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Suppose you want to add some functionality toAlarmClock
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 wantsAlarmClock
to beep at it every second. It would be preferable to register a single request withAlarmClock
, for two reasons:
- It's more efficient because it involves fewer method calls.
GUIClock
wouldn't have to maintain its ownAlarmClock
because it wouldn't risk losing its bed.AlarmClock
needs to differentiate such "beeps" from an actual wake up call. So now theSleeper
interface must include abeep
method:However, if you make this change topublic interface Sleeper { public void wakeUp(); public void beep(); public long ONE_SECOND = 1000; }Sleeper
, all classes that implement the oldSleeper
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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