Smalltalk and Object Orientation: an Introduction - Free

“Now calculate y “
y := x * 23.
“
The Smalltalk compiler would read this as a comment , followed by the code Now calculate y ,
followed by another comment. This is (almost) certainly going to cause an error.
The | temporaries | format is used to define variables which are local to the method. They must be
declared at the beginning of the method (just after the message pattern) and are initially nil.
The statements represents any le gal set of Smalltalk statements. These statements are used to
implement the behavior of the method.
One of the uses of methods is in providing an interface between an object’s internal data and the
outside world. Such methods are often termed accessor methods. Such a method retrieves the value of
an instance variable and makes it available to other objects. For example, the class Employee has two
instance variables age and name. A method implemented in Employee returns the age of an
employee. Thus in response to the message age, this method would be executed and the value of the
employee’s age returned to the object sending the message.
In this situation the employee’s age is held explicitly. An equally valid internal representation for an
Employee would be to have an instance variable dateOfBirth. The method age, would now need
to take the date of birth away from the current date, in order to obtain the employees' age.
Note that this would be a change to the implementation of Employee, but there would be no visible
change as far as any other object in the system is concerned. This illustrates the encapsulation possible
within Smalltalk (and other OOP languages).
6.4.1 The ^ (or return) operator
Once a method has finished executing, an answer is ret urned to the sender of the message. By default
the object returned is the receiver itself (i.e. self). However, other objects can be returned by use of a
return expression - an expression preceded by an up arrow (^). The return expression must be the last
expression executed in a method. This does not mean that it must be the last expression in the method,
merely that it is the last executed. For example:
C Version
Smalltalk
if (x == y) (x = y)
return x;
ifTrue: [^x]
else
return y;
ifFalse: [^y].
In this case, either the value of x or y will be returned depending upon whether x and y are equal or
not.
6.4.2 An example method
Let us compare a procedure definition in a language such as C with the Smalltalk equivalent. We will
assume that we wish to define a procedure to take in a number, add 10 to it and return the result.
int myAdd (int x)
myAdd: aNumber
{ int result; | result |
result = x + 10; result := aNumber + 10.
return result;
^result.
}
From this example you will see that although the format is different you should soon be able to get
used to it. Let us look at some of the constituent parts of the method definition. The method name (and
its message selector) is myAdd: . Note that because this meth od takes a parameter, the method name
must have a trailing colon. It has one parameter called aNumber. Just as in any other language, this
parameter variable is limited to the scope of this method. The method also defines a temporary variable
(result) which is also limited to the scope of this method.
Variable names are identifiers containing only letters and numbers which must start with a letter.
Some examples are:
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