Interfaces

An interface is a contract: the guy writing the interface says, "hey, I accept things looking that way", and the guy using the interface says "OK, the class I write looks that way".

An interface is an empty shell, there are only the signatures (name / params / return type) of the methods. The methods do not contain anything. The interface can't do anything. It's just a pattern.Implementing an interface consumes very little CPU, because it's not a class, just a bunch of names, and therefore there is no expensive look-up to do. It's great when it matters such as in embedded devices.