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Revision as of 17:53, 12 July 2011 by JaroslavTulach (Talk | contribs)
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Dissatisfied with relational or OOP mappings of the real world? Let's create our own world inside a computer. Let's write in Smalltalk! Smalltalk is unique/strange, but I like Smalltalk's Hotswap. Long time ago Smalltalk could do things Java is still dreaming about.

Extending Root Hierarchy Class

The Smalltalk systems (as far as I remember) start with a bunch of predefined classes which everyone can modify. One can create new subclasses (OK, that is the same in Java), but one can also add new methods to existing classes.

The classical Smalltalk style to find out what is a type of an object is based on a bunch of isSomething methods:

> 1 isNumber
> 'text' isNumber
> 'text' isString

What people do when they define new types like Person? They what to have their own isPerson method. Thus they add this method to Object to return false and override it in Person class to return true:

> Object subclass: #Person
> !Object methodsFor: 'Checking Person'! isPerson  ^false !!
> !Person methodsFor: 'Checking Person'! isPerson  ^true !!

One can then easily check on any object if it is a person or not:

> 1 isPerson
> Person new isPerson

As a result, if you want to understand a Smalltalk program, you should look at the added isSomething methods in the Object. This kind of programming creates not object oriented systems, but different flavors (in the above case a person oriented system), where capabilities of each object are driven by the variety of subclasses.

I don't like that each Smalltalk system ends up being completely unique. This is not how APIs should be designed, as that prevents merge of systems developed independently, as far as I can tell.

Is Java an OOP system?

But enough about Smalltalk and let's look at Java's root class Object. Can you guess what kind of oriented system Java is?



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