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Lesson 3Object diagram notation
ObjectiveCompare and contrast the object diagram and the class diagram notation.

Object Diagrams vs. Class Diagrams: A Comprehensive Examination for Object Modelers

In the realm of object-oriented modeling and UML (Unified Modeling Language), both object diagrams and class diagrams play crucial roles in representing different facets of a system. For an object modeler, understanding the nuances, similarities, and differences between these two diagrams is pivotal. This exploration offers a systematic comparison and contrast of the object diagram and the class diagram notations.
  1. Fundamental Definition:
    • Object Diagram: An object diagram depicts a snapshot of the instances in a system and the relationships between them at a specific point in time. It showcases the real-world instances of classes and their interrelations.
    • Class Diagram: A class diagram describes the static structure of a system. It represents the classes, interfaces, and their mutual relationships, without focusing on any specific instance or timeframe.
  2. Primary Purpose:
    • Object Diagram: To provide a real-time and practical view of a system by showcasing actual instances and their relationships. It is often utilized for making runtime decisions, debugging, or understanding specific scenarios within the system.
    • Class Diagram: To offer a high-level blueprint of a system, illustrating how classes and interfaces are structured and interrelated. It serves as a foundational tool during system design and architecture phases.
  3. Components
    • Object Diagram: Contains objects (instances of classes), links (instances of associations), and the values of the attributes of these objects at the represented moment.
    • Class Diagram: Comprises classes, interfaces, associations, generalizations, and possibly other relationships like aggregation and composition. Attributes and operations of classes are also depicted.
  4. Scope:
    • Object Diagram: Typically narrower in scope, focusing on a particular set of object instances and their state during a specific phase or operation in the system.
    • Class Diagram: Broader in scope, portraying an overarching view of the system's classes and how they relate and interact, irrespective of specific instances or states.
  5. Granularity:
    • Object Diagram: Delves into the finer details of specific object instances, including their attribute values and particular inter-object relationships.
    • Class Diagram: Stays at a higher granularity, showing the blueprint of classes and relationships without getting into specific instance details.
  6. Duration of Relevance:
    • Object Diagram: Represents a transient state of the system, relevant for a specific timeframe or scenario, making it inherently dynamic.
    • Class Diagram: Represents the more enduring, static structure of the system, remaining relatively stable across various states or scenarios.
  7. Use Cases:
    1. Object Diagram:
      Highly useful for:
      • Tracing system behavior through specific use cases.
      • Debugging by visualizing particular object states.
      • Analyzing system performance in specific scenarios.
    2. Class Diagram:
      Primarily employed for:
      • System and database design.
      • Documenting the static design of software.
      • Understanding and optimizing class hierarchies and relationships.
While both object diagrams and class diagrams are integral to object modeling in UML, they serve distinct purposes. The object diagram offers a detailed, instance-centric view, capturing a specific system state, whereas the class diagram paints a holistic, static picture, focusing on the blueprint of classes and their interrelations. An adept object modeler harnesses the strengths of both diagrams, employing them judiciously based on the task at hand.

The object diagram notation is largely a subset of the class diagram notation. The graphics below show examples of an object and a class for the object.
Class diagram
Class diagram

Object diagram
Object diagram

Note the differences between the two diagrams:
Class diagram Object diagram
The class has three compartments: name, attribute, and operation. The object has only two compartments: name and attribute.
The class name stands alone in the class name compartment. The format for an object name is object name, colon, class name (Tom : Employee). You will encounter this notation in other diagrams that model objects rather than classes.
Sometimes the object name is left off and only the colon and class name are used. This is referred to as an anonymous object.
The class attribute compartment defines the properties of the attributes. The object defines only the current value of each attribute.
Operations are listed in the class. Operations are not included in the object because they would be identical for every object of the same class.
The classes are connected with an association with a name and multiplicity. The objects are connected with a link that has a name and no multiplicity. Classes represent a "classification" of objects, so it is necessary to specify how many mayparticipate in the association. Objects represent single entities. All links are one to one, so multiplicity is irrelevant. Roles may be used on links and associations.

In the next lesson, you will learn how to use the object diagram for testing and verification.

Object Diagram Notation versus class Diagram Notation - Quiz

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Object Diagram versus Class Diagram - Quiz