2.3 interface

MethodReturn Id (Type Id ,...) ; 

The signature specifies a method named Id, expecting the listed arguments. All classes implementing the (scheme interface) must contain a method with the same name, return type, and argument types. A method that does not return a value uses the void designation instead of a Type.

abstract MethodReturn Id (Type Id ,...) ; 

A signature may be declared abstract. This does not impact the method behavior; all signatures are by default abstract.