Re: Understanding Implements

Hi Buck,

Sorry for the delay in responding. I've been knee-deep in an upgrade over
the last few days, and I thought someone else would have picked up the ball
by now.

> So "interface" is really a way round the single inheritance restriction?
We
> can define "utility" classes that aren't really a subclass of anything
else.
> Without "interface" we'd be locked in to creating artificial objects to
> contain utility classes.
>
> Closer?

Interfaces are not used to define utility classes because interfaces are
_not_ classes. Remember, an interface is simply a collection of method
_signatures_ (kind of like a procedure prototype in RPG). They describe a
standard protocol for interacting with objects that implement the interface.
However, there is no source code within the methods defined in the
interface. That code is the responsibility of whatever classes implement it.
Therefore, an interface may be created to define a certain set of behaviours
that a utility class _implements_, but the interface itself is not the
utility class.

Another way to think about it is to compare an interface to an electrical
outlet in your house. As an interface, the outlet defines a plug-shape,
voltage level, and polarity. But it doesn't supply the power! All it does is
provide a common interface for consumers of that power to access it. The
power could come from the local utility grid, your own generator, or
solar-panels. But where it comes from doesn't matter to your toaster or your
coffee-maker. The male power connector on the appliance is the means by
which the toaster "implements a" particular electrical "interface".

Hopefully, the previous example illustrates how interfaces help us to
provide more flexibility in our designs, by allowing "pluggability". They
loosen up the coupling between classes (the generator and the toaster),
which ultimately makes our design more interchangable. Wherever you add an
interface, you are in fact adding a plug-in point - a place where you can
plug in any object from any class that implements the interface.

The larger your system, the more important interfaces become.

Is it getting any clearer?


John Taylor
Canada



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