Smalltalk

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Dissatisfied with relational or OOP mappings of the real world? Let's create a better 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

OOP languages tend to have a RootClass - a super type of every object in the system. Java has Object. Scala has Any. Etc.

Smalltalk systems come with a bunch of predefined classes which everyone can use and modify (in the form of serialized heap, btw.). Of course, the nature of OOP system is to be extensible. Thus one can create new subclasses (OK, that is the same as in Java), but one can also add new methods to existing classes. Lovers of meta-programming may be excited by that, but on and on, this is not a system that could work in a distributed development environment easily. Imagine:

The classical Smalltalk style to implement instanceof (e.g. to find out if an object is instance of a certain type) is based on a bunch of isSomething methods:

> 1 isNumber
true
> 'text' isNumber
false
> 'text' isString
true

What can developers do when they define new types like Person and want to recognize their instances? They add isPerson 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
false
> Person new isPerson
true

As a result, if you want to understand a Smalltalk program/system, you should look at the added isSomething methods in the Object. Some might argue, that this is the way OOP should be done. On the other hand, such kind of programming creates not pure 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 isSomething methods on its RootClass.

I don't like that each Smalltalk system ends up being completely unique. Merging two such systems together may create a lot of accidental conflicts preventing 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?

I think the answer is clear. Java is a hash oriented language and also lock oriented language!