Systems Analysis II Class and Object Diagrams

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Systems Analysis IIClass and Object Diagrams

• INFO 355
• Glenn Booker

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More Class Diagram Concepts

• Beyond what was covered in week 3, there are
  additional concepts and corresponding notations
  which may be used in class diagrams
• They are grouped here into
• Class and Object Concepts
• Association Concepts
• Attribute Concepts

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Visibility

• Visibility applies to a class, its attributes,
  and methods
• Options are public, private, and protected, but
  their exact meaning differs by programming
  language
• Public is designated by a sign, and means
  things outside the class can see it and use it

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Visibility

• Private is shown with a prefix, and means that
  thing is only visible within that class
• Protected is shown with a prefix, and means
  something like it can be seen by that class
  generalization hierarchy, but not outside that
• By default, attributes are private classes and
  methods are public

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Class and Object Concepts
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Abstract Classes

• Abstract classes are used in implementation to
  show a superclass that is only used via its
  subclasses
• The abstract superclass is never instantiated
  into an object
• The abstract class is shown by putting its name
  in italics
• In Visio, check the box IsAbstract

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Interface Classes

• An interface is an abstract class with all
  abstract features
• A class provides an interface if it implements
  that interface
• A class requires an interface if it is dependent
  upon an object from that interface

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Interface Classes

• This is a different sense for interface than the
  model/view/ controller breakdown
• There is a special element in Visio for interface
  classes
• Interfaces are shown in interaction diagrams with
  a lollipop

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Interface Classes

• An interface class could refer to a connection to
  an external system
• Have that interface class be the gateway to that
  external system from anywhere within your system

Also called a façade pattern
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Interface Classes

• Or they could be a connection to somewhere else
  within your system, such as an interface class
• Treating human interfaces as an interface objects
  allows data objects to send output directly to
  them
• Have the data object send output to an abstract
  interface class, which in turn points to the
  actual user interface

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Generalization and Classification

• Generalization is often assumed to have an
  implied verb phrase of is a
• Beware that sometimes the intent is really
  classification
• Classification means the thing (object) is an
  example of the supertype
• My dog Hajime is an example of a Black Lab
  therefore, it is a classification
• Show classification with an ltltinstancegtgt
  constraint

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Generalization and Classification

• Generalization means the thing is a subtype of a
  supertype they have traits in common in the
  supertype, but each subtype has unique traits
• A poodle is a subtype of Dog all Dogs have
  common traits, but poodles have certain size and
  shape characteristics
• Use the generalization symbol

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Multiple Classification

• Conceptually, it is possible for one superclass
  to have many types of classifications (p. 77)
• In such cases, it is acceptable to choose one
  value from each generalization hierarchy
• Hard to define all legal combinations
• Difficult to implement in code, hence only used
  conceptually

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Association Classes

• An association class is a class which depends on
  the existence of an association between two other
  classes

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Association Classes

• Notice that there is no multiplicity shown on the
  association class line ends it is always assumed
  to be 1
• The association class adds a constraint
• You can only have one entry (instance of the
  association class) for each combination of the
  other two classes

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Association Classes

• Be wary of creating association classes
• Consider whether the same class could just be a
  normal class, or if the constraint adds meaning
• Often helps clarify many-many multiplicities

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Active Classes

• An active class is a controller class that
  manages its own threads based on inputs
• UML 2 has a double line symbol (p. 83)
• Check the IsActive box in Visio it thickens the
  lines a little in UML 1.x
• Good for an independently operating process or
  daemon

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Object Diagrams

• The class diagram represents all classes which
  could exist in your system
• If you draw the class diagram at one moment, and
  only show the objects which exist at that moment,
  you have an object diagram

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Object Diagrams

• This is used to help navigate complex use cases,
  when it isnt clear what objects have been
  created or destroyed
• You could consider the object diagram like a
  collaboration diagram without messages

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Object Diagrams

• Make sure to note the full object names,
  IvanPerson
• Actual values for attributes can be shown (p. 88)

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Association Concepts
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Association Navigability

• Now we can explicitly state whether an
  association can go in only one direction or
  another
• Ask yourself if either class might need to call
  upon a method in the other class
• An Invoice might add or remove LineItem(s), but
  will a LineItem ever create an Invoice?

