Smalltalk and Object Orientation: an Introduction - Free

Micro
Computer
1..
*
Monitor
System
Box
Mouse
Keyboard
Figure 17.9: Aggregation tree notation
A qualified association is an association which requires both the object and the qualifier to uniquely
identify the object involved in the association. It is represented as a box between the association and the
class. For example, in Figure 17.10, it is necessary to use the catalog and the part number to identify a
unique part. Note that the qualifier is part of the association, not the class.
Catalog
partNumber
1
0..1
Part
Figure 17.10: Qualified associations
Ternary (or indeed higher order) associations are drawn as diamonds with one line p ath to each of
the participating classes (for example see Figure 17.11). This is actually the traditional entity -
relationship model symbol for an association (the diamond is omitted from the binary form to save
space). Ternary associations are very rare and higher order associations are almost none existent.
However, the facility to model them is available.
Project
Language
Person
CAD : Project
Java : Language
Pheobe : Person
Figure 17.11: Ternary Associations
It is s ometimes useful to differentiate between by -value references and by -reference ones. This is
done using the aggregation symbol. If the aggregation symbol is hollow, it indicates a by -reference
implementation (i.e. a pointer or other reference); if the aggre gation symbol is filled, it indicates a by -
value implementation, i.e. a class that is embedded within another class (see Figure 17.12).
Polygon
1
Contains 3..
*
{ordered}
Point
1
GraphicsBundle
1
colour
texture
density
Figure 17.12: Implementation adornment
Inheritance of one class by a subclass is indicated by a solid line drawn from the subclass to the
superclass with a large (unfilled) triangular arrowhead on the superclass end (see Figure 17.13). For
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