Legacy Code Testing Skill (Java)

Battle-tested techniques for introducing tests into legacy Java code, based on Michael Feathers' "Working Effectively with Legacy Code." All examples use JUnit 4/5 and Mockito.

Core Philosophy

Legacy code = code without tests. The methodology is Cover and Modify — the opposite of "Edit and Pray."

• Edit and Pray: Change the code carefully, test manually, hope nothing breaks. Slow, risky, fear-driven.
• Cover and Modify: Build a safety net of tests first ("the Vise"), then change with confidence.

The golden rule: Cover → Modify → Refactor.

The Legacy Code Change Algorithm

Before any modification, follow these 5 steps: