Subjective behavior

ions in Modeling In programming languages we declare and invoke abstractions. An abstraction has a designation – the designation is given in the declaration of the abstraction and is used in the application of it. Invocation of an abstraction means that the corresponding description is interpreted according to the semantics of the language. In general, when we know the designation we also know which description is chosen. Interpretation is state dependent. The execution state determines how the interpretation evolves and what the result (in very general terms) of the interpretation is. As an example, in procedural programming we declare a procedural abstraction BinarySearch. We invoke the procedure by its name and we simply execute the body of the BinarySearch procedure. The execution is dependent of the actual values to search among and the key value to search for. Recursive invocations of the procedure behave differently, dependent on these values. But the description to be followed is the same. The situation is similar for a class abstraction. The class has a name and we instantiate objects by the description of the class. A method is also an abstraction. When we invoke a method we instantiate some kind of activation record by means of the method description. The method is chosen relatively to an object – its actual interpretation depends on the state of that object. Object-Oriented Modeling In most object-oriented languages method invocation differs form usual invocation of abstractions. An object invokes a method of another object. The object does this through a reference to the other object and the designation (the name) of the method. The difference is that the reference dynamically can denote different objects for which the method may be declared differently. In some object-oriented languages the reference is not typed, and may denote any type of object. In other languages the reference is typed but may still denote different types of objects (usually any object of a subtype of the type of the reference). The idea is that even if we give the designation of the method, then the actual choice of (a description of) a method is parameterized by the value of the reference (namely an object). That object solely determines which description is chosen, namely the method declared for that object. Still, objects of the same type (class) behave the same way – they interpret the same description.* In Figure 1 we illustrate a notation for classes, objects, methods and references. We can summarize the situation as follows – object a of class A invokes method m of object b of class B by means of reference rB (where reference rB can be implicit): 1. Type B of reference rB to object b is known. The type determines which types of objects can be referenced by rB. 2. The type of the object actually referenced by rB determines which description of m is used. 3. Object b interprets method m in its current state. 4. A type (or signature) for method m may or may not be given. Method m can have different implementations in different classes. 5. Type A of object a that invokes a method is not used, except that a can access reference rB. 6. The state of object a is not used, except that a invokes rB.m in the current state and the values of actual parameters of m reflect the current state. In Figure 2 we illustrate objects a and b of types A and B, respectively. By rB.m the implementation of a method in a invokes method m of b by use of reference rB. Definition of Subjective Behavior Given the type of the reference and a named designation of the method, we obtain different interpretations by means of different objects only. That fact, that objects of different types behave differently in terms of the description that is chosen, is fundamental in object-oriented modeling. In this article we are interested in situations where objects of same type behave differently in general (still in terms of the 14 computer systems science & engineering B B KRISTENSEN *An exception is a singular or one-of-a-kind object, where the class is given only implicitly. A singular object executes its unique, but fixed description upon invocation of a method. Figure 1 Classes, Objects, Methods and References description that is chosen) dependent of various other circumstances. Objects can legally display positive discrimination in favor of other objects under given circumstances. This includes as a special case that the same object (an object with one given identity) behaves differently under different circumstances. We shall understand this as subjective behavior because from the usual objective point of view, we cannot determine which kind of behavior to expect. Actual behavior is due to other circumstances, the kind and form of which are the topic of this article. We shall use the following definitions of subjective behavior in context of object-oriented modeling: