• Nested classes and interfaces

An example interface

• Java has single inheritance of implementation - you can extend only one class.
• Java has multiple interface inheritance.
• All methods in an interface are implicitly abstract.
• Each class that implements the interface must implement all its methods.
• Methods in an interface are always public.
• Fields in an interface are always static and final.

Nested classes and interfaces

• Nested classes and interfaces let you associate types that are strongly related to an interface inside that interface.
• Any class or interface inside an interface is public.
• Any classes nested inside an interface are also static.
• Any interface nested inside a class can be public, protected, package-accessible, or private.