Variables

In a Nutshell

• Local variables start with lowercase (temps, instance variables, arguments,...)
• Shared variables start with uppercase (class, class variables)

Local Variables Start With Lowercase

Temporary variables are local to the method

Example: c

CounterTest >> testIncrement
   | c |
   c := Counter new.
   ...

Remember: class names start with uppercase

Local Variables Start With Lowercase

Instance variables are local to the object

Example: x, y, count

Object subclass: #Point
   instanceVarNames: 'x y'

Object subclass: #Counter
   instanceVarNames: 'count'

Local Variables Start With Lowercase

Method arguments: aPoint

crossProduct: aPoint 
  "Answer a number that is the cross product of the receiver and the 
  argument, aPoint."

  ^ (x * aPoint y) - (y * aPoint x)

Block arguments: :x

[ :x | x + 2 ]

Special Variables

Special variables cannot be changed

• true, false, nil
• self, super, thisContext

Special Variables

• true, false are the Booleans
  • true is the unique instance of the class True
  • false is the unique instance of the class False
• nil is the unique instance of the class UndefinedObject

Special Variables

• self refers to the receiver of the message (this in Java)
• super refers to the receiver but the method lookup starts in the superclass of the class defining the method (see dedicated Lectures)
• thisContext refers to the current execution stack (advanced)

Shared or Global Variable starts with Uppercase

Object is a class globally accessible

Object subclass: #Point

Transcript is an object that is globally accessible (a kind of stdout)

Transcript cr.
Transcript show: 'hello world'.
Transcript cr.

ClassVariables are Shared Variables

• To share information between all the instances of a class and subclasses
• Use a classVariable

Object subclass: #CombinedChar
   instanceVariableNames: 'codes combined'
   classVariableNames: 'Compositions Decompositions Diacriticals'
   package: 'Kernel-BasicObjects'

• Here Compositions is shared between all the CombinedChar instances and instances of subclasses

Summary

• Lowercase are used for local/private/temporary variables
• Uppercase are used for shared or global variables