Object Modeling Techniques

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Object Modeling Techniques
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Overview

• Identify objects and classes
• Prepare a data dictionary
• Identify associations between objects
• Identify attributes of objects and links
• Structure object classes using inheritance
• Verify access paths for likely queries
• Iterate and refine the model

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Identify Objects and Classes

• Perform textual analysis.
• The following notes should be taken into
  consideration
• Not all objects can be identified from the use
  case description.
• A class should have a well-defined scope.
• A class need not be normalized.
• No need to be too selective at this stage
• Focus on identifying the objects first, forget
  about the other issues, such as inheritance.

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Prepare a Data Dictionary

• Write a paragraph describing each modeled
  elements in the class diagram.
• Describe the scope of the class and associations.
  

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Identify Associations

• Look for verbs or verb phrases linking 2 or more
  objects. e.g. Customer Service Assistant enters
  the sale order into the system
• Look for ownership or containment, such as
• e.g. A sale order can contain several items
• e.g. A student has one or two supervisors

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Maintain the Right Associations

• N-ary Associations
• It is often possible to decompose N-ary
  associations among 3 or more classes into two or
  more binary associations or re-phrased as
  qualified associations
• e.g. Decompose Customer service assistant enters
  a sale order for a member into
• customer service assistant enters a sale order
• a sale order is issued by a member
• Sometimes, it is not possible to decompose a
  N-ary association without losing information,
  e.g. A lecturer teaches a group of students in a
  room on a day at a time.

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Keep the Right Associations (Cont)

• Derived Associations
• Remove associations that can be derived
  indirectly from other associations, such as
• e.g. A deposit request is sent to the member
  can be derived from A member issues an order.
  and A deposit request is issued for an order
  which has a re-order amount exceeds the members
  re-order limit.
• Conditions on attributes
• e.g. more than is defined as a condition of the
  order amounts.
• Misnamed Associations
• Names need not reflect historical events
• e.g. Member has an order vs Member issued an
  order ?
• For many multiplicity, consider whether a
  qualifier is needed

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Keep the Right Associations (Cont)

• Multiplicity
• Specify multiplicity, but dont spend too much
  effort on it.
• Multiplicity is often changed in the analysis.
• Double check for multiplicity of 1.
• For many multiplicity, consider whether the
  objects are ordered.

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Identify Attributes

• Simple values
• Attributes are properties of individual objects
• Look for nouns followed by possessive phrases,
• e.g. the description of an item
• the address of a member
• Adjectives often indicate attribute values
• e.g. red, on, expired

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Structure with Inheritance

• Identify inheritance by bottom up approach
• Look for similar classes and generalize classes
  into a superclass
• Identify inheritance by top down
• Refine a class into specialized subclasses
• Look for the following phases
• is-a-kind-of
• is-a
• Search for classes with similar attributes,
  associations, or operations

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Test Access Paths

• Verify the class diagram can support the required
  queries.
• Trace the access paths through the class diagram
  to see whether they give results consistent with
  the expected results.
• e.g. Find all members which are affected by the
  deposit request. It is not current supported by
  the class diagram as the navigation direction is
  from reorder request to deposit request. It would
  give one member only if the reverse direction of
  the navigation is supported.
• Unique result for 1 associations?
• For many associations, can we pick up a unique
  result when so required?

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Iterative Object Modeling

• Like other models, the class diagram is developed
  in an iterative and incremental manner. Different
  parts of the class diagram may be at different
  stages of completion
• Refine the class diagram after creating the
  interaction diagrams and state diagrams.

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Example Mail Order System

• Data Dictionary
• Member an individual or an organization that
  have been registered for the use of mail ordering
  service. A register member can order goods by
  sending a mail order form to the customer service
  assistant. Each member is assigned an re-order
  credit limit. A member who has not been active
  for a year will be archived.
• Order an order is entered by the customer
  service assistant when a mail order form is
  received from a member. An order contains a list
  of items, prices and quantities of the items.

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Example Mail Order System (Cont)

• Identify Associations
• The Customer Service Assistant will first check
  for a valid membership and enter the sale order
  into the system.
• gt entered by association between Customer
  Service Assistant and Order
• The Order Process Clerk checks the availability
  of items of the sale order and hold the items for
  the order. When all items are available, the
  Order Process Clerk will schedule the delivery of
  the items.
• gt handled by association between Order Process
  Clerk and the Order.

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Example Mail Order System (Cont)
Initial Class Diagram
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Example Mail Order System (Cont)
Design sequence diagram
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Example Mail Order System (Cont)
Partly Revised Class diagram