Interfaces, Anonymous Classes & Enums

An interface is a contract: a list of methods a class promises to provide.

interface Shape { double area(); double perimeter(); } class Square implements Shape { double side; public double area() { return side * side; } public double perimeter() { return 4 * side; } }

A class can extend only one class, but implement many interfaces: class Duck extends Animal implements Flyer, Swimmer. Code can depend on the interface (Shape) rather than a specific class.