What is the difference between interface and abstract class in depth?

Interfaces define contracts, while abstract classes provide partial implementation and shared state.

Interfaces enforce a pure behavioral contract with no state (in most languages), while abstract classes allow shared fields, constructors, and partial method implementations. Abstract classes are used when there is a strong is-a relationship and shared logic, whereas interfaces are used for capability-based design. Overuse of abstract classes can lead to rigid hierarchies, while interfaces promote flexibility and multiple inheritance of behavior.