How to override equality method in Smalltalk?

Question

I'm reading a book on Smalltalk and I have an exercise about the anomaly of disappearing element I'm not able to solve.

Object subclass: Book [
    | isbn |
    <comment: 'A book class'>

    setIsbn: anIsbn [
        isbn := anIsbn.
    ]

    getIsbn [
        ^isbn.
    ]

    = anotherBook [
        ^self getIsbn = anotherBook getIsbn.
    ]
]

| Library |

Library := Set new: 100.
Library add: (Book new setIsbn: '0-671-2-158-1').
(Library includes: (Book new setIsbn: '0-671-2-158-1')) printNl.

I read I have to override the hash method too, but I don't know how to do it. How do I amend the Book class in order to avoid the anomaly?

Answer 1

I can't really tell what are you asking about, but to override hash, you should do the same as with =, which you have overridden as well, just by including different definition. So you do something like:

hash [
  "return hash here"
]

If you are asking what hash should be like… well think about it like that: objects that are equal have to have the same hash (but this doesn't have to work the other way around). So I'd suggest you to do something like

hash [
  ^ self getIsbn hash
]

Also about disappearing element. Set is a hashed collection. This means that before comparing it's element with the one you are looking for, it select's a subset by hash. So if you are not overriding hash it may select a subset that won't contain your desired element.

In the end I'd suggest you to use some different implementation of Smalltalk, because my head hurt when I was starting to learn it with gnu-smalltalk. Personaly I use pharo it provides a nice UI and allows you to see what you override, allows you to debug, etc.

Answer 2

There are a few further Smalltalk idiomatic issues with your code:

• normally accessors in Smalltalk don't use get and set. So you'd have isbn: anIsbn and isbn.

• you probably want to create an extra constructor that has the ISBN as a parameter so you don't have to send both new and the setter yourself:

  Book>>onIsbn: anIsbn    
  
      ^self new
           isbn: anIsbn;
           yourself
  

Answer 3

The basic rule is never override #= without overriding #hash