CODEWITHSUNDEEP
oopsmalltalkpharo
Christophe De Troyer
Christophe De Troyer

Reputation: 2922

Inheritence method call design issue

I'm in a bit of a pickle with designing an OO application using Pharo.

I have a class Household, this class has an attribute named tvConnection. This object is an instance of either Digital or Analog. Both these classes inherit from the super named TVSubscription.

If I want to add a television to my analog cable subscription I can simply call addTV: aTelevision on my tvConnection property, which will then properly call the addTV: method I have implemented in my Analog class.

A digital connection can not have televisions connected. These have to be SetTopBox devices, which in turn can have a television connected to them.

Now, I have to throw an error/show some output in case a user tries to add a television to a household which has a digital connection.

I figure I can do two things:

1) I can implement the addTV: method in the Digital class, which will simply perform the action wanted (eg. show a message "YOU CAN NOT DO THIS!"). But this seems wrong in so many ways..

2) I can just not implement the method and catch the MessageNotUnderstood error that gets thrown. This would happen when I do the following:

**Adding a TV**
--> call `addTelevision` in `Household`
----> this `addTV:` on the `tvConnection` property
----> catch error if any, which implies that a TV was being added to a digital connection

Both of these solutions look REALLY dirty to me.

Upvotes: 2

Views: 107

Answers (2)

Max Leske
Max Leske

Reputation: 5125

Maybe you should rethink your design. In my opinion the tvConnection should be a connection object, let's say TVConnection. TVConnection could have two subclasses DirectConnection and SetTopBoxConnection. Digital and Analog would know their connection type and the connection type knows how to addTV: or more conveniently, Digital and Analog implement addTV: and simply delegate to their TVConnection object.

Upvotes: 2

Uko
Uko

Reputation: 13386

If I were you I'd probably implement addTV: in digital which would either add TV to a box or say that box is missing.

This looks quite nice. Think about it like:

TVSubscription>>addTV: aTV
  self subclassResponsibility

TVSubscription>>tvs
  self subclassResponsibility

Analog has tvs var.

Analog>>addTV: aTV
  tvs add: aTV

Analog>>tvs
  ^ tvs

Digital has box var.

Digital>>box: aBox
  box := aBox

Digital>>box
  ^ box

Digital>>addTV: aTV
  box 
    ifNil: [ Exception signal: 'Connect box' ]
    ifNotNil: [ box addTV: aTV ]

Digital>>tvs
  ^ box tvs

This is just a first best thought. Now I'll go to sleep and maybe I'll dream about some better solution

Upvotes: 2

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