Checklist: how to introduce changes alongside refactoring

A structured set of steps to safely add new functionality while refactoring existing code.

1.  Check test coverage

Ensure that your code has a high enough level of test coverage.

2.  Evaluate test quality

Run mutation testing (e.g., using PIT) to assess the resilience and effectiveness of your tests.

3. Improve tests if necessary

Add or improve tests to:

• increase code coverage,
• strengthen resistance to mutations.

4. Organize unit tests

If the class under test contains a lot of logic:

• use JUnit’s @Nested annotation to group unit tests by functionality.

5. Rethink architecture

Check whether these @Nested test groups suggest logical separations in your production code — maybe some parts should be split into distinct classes.

6. Extract new classes

• move relevant @Nested test groups into separate test classes.
• implement a new class that allows these tests to pass.

7. Refactor and Integrate

• inject or integrate the new class into the original implementation.
• adapt the remaining tests to align with the updated architecture.

8. Add New Functionality Using TDD

Follow the test-driven development process:

• first write a failing test,
• then implement the corresponding functionality.