How to distinguish between unit testing and integration testing, and what are their key differences?

Unit testing checks individual, isolated parts of the code — usually individual methods or classes — without interaction with external components. The goal is to ensure that each part works correctly on its own.

Integration testing checks the interaction between multiple modules or system components to ensure they work together correctly.

Key differences:

• Object of testing: unit — a single module, integration — multiple modules together.
• Isolation: unit tests are isolated, integration tests are not.
• Purpose: unit tests identify errors within a module, integration tests identify interaction errors.

Example:

• A unit test checks a method that calculates a sum.
• An integration test checks that the service calling this method correctly handles the result.