Smalltalk and Object Orientation: an Introduction - Free

Object
Class1
Class 2
method
search
Class 3
message
Figure 6.2: Class inheritance in Smalltalk
This search process is repeated every time a message is sent to the instance of Class3. Thus, if the
method which matches the original message sends a message to itself (i.e. the instance of Class3),
then the search for that method starts again in Class3 (even if the original metho d was found in
Class1).
6.3.4.2 The Yo-Yo problem
The process described above can pose a problem for a programmer trying to follow the execution of the
system by tracing methods and method execution. This problem is known as the Yo -Yo problem (see
Figure 6.3). It occurs because, every time you encounter a message which is sent to “self” you must
start searching from your own class. This may result in the programmer jumping up and down the class
hierarchy.
Object Class
Class1
Class 2
Class 3
Figure 6.3: The Yo-Yo Problem
The problem occurs because you know that the execution search will start in the current instances
class, even if the method which sends the message is defined in a superclass of the current class. In the
figure, the programmer starts the search in Class3, but finds the method definition in Class1,
however this method sends a message to “self” which means that the programmer must restart the
search in Class3. This time, the method defi nition is found in the class Object etc. Even with the
browsing tools provided, this can still be a tedious and confusing process (particularly for those new to
Smalltalk).
6.3.5 Instance creation
Classes are also the things which construct the instances. They do so in response to a class message. It is
probably confusing, but classes can possess class specific methods as well as class instance variable s.
These are often referred to as class side methods and variables. They can the n respond to a message as
an instance would (this is because classes are in fact special instances - we will discuss this in a later
chapter. For the moment merely accept that classes can be sent messages).
The message most commonly sent to a class is the message new. This message causes a method to
execute which constructs a new instance of the class. This process is referred to as instantiation. You do
not need to know the details of this process. An example of sending the message new to a class
Person is presented below:
Person new.
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