Summary

Conclusions

An abstract class combines shared state with extension points, an interface defines a role, and nested classes localize helper implementations. Reliability depends on a clear contract: comparison order, iteration completion, and the independence of the completed result.

Self-check questions

1. Why does an abstract class need a constructor?
2. Which interface members are implicitly public?
3. How does default differ from static?
4. How do you resolve a conflict between two default methods?
5. When do you need an abstract class instead of an interface?
6. Why should you avoid implementing compareTo with subtraction?
7. What should next do after iteration ends?
8. What reference does an inner class have that a static nested class does not?
9. What does effectively final mean?
10. How does an interface let you replace a strategy?