Abstraction & Interfaces

Interface provides a contract with multiple inheritance and no state. Abstract class provides partial implementation with single inheritance and can have state and constructors. Prefer interface unless you need shared mutable fields.

Yes: an abstract class can extend one class (abstract or concrete) and implement multiple interfaces simultaneously. It can leave interface methods unimplemented for concrete subclasses. The concrete child at the bottom must implement all remaining abstract methods.

new AbstractClass() is a compile error. new AbstractClass() { implement abstract methods here } creates an anonymous subclass. Lambda replaced anonymous for single-method interfaces. Anonymous is still needed for abstract classes and multi-method interfaces.

Interface for capability contracts (multiple inheritance, no state). Abstract class for family hierarchies (shared state, constructors). Java 8 default methods help but still no constructors, no instance fields, no protected methods.