CHAPTER 4
Topic: Static View
After reading this Chapter, students should appreciate the following:
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Understand the class, object and data types.
Understand the concepts of generalization, inheritance and multiple
inheritances.
Discuss the Realization, dependencies and constraints
Instructor Notes
This chapter discusses in detail the static view and the components of static view.
The static view captures the object structure i.e. traditional data structure as well as
the organization of the operations of data. The static view describes behavioral
entities as discrete modeling elements but does not contain the details of a dynamic
behavior.
The key elements in static view are classifiers and relationships.
Classifiers:
A classifier is a discrete concept in the model, having identity, state, behavior and
relationships. Different types of classifiers are classes, interfaces, and data types.
Please refer to Table 4.1 on page 43 to learn about different types of classifiers and
their functions.
Class:
A class represents a discrete concept within the application modeled – a
physical thing, a business thing, a logical thing, an application thing , a
computer thing or a behavioral thing. A class is the descriptor for a set of
objects with similar structure, behavior and relationships. All attributes and
operations are attached to classes or other classifiers.
Interface:
An interface is the description of behavior of objects without giving their
implementation or state; an interface contains operations but no attributes.
Data Type:
A data type is the description of primitive values that lack identity. Data type
includes numbers, strings and enumerated values.
Relationships
Relationships among classifiers are association, generalization, flow and various kind
of dependency including realization and usage. (refer to table 4.2 on page 46)
Associations:
An association describes discrete connections among objects or other instances in a
system.
- An association relates an ordered list of two or more classifiers, with repitiions
permitted.
- The most common kind of association is a binary association between a pair
of classifiers.
- An instance of an association is a link.
- A link comprises a tuple of objects, each drawn from its corresponding class.
- A binary link comprises a pair of objects.
- Associations carry information about relationships among objects in a system.
- Without associations there are nothing but isolated classes that do not work
together.
- Each connection of an association to a class is called an association end.
- The most important property os association end is multiplicity i.e. how many
instances of one class can be related to one instance of the other class.
- Multiplicity is most useful for binary associations because its definition for nary associations is complicated.
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An association can also have attributes of its own, in which case it is both an
association and a class – an association class.
Aggregation and composition
o An aggregation is an association that represents a part-whole
relationship.
o A composition is a stronger form of association in which the composite
has sole responsibility for managing its parts – such as their allocation
and deallocation.
Links
o An instance of an association is a link.
o A link is an ordered list of object references, each of which must be an
instance of the corresponding class in the association or an instance of
a descendant of a class.
Bidirectionality
The different ends of an association are distinguishable, even if two of them
involve the same class.
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An association is sometimes said to be bi-directional i.e. logical relationships
work both ways.
Generalization
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The generalization relationship is a taxonomical relationship between a more
general description and a more specific description that builds on it and
extends it.
Generalization has two purposes
o To define the condition under which an instance of one class can be
used when a variable is declared as holding values of a given class.
This is called substitutability principle
 Generalization enables polymorphic operations i.e. operations
whose implementation is determined by the class of object they
are applied to rather than explicitly stated by the caller.
o To permit the incremental description of an element by sharing the
descriptions of its ancestors. This is called inheritance.
Inheritance
Inheritance is the mechanism by which a description of the objects of a class is
assembled out of declaration fragments from the class and its ancestors. Inheritance
permits shared parts of the description to be declared once and shared by many
classes, rather than be repeated in class.
Multiple Inheritance: If a classifier has more than one parent, it inherits from each
one. Its features are union of both parents.
Static and Dynamic Classification
- When multiple classifications are combined with dynamic classification an
object can gain and lose classes during its life.
- The dynamic classes are called roles or types.
- The requirement is that there should be a single static inherent class plus
zero or more role classes that may be added or removed over the lifetime of
the object.
Realization
- The realization relationship connects a model element, such as a class to
another model element such as an interface, that supplies its behavioral
specification but not its structure or implementation.
- Realization can be used with specification elements such as interfaces as well
as with concrete implementation element.
Both generalization and realization relate a more general description to more
detailed versions of it.
Dependencies
- A dependency indicates a semantic relationship between teo or more model
elements.
- It relates the model elements themselves and does not require a set of
instances for its meaning.
- It indicates a situation in which a change to the supplier element may require
a change to or indicate a change in meaning of the client element in
dependency. (Please refer to table 4.3 on page 57 for knowledge of all
dependencies)
Constraint
- A constraint is a Boolean expression represented as a string to be interpreted
in a designated language.
- Natural language, set theoretic notation, constraint languages or various
programming languages may be used to express constraints.
- The UML includes the language of constraints called OCL.
- Constraints can be used to state various nonlocal relationships, such as
restrictions on paths of associations etc.
Instances
- An instance is a run-time entity with identity i.e. something hat can be
distinguished from other run-time entities.
- It has a value at any moment of time.