

- Additional Features: `@GenerateNiceMocks` only allows `MockSpec`s in its argument list, providing more flexibility in customizing the generated mock classes. For example, you can specify a custom name for the mock class using the `as` parameter in `MockSpec`.

Key Differences

1. Handling of Unstubbed Methods: The most significant difference is how each annotation handles method calls without explicit stubs. `@GenerateMocks` throws an exception, while `@GenerateNiceMocks` returns a default value.

2. Code Simplicity and Robustness: `@GenerateNiceMocks` generally leads to simpler and more robust test code by reducing the need for exhaustive stubbing.

3. Customization: `@GenerateNiceMocks` offers more customization options through `MockSpec`, allowing for custom mock class names and other advanced configurations.

In summary, `@GenerateNiceMocks` is generally preferred for its ability to simplify test code and reduce errors due to unstubbed methods, while `@GenerateMocks` provides a stricter approach that may be useful in specific scenarios where explicit control over mock behavior is required[1][4][11].