Foundations of Object-Oriented Analysis and Design

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Foundations of Object-Oriented Analysis and Design

• Lecture 06 Design Principles Public Interfaces,
  Private Implementations

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Objectives

• Learn the principles of good OO class design
• Learn to use UML Class Diagrams to model the
  classes in a system

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Elements of UML Class Diagrams

• Classes
• Three levels of detail
• Name only
• Name and Attributes
• Name, Attributes and Operations
• Level of detail depends on how we are using a
  diagram at the time
• Associations

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Modeling Classes with UML Class Diagrams

• Attributes
• Operations (Methods, Member Functions)
• Stereotypes

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Attribute Elements

• Visibility
• Name
• Type
• Default value (optional)
• Example
• private nameString "Bob"

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Attribute Types

• Instance attributes
• Each object of the class has its own attribute
• Changing the value of the attribute affects only
  the containing object
• Class attributes
• Only one instance of the attributes for all
  objects of the class
• Changing the value of the attribute affects all
  objects of the class

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Operation Elements

• Visibility
• Name
• Parameter list
• Return type
• Example
• public getName(idInteger)String

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Operation Types

• Instance operations
• Performed only on behalf of an individual object
• May access and alter instance attributes
• Class operations
• Performed on behalf of the class and all
  instances of the class
• May only access and alter class attributes

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Common Operations

• Constructors
• Executed on creation of an object of the class
• Usually have several
• Accessors (queries, getting operations)
• Return values of attributes
• Mutators (modifiers, setting operations)
• Alter the values of attributes
• Destructors
• Executed when an object of the class goes out of
  existence
• Methods Mapped to Class Responsibilities

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Modeling Associations with UML Class Diagrams

• Association types
• Peer-to-peer
• Aggregation
• Composition
• Association elements
• Multiplicity
• Roles
• Constraints
• Navigability

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UML Class Diagram Example
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Aggregation and Composition
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Public Interface vs. Private Implementation

• Public interface
• The totality of a class's public operations
• The responsibilities of a class are fulfilled by
  its public interface
• Should be specified such that changes to the
  private implementation don't change the interface

• Private implementation
• How the responsibilities are actually performed
• Should be changeable without affecting the public
  interface

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Generalization

• Models the concept of inheritance
• Subclasses inherit the attributes and operations
  of all their superclasses
• Objects of a subclass should be substitutable for
  objects of their superclasses
• Subclasses must conform to the public interface
  of their superclasses

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Special Class Stereotypes

• Interfaces
• Specify public operations, but must not have
  implementation
• May have attributes but they must be class
  attributes

• Abstract Classes
• Specify operations, may or may not have
  implementation
• May have class or instance attributes

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Internal Design Principles

• Preconditions
• What must be true before a particular operation
  can execute successfully?
• Postconditions
• What must be true after sucessful execution of an
  operation?
• Class invariants
• What must always be true of the attributes of an
  object of the class

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Example A Date Class

• Class invariants
• Year must be positive number
• Month must be between 1 and 12 inclusive
• Day must be between 1 and 31 inclusive
• Day must be less than or equal to the maximum for
  a given month
• Leap year anomalies must be correct
• Preconditions
• No operation may take a parameter that would
  cause violation of the class invariants
• Postconditions
• No operation may leave the object in a state that
  violates the class invariants

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Deriving Invariants

• Business rules
• Data integrity constraints