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An object is a computer representation of an entity, either real world or invented. An object is a concept, abstraction, or thing with sharp boundaries and meaning for an application. Each object in a system has three characteristics: state, behavior, and identity. The state of an object is one of the possible conditions in which it may exist. Behavior determines how an object acts and reacts to requests from other objects. Identity states that each object is unique ”even if its state is identical to that of another object.
A class is a description of a group of objects with common properties (attributes), common behavior (operations), common relationships to other objects (associations and aggregations), and common semantics. In the UML a class is drawn as a compartmented rectangle. The compartments show the class name , its structure, and its behavior. As classes are created, they should also be documented. The documentation should state the purpose of the class and not the structure of the class.
Stereotypes provide the capability to create a new type of modeling element. Stereotypes must be based on elements that are part of the UML metamodel. During analysis, three common stereotypes for a class are entity, boundary, and control. These stereotypes are useful in defining the classes needed for a system under development.
A package in the logical view of the model is a collection of related packages and/or classes. By grouping classes into packages, we can look at the "higher" level view of the model (i.e., the packages) or we can dig deeper into the model by looking at what is contained by the package.
Class diagrams are created to provide a picture or view of some or all of the classes in the model. Class diagrams may also be created in the use case view of the model. These diagrams typically are attached to use cases and contain a view of the classes participating in the use case.
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