Object and Class Diagrams

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Object and Class Diagrams

• CSE301
• University of Sunderland

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Purpose

• Describes the static structure of a system, not
  how it behaves.
• The objects/classes you choose to show should be
  the important onesthose in the use case and
  behavioural diagrams (later).
• Class diagrams show the long-term structure.
• Use separate diagrams for associations and
  inheritance.
• Object diagrams then show the structure at a
  specific time. Should show only links, not
  inheritance.
• May be high- or low-level.

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Class Diagram Notation

• Simple.
• Made up of
• Classes (or subsystems)
• Associations

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Technique for Construction

• Identify the classes
• (Object diagrams can be used to explore class
  relationships. Eventually you replace the whole
  mess with a class diagram.)
• Identify the associations
• Refine and elaborate.
• Keep it simple (silly!)

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Sample Class Diagram
Association Class (optional)
Skill

Class
Resource-Skill

Resource
Association
Inheritance
Hourly
Professional
(Note that this should be two diagrams, not one.)
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Class

• Abstract classes have a name in italics.
• May have interfaces
• May be active (Runnable/Thread) or passive

ClassName (reqd)
Attributes (opt)
Operations (opt)
Other Elements (opt)
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Attributes and Operations

• Public (), protected (), and private (-) are
  usually all you need to indicate.
• Default values for attributes can be specified
  (value)
• Return types ( type) and arguments (args) can be
  specified for operations.
• Underline to indicate class or static scope.

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Association

• May relate two or more classes.
• Paths lie between class symbols
• May be a class in its own right, with attributes
  and operations
• May be recursive from the class back to itself.
• May have role names.
• May have multiplicity. (Collections are usually
  required to implement multiplicity.)

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Object Diagram Notation

• Objects (instances of classes)
• Links (instances of associations)
• In other words, an object diagram instantiates a
  class diagram.
• You dont show inheritance in object diagrams.

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Sample Object Diagram
ProgrammingSkill
Object
ProgrammerResource
Link
MaintainerHourly
EntryLevelProfessional
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Object

• Unique and has identity.
• May be active or passive.

InstanceNameClass
Attribute values
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Link

• Is an instance of an association.
• Shown as paths between objects
• Can be objects in their own right.

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Some Added Class Stereotypes

• Boundary (input and output)
• Entity (business object)
• Control (applications model)
• You can use these instead of
• or inside the box

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Using Class Diagrams

• Issues
• The layout of a GUI does not usually match the
  structure of the applications model or business
  objects. It can, but it need not. (It is a good
  idea for the GUI to be designed by a GUI
  specialist.)
• The rules (business logic) in an applications
  model are often arcane and volatile. Hence the
  applications model should not be tightly coupled
  with the GUI or business objects.
• Business objects (external interfaces and
  database tables) tend to be very rigid and should
  be separate from the GUI and business logic.

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A Posting on comp.object
Hi It seems generally accepted in Java that
Swing GUI builders are not the best way of
constructing your GUI, IOW hand-coding seems to
be the best practice. Two reasons for this seem
to be 1. the type of code that is
automatically generated from a GUI builder is not
so nice and 2. using a GUI builder can encourage
the developer to write business logic code into
the GUI. Now I just wondered if this is the
general case for most OO languages or do other
languages provide better GUI building
facilities? Regards Shane
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A Response (editd)
It is not the case for all GUI builders but it is
common. The Borland Delphi/Kylix IDE is a case
in point. While it is perfectly possible to
separate out business logic the newbie Delphi
programmer almost invariably puts it in with the
GUI - because the tool encourages that style. VB
is the same and so are the various Visual C GUI
tools. (Note this is not a rant against Delphi -
I've been using it for most of my Windows GUI
work since Version 2) Many open-source tools are
based on the same ideas as Delphi etc (GLade
being an example) and thus lead to similar
problems. ltcutgt My experiences, FWIW, Alan G.
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Bob Martins Comments (editd)
gtHi gtIt seems generally accepted in Java that
Swing GUI builders are not the best gtway of
constructing your GUI, IOW handcoding seems to be
the best practice. I hold out high hopes for the
new IntelliJ 4.0 GUI builder. I haven't tried it
yet, but from what I've heard it's intriguing.
If it's as good as the rest of IntelliJ, then
it'll be pretty good. gtTwo reasons for this
seem to be 1. the type of code that is
automatically gtgenerated from a GUI builder is
not so nice and 2. using a GUI builder
can gtencourage the developer to write business
logic code into the GUI. Most GUI builders I
have worked with leave the programmer hanging.
They provide a lot of assistance for simple
GUIs, but less and less as the GUI's get more
interesting. There comes a point where the
builder ceases to be useful, and the poor
programmer is left with a mass of opaque
generated code, and without the practice to
manipulate it properly.
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Think About This for a Minute

• Take a piece of paper and write down what this
  seems to be telling you

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So How Do You Use Class Diagrams Professionally?

• Make sure you model your GUI, applications model,
  database tables, and external interfaces
  separately.
• It is OK to use a GUI-builder, but you must
  separate your GUI from your applications model.
• Learn how to handle events and how to set up the
  Observer/Observable pattern.