Figures 7 to 10 illustrate pictorially the various relationships between classes in the system. They will be described in detail in Section 4.2.
The notation used in Figures 7 to 10
is a subset of OMT (Object Modeling Technique), as described in Appendix B of
[4]. The key points to note are that methods are followed
by empty parenthesis, boldface text is used for class names, italics for abstract
classes and methods, a triangle 
indicates inheritance and
the symbol
indicates
containment (with the optional dot next to the arrow indicating the containment
of multiple objects of the indicated class).