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Trail: Learning the Java Language
Lesson: Classes, Interfaces, and Packages

## Warning! Interfaces Cannot Grow

> Suppose you want to add some functionality to `AlarmClock` for its 2.0 release. For example, `GUIClock` needs to update its display every second so that it makes continual requests to be woken up. Really, `GUIClock` just wants `AlarmClock` to beep at it every second. It would be preferable to register a single request with `AlarmClock`, for two reasons:
> - It's more efficient because it involves fewer method calls.
> - `GUIClock` wouldn't have to maintain its own `AlarmClock` because it wouldn't risk losing its bed.
> `AlarmClock` needs to differentiate such "beeps" from an actual wake up call. So now the `Sleeper` interface must include a `beep` method:

> >     public interface Sleeper {
> >         public void wakeUp();
> >         public void beep();

> >         public long ONE_SECOND = 1000;
> >     }

> However, if you make this change to `Sleeper`, all classes that implement the old `Sleeper` will break because they don't implement the interface anymore! Programmers relying on this interface will protest loudly.

> Try to anticipate all uses for your interface up front and specify it completely from the beginning. Given that this is often impossible, you may need either to create more interfaces later or to break your customer's code.

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