Smalltalk Best Practice Patterns Volume 1: Coding - Free

Query MethodA Composed Method has had to execute a boolean expression.How do you write a method that tests a property of an object?There are actually two decisions here. The first is deciding what to return from a method that tests aproperty. The second is what you should name the method.Designing the protocol for a Query Method provides you with two alternatives. The first is toreturn one of two objects. For example, if you have a switch that can be either on or off, you couldreturn either #on or #off.Switch>>makeOnstatus := #onSwitch>>makeOffstatus := #offSwitch>>status^statusThat leaves clients needing to know how Switch stores its status:WallPlate>>updateself switch status = #on ifTrue: [self light makeOn].self switch status = #off ifTrue: [self light makeOff]A maintenance programmer who innocently decides to change the Symbols to #On and #Off willbreak the client.It is far easier to maintain a relationship based solely on messages. Rather than status returning aSymbol, it is better for Switch to provide a single method that returns a Boolean, true if the Switchis on and false if the Switch is off.Whether this is represented in the Switch as a variable holding a Boolean or a variable holding oneof two Symbols is irrelevant to designing the protocol.The naming question is a bit more sticky. The simplest name for a method that tests a property andreturns a Boolean is just a simple name. In example above, I’d be tempted to call the method “on”:Switch>>on“Return true if the receiver is on, otherwise return false.”However, this leads to confusion. Does “on” mean “is it on?” or “make it on”?Return a Boolean. Name the method by prefacing the property name with a form of “be”- is,was, will, etc.Here are some examples from Smalltalk:isNilisControlWantedisEmptyLooking at the code above, I’m not sure it’s the greatest design in the world. I’d probably want tointroduce the switch directly to the light.Coding Patterns page 32 of 147 9/30/2006