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Generally speaking, classes are used to describe
structures and behaviors of virtual or real-world objects. Look at your current
environment, for example. If you are presently in a room, then the class diagram in
Figure 3.5 can be thought of as a simple model of your environment. A room can
contain several tables, with up to two chairs at each table, where each chair has a
defi ned number of legs. Chair and table are types of furniture.
Classes and objects represent the core in object orientation. A class is the
smallest building block of an object-oriented system. Classes are interconnected
by associations to form larger structures. Objects are concrete elements that are
created based on the building plans of classes.
SysML does not know the term class , but calls it block . Everything I write
about classes in this chapter applies just as well to blocks in SysML. You just have
to swap the two terms in your mind. Instead of a class diagram, SysML knows
the block defi nition diagram . Objects play a subordinate role in SysML.


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