Smalltalk with James Robertson

Today's Smalltalk 4 You looks at how it's easy to use - and abuse - #doesNotUnderstand: in Smalltalk. If you have trouble viewing it here in the browser, you can also navigate directly to YouTube. To watch now, click on the image below:

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Today we'll look at #doesNotUnderstand: - and how you:

• Can override it to change the way an object behaves
• Should never do this lightly - it can create situations where your application fails in very hard to understand ways

First, we'll load the SimpleCounter application we used as an example for WebServices, and add this method:

doesNotUnderstand: aMessage
	| arg dict |
	
	dict := Dictionary new.
	dict at: #one put: 1.
	dict at: #two put: 2.
	dict at: #three put: 3.
	arg := dict at: aMessage selector ifAbsent: [nil].
	arg := arg isNil
		ifTrue: [0]
		ifFalse: [arg].
	self perform: #addValue: with: arg

Notice what this does - it treats a few strings as numeric arguments, and then converts anything it doesn't understand into a send of #addValue: with an argument of zero. That seems great - no more MessageNotUnderstood exceptions here. However, consider a more interesting object doing something like this, and passing values down the chain (maybe to a database, eventually). If you're lucky, you only get bad data. If you're unlucky, you get bad data and very, very odd application behavior.

Note the following workspace code:

counter := Counter new.
counter one.
counter two.
counter three.
counter thousand.
counter goAway

All of that now "works" - or at least, none of those message sends break immediately. However, a follow on developer who sees the sending of #goAway, and looks for an implementor will find 26 nothing. That's the danger here. The code is now much more difficult to understand.

Need more help? There's a screencast for other topics like this which you may want to watch. Questions? Try the "Chat with James" Google gadget over in the sidebar.

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