Suppose you want to add some functionality to AlarmClock
for its 2.0 release.
For example, GUIClock needs to update its display every minute
so that it makes continual requests to be woken up.
Really, GUIClock
just wants AlarmClock to beep at it every minute.
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;
public long ONE_MINUTE = 60000;
}
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.