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Association Navigability

• If its a one-way association, in Visio, check
  the correct box in the line properties for
  IsNavigable
• Leave the line with no arrows if its
  bidirectional

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Association End Labels

• The ends of associations can be labeled
  separately
• Frequently helpful for hierarchical associations

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Aggregation

• Aggregation is used to show when parts make up
  the whole of something else
• A Club class is made up of Member(s)
• A Car is made up of Part(s)
• The notation for aggregation is an open diamond
  on the collective side of the association (Club
  or Car)

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Aggregation

• To get this line in Visio
• Start with a Composition line
• Double click on it
• Change the Aggregation type from Composite to
  Shared
• Edit multiplicity and line ends

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Composition

• A concept similar to aggregation is composition
• Composition is used to show properties of a class
  that are strongly owned by each object
• The composite does not imply ALL properties of
  that class, just an important one
• The diamond is filled in for composition

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Composition

• For examples
• A Polygon is defined by three or more Point(s) in
  some ordered sequence
• A Circle has a Point which is the center of that
  circle
• Hence the example in figure 5.4 (p. 68)

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Aggregation and Composition

• Aggregation doesnt affect a class diagram much,
  but its nice to show the relationship where it
  applies
• Composition is fairly rare, but can show critical
  connections among classes
• What do Class A and Class B have in common? They
  share a composition with Class C.

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Qualified Associations

• A qualified association can be used to refine the
  multiplicities of an association by specifying an
  attribute through which the association is made

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Qualified Associations

• The multiplicity of the qualified association
  (itemID) is always 1
• The other multiplicity is based on one example of
  the qualified association (one itemID associates
  with exactly one ProductSpecification)
• Could express as there is one ProductSpecificatio
  n per itemID in the ProductCatalog

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Attribute Concepts
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Static Attributes and Methods

• Static means the attribute or method applies to
  the class, not its instances (objects)
• Analogous to the C concept of static, if an
  attribute or method is static, its name is
  underlined on the class diagram
• I cant find a Visio way to show this add a
  stereotype of ltltstaticgtgt?

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Derived Attributes

• In a class diagram, attributes which are derived
  (calculated) from other attributes can be shown
  if they are significant
• Normalization doesnt apply to classes!
• Put a front slash / in front of the attributes
  name to show you are deliberately using a derived
  attribute

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Derived Attributes

• A Note box (text box with upper right corner
  dog-eared) can be used to show how the derived
  attribute is calculated (Fig 5.5)

Note box
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Attribute Data Types

• As shown in the example on slide 35, each
  attribute can show its data type, using the
  format
• attributename datatype
• startdate Date
• This is easy for primitive, or built-in data
  types how deal with non-primitive data types?

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Primitive vs. Non-Primitive Data

• Recall primitive data (attribute) types are the
  most basic ways to represent data in a computer
• Boolean, Date, Number, String, Time
• More elaborate data types are considered
  non-primitive

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Attribute Types
The primitive data types are in bold others are
non-primitive
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Non-Primitive Data Types

• Use non-primitive if any of the following
  guidelines apply
• Data has separate sections (phone)
• There are operations associated with it (SSN)
• It needs other characteristics (start and end
  date, for example)
• It has units (height, , seconds)
• Or any other complex concept (UPC)

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Non-Primitive Data Types

• So a credit card number is a non-primitive data
  type, since it has
• Type of card (Visa/MC/Discover)
• Fixed length depending on type
• Validation rules based on first digit
• Non-primitive data can be shown as separate
  conceptual classes, or just flagged as specific
  data types

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Non-Primitive Data Types
Example Class is called ProductSpecification. One
attribute is called id, which is of the
non-primitive data type ItemID. This is the
equivalent
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Attributes with Units

• Things with units (, ft., lb., etc.) need to be
  modeled with non-primitive data types
• This helps support international-ization and
  conversion to other measurement systems (e.g.
  dollars to Euros, or English to metric units)

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Non-Primitive Data Types

• Non-primitive data type classes can be shown
  separately from the rest of the class diagram
  (to avoid clutter)
• This is the only exception to the need for every
  class to be explicitly associated with at least
  one other class, since the association is
  implied

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Enumeration Classes for Attributes

• A class can be defined to contain only the
  elements of a list or enumerated attribute (p.
  83)
• The ltltenumerationgtgt stereotype is added before
  the class name
• This is like using a class to define any other
  non-primitive attribute
• In Visio, based on the Data Type class