"Overloading" method in Smalltalk according to parameter's class?

I know you can't overload a method in Smalltalk according to the parameter's class. I am left with this design problem:

I have three classes: one that implements a Mail, one that implements a Sentence and one that implements a Word. These three classes have a method called addContent that receives as an argument a String. Mail and Sentence have another method called addContent that receives a Word, and Mail has another addContent that receives a Sentence. On the side, all of them have a method called returnAsString that return the content as a string.

I am left to the following possibilities:

Implement them through dependencies: Mail has a collection of Sentence and Sentence has a collection of Word (and Word just has a string). Then that addContent could be implemented by asking the parameter to return its contents in a string, and make the object's attribute use the addContent(String) to load it. The problem I find with this approach is that I'd have to add a method for a String object to return itself as a String, and for the rest that method should be returnAsString
Make them all inherit form a base abstract class. I just don't know how this would work, because I'd also make Word have a method to addContent through a Sentence, and that is wrong.

Any ideas?

Thanks

Solution

Your friend is Double Dispatch.

You will have to add a specialized addContent: method on each of the classes Mail, Sentence and Word. I give you the example for the combination Mail and String.

Mail >> addContent: content
    content addToMail: self

String  >> addToMail: mail
    mail addStringContent: self

Mail >> addStringContent: aString
   "here you have the explicit type encoded in the selector"
   self todo: 'Add a string to the mail"

Similarly you can fix the combinations for adding Strings to Sentences, Words to Sentences, and Sentences to Mails.