Home Site

    Page 46
16.4 Table of Contents Bottom of Page Site Map
The last and most complex case uses named parameters,
where beginsWith: is both the name of the method and the colon signifies that a parameter will follow. The parameter may be any object. Here is another named parameter example:

aDictionary at: 'Bunny' put: 'Rabbit'.

Here the object aDictionary is sent the message at:put: with each parameter inserted after the colon.

Classes and Variables

To extend and modify a Smalltalk image, add, remove,
or modify class objects. This is not the same as writing a class source-code file in Java. In most Smalltalk products, a real object instance within the image represents a class. While it is possible in most Smalltalks to define a new class via Smalltalk code, it is rarely done. Instead, the Class Browser, the Smalltalk equivalent to an IDE, creates the classes and upon creation, it is alive immediately.
A class has two sides: class and instance. The closest
analogy with Java and C++ is the difference between statically and non-statically scoped method functions or variables. However, this is not a completely accurate comparison because Smalltalk treats classes as any other object. Therefore, when you inherit from another class within an image, both sides of the class are inherited, not just the instance side. Both sides of a class support variables, which are private to that side of the class.
Keeping variables in their own dictionaries prevents name
clashing between method names and variable names. It is common to see getters and setters without the explicit get and set prefixes.
Many Smalltalks support the concept of a Shared
Variable or Pool Variable. These named aliases in the main symbol dictionary can arbitrarily refer to any object instance (including class instances) within a Smalltalk image.

PC AI Reprints
A Great Marketing Tool
• Promote your company with an excellent article in a targeted publication such as PC AI.

• Our staff will work with you to re-layout your articles into full-color, customized reprints. (Adding promotional material, technical details, contact information and more).

• Reprints are great as marketing tools, trade-show handouts and in media kits.

For reprint price, quotes, and policy
information call:


Only one instance of any classes' class is instantiated,
making it act similar to a Singleton. Memory is the only limit to instance side instantiation.
An interesting difference between Smalltalk and
Java/C++ is that the class side is responsible for instantiating the instance side of a class. Therefore, Smalltalk has no official constructor syntax similar to Java or C++. The Named Constructor Idiom is the closest equivalent in the Java and C++ world to the way a Smalltalk programmer instantiates a class instance.
In practice, Smalltalk's classes have so few restrictions
that they are much more powerful and robust than their Java or C++ counterparts. Since Smalltalk is dynamically typed, it is easy to treat the class hierarchy as a type of super-schema, modified in real-time. While most Smalltalk's do not directly support multiple inheritance, many do support adding the functionality if desired. Java does not support multiple implementation inheritance, and multiple interface inheritance has many drawbacks.
Since there is no arbitrary limits on implementation
inheritance within Small-talk, as there is in Java, so it is possible to change String or inherit from it.

Other Syntax
The remaining set of syntactical notations follows. The
first is variable assignment:

firstName := 'Hello World'.

Note that most Smalltalks implements variables as a
Dictionary with named aliases to their r-value. It is possible to define context local variables by using the pipe symbol, known as fence posts in Smalltalk. Here is an example:

| firstName lastName age weight |
firstName := 'Jeff'.
lastName := 'Panici'.
age := 127. "Feels that way sometimes"
weight := 175. "Well, that's what I
should weigh"

These variables persist through the lifetime of a context.
A context in Smalltalk is either a method or a BlockClosure, covered later in the article.
To return a value from a method, code something like

" Seems really old… "

The words in double-quotes are comments and the
carrot tells Smalltalk to return the object following it. Although some say this breaks the object message pattern, it really does not:

" Seems really old… "
thisContext ^ 30

This is the exact same code as the previous example;
with the implicit thisContext in the code, although most Smalltalks do not require the explicit inclusion.
Parentheses serve a different purpose in Smalltalk than
most other languages. They act as a type of stack, where each object returned from a previous send receives the next message sent, as in this example:

To Page 45

16.4 Table of Contents
Top of Page

To Page 47

16.4 2002

PC AI Magazine - PO Box 30130 Phoenix, AZ 85046 - Voice: 602.971.1869 Fax: 602.971.2321
e-mail: info@pcai.com - Comments? webmaster@pcai.com