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These six different kinds of class relationships, associations are the most general but also the most semantically weak. Inheritance is perhaps the most semantically interesting of these concrete relationships, and exists to express generalization/specialization relationships. In our experience, however, inheritance is an insufficient means of expressing all of the rich relationships that may exist among the key abstractions in a given problem domain. We also need aggregation relationships, which provide the whole/part relationships manifested in the class's instances. Additionally, we need using relationships, which establish the links among the class's instances
We may show a simple association between these two classes: the class Product denotes the products sold as part of a sale, and the class Sale denotes the transaction through which several products were last sold. By implication, this association suggests bidirectional navigation: given an instance of Product, we should be able
to locate the object denoting its sale, and given an instance of Sale, we should be able to locate all the products sold during the transaction.
An association only denotes a semantic dependency and does not state the direction of this dependency (unless otherwise stated, an association implies bidirectional navigation, as in our example), nor does it state the exact way in which one class relates to another (we can only imply these semantics by naming the role each class plays in relationship with the other). However, these semantics are sufficient during the analysis of a problem, at which time we need only to identily such dependencies. Through the creation of associations, we come to capture the participants in a semantic relationship, their roles, and, as we will discuss, their cardinality.
Cardinality Our example introduced a one-to-many association, meaning that for each instance of the class Sale, there are zero or more instances of the class Product, and for each product, there is exactly one sale. This multiplicity denotes the cardinality of the association.
In practice, there are three common kinds of cardinality across an association:
A subclass may inherit the structure and behavior of its super class. Quality Classes and
"A system should be built with a minimum set of unchangeable parts; those parts should be as general as possible; and all parts of the system should be held in a uniform framework"
We suggest five meaningful metrics:
Message passing may thus take one of the following forms:
• Synchronous An operation commences only when the sender has initiated the action and the receiver is ready to accept the message; the sender and receiver will wait indefinitely until both parties are ready to proceed.
• Balking The same as synchronous, except that the sender will abandon the operation if the receiver is not immediately ready.
• Timeout The same as synchronous, except that the sender will only wait for a specified amount of time for the receiver to be ready.
• Asynchronous A sender may initiate an action regardless of whether the receiver is expecting the message.
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