Unit 4 designing classes

Designing Classes

1. Designing classes
2. Designing protocols and class visibility
3. The UML object Constraint language
4. Designing methods

Introduction
• Designer has to know
– Specification of the class
– interaction of that class with other classes

Object oriented design philosophy
• Designing class with reusability in mind ->
more gain in productivity and reduce the time
for developing new application

UML Object Constraint Language(OCL)
• The rules and semantics of UML are expressed using OCL
• OCL
– Specification language
– Uses simple logic to specify the properties of the system
• UML modeling constructs requires expression. some example as follow
– Item.selector
• Item -> object
• Selector -> attribute
• Ex : kathir.regno
– Item.selextor[qualifier-value]
• Qualifier is uesd to select related values
• Ex:
– Kathir.phone[2]
– Set->select ( Boolean expression)
• Company.employee->salary>20000

Designing classes : process
• Apply design axioms to design calsses , their
atribute, methods and association , structures,
amd protocols
– Refine and complete the static uml class diagrams by
adding details to that design
• Refine attribute
• Design methods and protocols
– Uses uml activity diagram to represent algorithm
• Refine the association bw classes
• Refine class hierarchy and design with inheritance
– Iterate and refine

Class visibility: desiging well defined
public , private and protected
protocols
• 2 problems in designing methods or attributes
of class
– Protocol or interface to the class operation and
its visibility
– How it is implemented
• Protocol layers
– Private protocol
– Protected protocol
– Public protocol

Internal layer
• Defines the implementation of the object
• Apply axioms and corollaries ( corollary 1) to
decide what to private
• Private protocol
– Includes messages that should not be sent from other
objects
– Accessible to only operations of that class
• Protected protocol
– Methods and attributes can be used by class itself or
its subclass

External layer
• Design the functionality of the object
• Public protocol
– Defined to pass message bw associated classes
– Interpretation and implementation of each
message is up to the individual classes

Encapsulation leakage

• lack of well designed protocol leads to this
problem
• Encapsulation leakage occurs when details
about classes internal implementation are
disclosed through the interface

Refining attribute
• Attribute
– Represents the state of the object
• Analysis phase -> name of the attribute is sufficient
• Design phase -> detailed information must
be added
• Attribute types
– Single valued attribute
• Ex: dob of the student
– Multiplicity or multivalue attribute
• Subjects handled by the staff
– Reference to another object or instance connection
• Person hold the account

UML attribute presentation
• OCL is used in the design phase to define the
class attributes
• Attribute presentation
– Visibility name : type-expression = initial-value
– Visibility
• Public visibility : +
• Protected visibility : #
• Private visibility : -
– Type expression
• Language dependent specification of the type
– Initial value
• Language dependent expression for the initial value

Designing methods and protocols
• Specifying the algorithm for methods
– By using formal structure (ex uml activity diag) with
OCL
• Types of methods
– Constructor
– Destructor
– Conversion method
– Copy method
– Attribute set
– Attribute get
– IO methods
– Domain specific

Design issues : avoiding pitfalls
• Better to have Large set of simple classes than
few large , complex classes
– Initially class might be too big
– Apply design axioms and corollary
• to reduce the size
• Improve reusability

UML Operation presentation
• Visibility name: (Parameter list ) : return-type-expression
– Visibility
• Public visibility : +
• Protected visibility : #
• Private visibility : -
– Name
• Name of the operation
– Parameter list
• List of parameters each specified by
– Name : type expression = default value
– Return type expression
– Ex:
• + withdraw(amount:type):

Packaging and managing classes

Unit 4 designing classes

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Unit 4 designing classes