Definition: Object Modeling

Definition: Object Modeling
• Main goal: Find the important abstractions
• What happens if we find the wrong abstractions?
 Iterate and correct the model
• Steps during object modeling
 1. Class identification
 Based on the fundamental assumption that we can find abstractions
 2. Find the attributes
 3. Find the methods
 4. Find the associations between classes
• Order of steps
 Goal: get the desired abstractions
 Order of steps secondary, only a heuristic
 Iteration is important

Class Identification
• Identify the boundaries of the system
• Identify the important entities in the system
• Class identification is crucial to object-oriented modeling
• Basic assumption:
 1. We can find the classes for a new software system (Forward Engineering)
 2. We can identify the classes in an existing system (Reverse Engineering)
• Why can we do this?
 Philosophy, science, experimental evidence

Class identification is an ancient problem
• Objects are not just found by taking a picture of a scene or domain
• The application domain has to be analyzed.
• Depending on the purpose of the system different objects might be found
 How can we identify the purpose of a system?
 Scenarios and use cases
• Another important problem: Define system boundary.
 What object is inside, what object is outside?
Pieces of an Object Model
• Classes
• Associations (Relations)
 Part of- Hierarchy (Aggregation)
 Kind of-Hierarchy (Generalization)
• Attributes
 Detection of attributes
 Application specific
 Attributes in one system can be classes in another system
 Turning attributes to classes
• Methods
 Detection of methods
 Generic methods: General world knowledge, design patterns
 Domain Methods: Dynamic model, Functional model

Object vs Class
• Object (instance): Exactly one thing
 The lecture on September 7 on Software Engineering from 9:00 -10:20
• A class describes a group of objects with similar properties
 IETM, Author, Corrosion, Work order
• Object diagram: A graphic notation for modeling objects, classes and their relationships ("associations"):
 Class diagram: Template for describing many instances of data. Useful for taxonomies, patters, schemata...
 Instance diagram: A particular set of objects relating to each other. Useful for discussing scenarios, test cases and examples
• Together-J: CASE Tool for building object diagrams, in particular class diagrams
 Lecture on September 2

UML: Class and Instance Diagrams

Links and Associations• Links and associations establish relationships among objects and classes.
• Link:
 A connection between two object instances. A link is like a tuple.
 A link is an instance of an association
• Association:
 Basically a bidirectional mapping.
 One-to-one, many-to-one, one-to-many,
 An association describes a set of links like a class describes a set of objects.