Computer Programming with GNU Smalltalk - Free

60 Computer Programming with GNU Smalltalk
it does not look messed up.
Other than the indentation, also most of the white spaces we used were programmer-dependent. You
can choose not to place or even place more white spaces around anything. If it is a wrong usage
(whenever it causes an ambiguity) then the interpreter will give you an error. For example, you cannot
join the variable name and a keyword selector because interpreter cannot detect where the variable
name ends and where the keyword selector begins. So you should put at least one space there.
Let's look at the code above line by line. The first thing we do is to define a class with the code piece
below:
SuperclassName subclass: SubclassName [
...
]
The three dots in our code always mean that there are more expressions there but we cut it short to
emphasize the really important part. So you should not put three dots in your code. With the part
above, we told SuperclassName class to create a subclass named SubclassName. We did this to
inherit the properties and behavior of the superclass so we don't have to create them again. After doing
that, we opened a bracket to specify the class variables, instance variables, class methods and instance
methods.
The part up to the opened bracket is called the header of the class. The part between the square
brackets is called the body of the class.
The names we put between pipes are instance variables of our class:
| instanceVariable1 instanceVariable2 |
Instance variable names are separated by white spaces. We have two instance variable here named
instanceVariable1 and instanceVariable2.
Then we define class variables by using assignment syntax:
classVariable1 := anObject.
classVariable2 := anotherObject.
:= is another message which makes the receiver an object referring to the argument object. So
whenever we use classVariable1 in our code from now on, the interpreter will understand that we
are actually referring to the anObject object. We will use this assignment message also for changing
the instance variables.
Mentioning about a variable for the first time is called declaring a variable. We should first declare a
variable before being able to use it.
Next expression adds a comment to the class: