An abstract class can have both abstract and concrete methods, while an interface can only have abstract methods (until Java 8, which allows default and static methods). Abstract classes allow you to define fields whereas interfaces cannot have instance fields. A class can implement multiple interfaces but can only extend one abstract class.

I use Git for version control. I regularly commit changes with meaningful messages, create branches for new features or bug fixes, and perform code reviews before merging code into the main branch. Additionally, I use tools like GitHub or GitLab to collaborate with team members.

A RESTful API adheres to REST (Representational State Transfer) principles, using stateless communication and standard HTTP methods like GET, POST, PUT, DELETE. To design a RESTful API, clearly define resource URIs, use appropriate HTTP methods, handle errors gracefully, and document the endpoints. Each resource should have a unique identifier and URLs should be designed to be both human and machine-readable.

My approach to debugging involves first reproducing the issue, then using print statements or a debugger to trace the problem’s origin. I look at logs for clues and divide and conquer by isolating sections of the code to identify where the issue lies. Once identified, I test potential solutions until the issue is resolved, then write tests to ensure it doesn’t recur.

I ensure code quality by following best practices such as writing clean, maintainable code, adhering to style guides, and conducting thorough code reviews. I also write unit tests, integration tests, and make use of automated testing frameworks. Additionally, I use static code analysis tools to catch potential issues early.

I worked on a challenging project where we had to refactor a legacy codebase with little documentation. I started by writing integration tests to understand the existing functionality. Then, I gradually refactored small parts of the code while continually running tests to ensure nothing broke. We held regular team discussions to tackle difficult sections and documented the changes thoroughly to aid future maintenance.