Which testing method focuses on applying invalid or unexpected inputs to validate responses?

The testing method that focuses on applying invalid or unexpected inputs to validate responses is negative testing. This approach is critical in identifying how applications respond to incorrect data, whether it be through invalid formats, values that fall outside expected ranges, or any data designed to push the boundaries of the system's handling capabilities.

By intentionally introducing such inputs, negative testing assesses the application's resilience and error-handling capability. The results can highlight potential vulnerabilities, allowing developers to fortify the application against malicious attempts that may exploit these weaknesses. Ultimately, negative testing ensures that the application behaves as expected under abnormal conditions and reinforces the importance of robust error management strategies.

In contrast, other testing types like regression testing focus on verifying that existing functionalities work correctly after changes, positive testing checks if the application behaves correctly with valid inputs, and functional testing ensures that the system's functionalities adhere to defined requirements. Each has its role, but negative testing is uniquely aimed at assessing an application's weak points through the use of unexpected data.