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EX2 Ch 9 Class and Method Design

Terms in this set (15)

Characteristics of Object-oriented Systems Analysis and Design
Classes
Instantiated classes are objects
Classes are defined with attributes, states & methods
Classes communicate through messages
Encapsulation & information hiding
Polymorphism & dynamic binding
Inheritance
Polymorphism
The ability to take on several different forms
Same message triggers different methods in different objects
Dynamic binding
Methods—the specific method used is selected at run time
Attributes—data type is chosen at run time
Implementation of X is language specific
Decisions made at run time may induce run-time errors
Need to ensure semantic consistency
Inheritance
Permits reuse of existing classes with extensions for new attributes or operations
Types
Single inheritance -- one parent class
Multiple inheritance -- multiple parent classes (not supported by all programming languages)
Redefinition of methods and/or attributes
Not supported by all programming languages
May cause inheritance conflict
Designers must know what the chosen programming language supports
Object Interaction
Dynamic modeling is about objects—their interactions and how the objects change through interaction.

In an object-oriented virtual system the only possible way of interaction is through sending and receiving messages.
Messages
X are instructions and information sent to objects in the expectation that the recipient objects will carry out certain actions.
Operations (aka Messages, Functions)
X are actions that an object performs. X are invoked when the object receives a message to perform an action.
An X is implemented or actually carried out by the object responsible for the X.
polymorphism; one X => more than one method.
Parameters
or arguments, specify the data that must be supplied to an object to carry out a specific operation:
Examples:
orderDrink(drink, quantity)
orderAppetizer(Appetizer)
getMenu()
orderMainCourse(whisky, on the rocks, many)
Accessor Operations
**X change or return the value of an attribute.
The get operation returns the value of the attribute to the public.
The set operation changes the value of the attribute.
Examples
getName( )
getAddress( )
setName (name)
setAddress (address)
Return Value
is the reply that a message may get from the receiving object after an operation is complete.

Example
name = getName( )
address = getAddress( )
Design Criteria
***A set of metrics to evaluate the design

GOAL:
Low coupling, high cohesion, low connascence
Coupling
refers to the degree of the closeness of the relationship between classes

Close X means that changes in one part of the design may require changes in another part
Cohesion
refers to the degree to which attributes and methods of a class support a single object

Types:
Method X
Does a method perform more than one operation?
Performing more than one operation is more difficult to understand and implement
Class X
Do the attributes and methods represent a single object?
Classes should not mix class roles, domains or objects
Connascence
refers to the degree of interdependency between objects

For example, if I change something in module A that requires a change in module B, then module A and B are connected via some kind of X.

X literally means "Born Together".
Adding Specifications
***Review the current set of analysis models
All classes included are both sufficient and necessary to solve the problem
No missing attributes or methods
No extra or unused attributes or methods
No missing or extra classes
Examine the visibility of classes
Private—not visible
Public—visible to other classes
Protected—visible only to members of the same super class
Decide on method signatures:
Name of the method
Parameters or arguments to pass
Type of value(s) to be returned
Define constraints that must be preserved by the objects
Preconditions, post-conditions, & invariants
Decide how to handle constraint violations
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