What is an Interface? How is it different from a class?

An interface in Java is a reference type that contains only abstract methods (method signatures with no body/implementation) and constants (implicitly public, static and final fields). It defines a contract that any class implementing it must fulfil by providing the actual code for every one of its methods. It is declared with the keyword 'interface' and a class uses it via the keyword 'implements'. Difference from a class: A class defines both the data (instance variables) and the actual implementation (method bodies) of its behaviour, and objects can be created directly from it using 'new'. An interface has no instance variables and no method bodies (only method headers) and cannot be instantiated on its own - it must be implemented by a class. Also, a class can extend only one superclass (single inheritance), whereas a class can implement any number of interfaces, so interfaces are used to achieve a form of multiple inheritance in Java.