At any time in the life of the object, when no process is acting on it, that object is at one of its states. If objects and processes are the building blocks of OPM, and links are the mortar, states can be considered as the finish of the house: the paint job, the furniture, and architectural elements. We have been using the terms states and values quite intuitively since the early chapters of this book. States and values enable modeling change in an object while that object retains its identity. As such, it is a specialization of state: Whereas objects can have states, only states of attributes, which are objects that describe other object, are called values. States and values add expressiveness to OPM. Hence, a state is a situation an object can be at. To be able to talk explicitly about a change in an object over time, we assign to it a number of possible, “legal” states. This is graphically represented via forks, as defined, discussed and demonstrated in this chapter. We then turn to another useful notation-the fork-which is based on the observation that structural relations are distributive in a sense analogous to the distributive law in algebra. Process participation constraints and link cardinalities are designed to take care of this. However, it is sometimes required to model the fact that more than one object takes part in a process. Similarly, our models so far have tacitly assumed that a process involves one object instance of each object class to which it is linked. In general, however, we may wish to specify a certain number or a range of numbers of instances of the same class of things that participate in the relation. Indeed, the convention in OPDs is that when no quantity is explicitly recorded by the side of a structural link, it is taken to be 1, which is the default value. In all the examples and discussions so far we have tacitly assumed that each thing, be it object or process, participates in the relation singly, i.e., in a quantity of exactly 1.
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