Re: [Newbies] enumerated types

On Sun, 2008-10-05 at 20:19 -0500, Mark Volkmann wrote:

Have I summarized this correctly? Smalltalk doesn't support the concept of enumerated types like in Java 5 and above. Instead, the Smalltalk way is to:

 1. create a class that represents the enumerated type
 2. add a class variable for each enumerated value (must start
    uppercase)

I wouldn't create class variables for the enumerated stuff. You can create a dictionary that holds the instances. It is more flexible to.

cache ^ cache ifNil: [ cache := Dictionary new ]

red ^ self cache at: #red ifAbsentPut: [ self new ... .... ]

blue ^ self cache at: #blue ifAbsentPut: [ self new ... .... ]

 1. add a class-side initialize method that creates an instance of
    the class for each enumerated value using basicNew and assigns
    it the corresponding class variable

see above

 1. prevent creation of additional instances by overiding the
    class method new with "self error: 'new instances cannot be
    created'"

see above

 1. add class-side getter method for each enumerated value that
    simply returns it

see above

I can see you have singletons in mind :) This approach is good if you need constant objects that should provide a richer protocol than just identity.

Norbert

Here's an example ColorEnum class.

Object subclass: #ColorEnum instanceVariableNames: '' classVariableNames: 'Blue Green Red' poolDictionaries: '' category: 'SomeCategory'

initialize Red := self basicNew. Green := self basicNew. Blue := self basicNew

new self error: 'new instances cannot be created'

red ^Red

green ^Green

blue ^Blue

---

Mark Volkmann

---

Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners