1. SOFTWARE DESIGN RAJIKA TANDON
CIS Software Specification & Design
Instructor: Dr. E. Sibert

2. DESIGN
The design of a system is essentially a blueprint or a plan for a solution for the system.
Here we consider a system to be a set of modules with clearly defined behavior which interact with each other in a defined manner to produce some behavior or services for its environment.
The design process for software systems often has two levels. At the first level the focus is on deciding which modules are needed for the system, the specifications of these modules, and how the modules should be interconnected.
This is what may be called the module design or the high-level design.
In the second level, the internal design of the modules, or how the specifications of the module can be satisfied, is decided. This design level is often called detailed design or logic design.
Detailed design essentially expands the system design to contain a more detailed description of the processing logic and data structures so that the design is sufficiently complete for coding.

3. Object model?
An object model defines the structural relationships and dynamic interaction between a group of related objects.
An object diagram represents a specific collection of objects,
with specific attribute values and relationships.
Therefore it looks like a snapshot of an O-O system at some instant in time.

4. Outline Object modeling Class vs instance diagrams Attributes
Operations and methods
Links and associations
Examples of associations
Two special associations
Aggregation
Inheritance

5. Definition: Object Modeling
Main goal: Find the important abstractions
Steps during object modeling
1. Class identification
Based on the fundamental assumption that we can find abstractions
2. Find the attributes
3. Find the methods
4. Find the associations between classes
Order of steps
Goal: get the desired abstractions
Order of steps secondary, only a heuristic
Iteration is important
What happens if we find the wrong abstractions?
Iterate and correct the model

6. Class Identification Identify the boundaries of the system
Identify the important entities in the system
Class identification is crucial to object-oriented modeling
Basic assumption:
1. We can find the classes for a new software system (Forward Engineering)
2. We can identify the classes in an existing system (Reverse Engineering)

7. Pieces of an Object Model
Classes
Associations (Relations)
Part of- Hierarchy (Aggregation)
Kind of-Hierarchy (Generalization)
Attributes
Detection of attributes
Application specific
Attributes in one system can be classes in another system
Turning attributes to classes
Methods
Detection of methods
Generic methods: General world knowledge, design patterns
Domain Methods: Dynamic model, Functional model

8. Object vs Class Object (instance): Exactly one thing
A class describes a group of objects with similar properties
Object diagram: A graphic notation for modeling objects, classes and their relationships ("associations"):
Class diagram: Template for describing many instances of data. A class diagram describes object classes.
Object class: group of objects with similar properties (attributes), common behavior(operations), common relationships to other objects)
Useful for taxonomies, patters, schemata...
Instance diagram: A particular set of objects relating to each other. Useful for discussing scenarios, test cases and examples

9. Class and Instance Diagrams
Inspector
Class Diagram
joe:
Inspector
mary:
Inspector
anonymous:
Inspector
Instance Diagram

10. Common uses of object diagrams are:
to illustrate the system state at a high level of abstraction, before and after some action (e.g. instruction execution)
i.e. dynamic modeling !!
to analyze the structure of recursive data types (classes) by unfolding a class diagram.

11. Attributes – A data value held by the objects in a class.
name:string
age: integer
Inspector
name = “Joe”
age = 24
joe:Inspector
name = “Mary”
age = 18
mary: Inspector

12. Operation, Signature or Method. What when
Operation, Signature or Method? What when? ( Feature –A generic word for either attribute /operation)
Operation: A function or transformation applied to objects in a class. All objects in a class share the same operations (Analysis Phase)
Signature: Number & types of arguments, type of result value. All methods of a class have the same signature (Object Design Phase)
Method: Implementation of an operation for a class (Implementation Phase)
Polymorphic operation: The same operation applies to many different classes.
Workorder
File_name: String
Size_in_bytes: integer
Last_update: date
Stickies: array[max]
print()
delete()
open()
close()
write()
read()

13. Links and Associations
Links and associations establish relationships among objects and classes.
Link:
A (physical/conceptual) connection between two object instances. A link is like a tuple.
A link is an instance of an association
Association:
An association describes a set of links like a class describes a set of objects.
Basically a bidirectional mapping.
One-to-one, many-to-one, one-to-many.

14. Notation for multiplicity:
one and only one exactly 1
zero or one or 1 (optional)
one or more > = 1
M- N from M to N M to N
greater than or equal to zero > = 0 (many)

15. 1-to-1 and 1-to-many Associations
Country
name:String
City
name:String
Has-capital
One-to-one association
Workorder
schedule()
StickyNote
x: Integer
y: Integer
z: Integer
One-to-many association

16. Object Instance Diagram
Example for 1-to-many
:Sticky
x,y,z=(-1,0,5)
:WorkOrder
:Sticky
x,y,z=(1,10,1)
:Sticky
x,y,z=(10,1,2)

17. Many-to-Many Associations
Mechanics
Plane
Work on

18. Ternary and n-ary relations are represented
using the diamond as before, e.g.
Object_1 :
Class_1
Object_2 :
Class_2
Object_3 :
Class_3

19. We can use object diagrams to understand
recursively defined classes. For example,
the class diagram
Binary_Tree
0,1
0 .. 2

20. T1 : Binary
Tree
T2 : Binary
Tree
T3 : Binary
Tree
T4 : Binary
Tree
T5 : Binary
Tree

22. Do associations have direction?
A association between two classes is by default a bi-directional mapping.
If you want to make A a client, and B a server, you can make the association unidirectional. The arrowhead points to the server role:
Class A can access class B and class B can access class A
Both classes play the agent role. A B
Name of association
Name Direction A B
accesses
Association Direction
Class A ( the “client”) accesses class B (“the server”). B is also called navigable.

23. Relation R between classes X and Y is:
n-to-… if for every object y :Y there are
n distinct objects x1 ,…, xn : X with
xi R y for i = 1 ,…, n
…-to-m if for every object x :X there are
m distinct objects y1 ,…, ym : Y with
x R yi for i = 1 ,…, m

24. Roles
A role name is the name that uniquely identifies one end of an association.
A role name is written next to the association line near the class that plays the role.
When do you use role names?
Necessary for associations between two objects of the same class
Also useful to distinguish between two associations between the same pair of classes
When do you not use role names?
If there is only a single association between a pair of distinct classes, the names of the classes serve as good role names

25. Example of Role Pr oblem Statement : A
person assumes the role of repairer
with respect to another person, who assumes the role of
inspector with respect to the first person.
Person
Person
Creates Workorders
Person
inspector
Person
Creates Workorders
repairperson

26. Qualification
The qualifier improves the information about the multiplicity of the association between the classes.
It is used for reducing 1-to-many multiplicity to 1-1 multiplicity
Without qualification: A directory has many files. A file belongs only to one directory.
Directory
File
filename 1 *
0..1
With qualification: A directory has many files, each with a unique name

27. Example Pr oblem Statement : A stock exchange lists many companies.
However
, a stock exchange lists only one company with a
given ticker symbol. A
company may be listed on many stock
exchanges, possibly with different ticker symbols.
Find company with ticker symbol AAPL.
StockExchange
Company
tickerSym
lists

28. Use of Qualification reduces multiplicity
StockExchange
Company
tickerSym
StockExchange 1
0..1
Company
tickerSym
Directory
FILE
filename

29. officer
Company
Person
office
Organization
Many-to-Many Qualification

30. Aggregation Models "part of" hierarchy
Useful for modeling the breakdown of a product into its component parts (sometimes called bills of materials (BOM) by manufacturers)

31. Aggregation Plain association: peer relationship.
Aggregation: whole/part relationship.
- An object may be a part of ONLY one aggregate at a time.
Company
whole 1 *
Department
part

32. Aggregation Special kind of association For “part-whole” relationships
For “has-a” relationships
Aggregation-by-Value: Composition
Aggregation-by-Reference: Aggregation

33. Composition In Composition (composite aggregation):
- An object may be a part of ONLY one composite at a time.
- Whole is responsible for the disposition of its parts.
whole
Window 1 *
Frame
part

34. Composition (by Value)
gfdgfdgfdgfdgfdg

35. Aggregation (by Reference)
Product Catalog
Product 1 *
Roll Call
Student

36. Aggregation vs. Composition
Both denote whole-part relationships
Both enable modeling at multiple levels of abstraction: whole or part

37. Aggregation vs Composition (contd.)
Aggregation: does not link the lifetimes of the whole and its parts. Aggregation can occur when a class is a collection or container of other classes, but where the contained classes do not have a strong life cycle dependency on the container—essentially, if the container is destroyed, its contents are not.
Composition: usually has a strong life cycle dependency between instances of the container class and instances of the contained class(es): If the container is destroyed, normally every instance that it contains is destroyed as well. Note that (where allowed):
Parts with non-fixed multiplicity can be created after the composite itself.  lifetime
Such parts can be explicitly removed before the death of the composite.  lifetime
An object may be a part of only one composite at a time.  strong ownership

38. Generalization Also called Inheritance or Specialization.
Connects generalized classes to more specialized classes.
Sometimes called the “is-a” relationship because each instance of a subclass is an instance of the superclass as well.
NOTATION :
A triangle connecting the superclass to its subclasses. The superclass connected by a line to the apex of the triangle. The subclasses are connected by lines to a horizontal bar attached to the base of the triangle.

39. Aggregation Vs Generalization
Both associations describe trees (hierarchies)
AGGREGATION:
Aggregation relates instances (involves two or more different objects).
Two distinct objects are involved; one of them is a part of the other.
An aggregation tree is composed of object instances that are all part of a composite object.
“part-of” relationship.
GENERALIZATION:
Generalization relates classes (a way to structure the description of a single object).
It is a way of structuring the description of a single object.
An generalization tree is composed of classes that describe an object
“a-kind-of”,”is-a” relationship.

40. Class relationships Aggregation: Represented by has-a relationship
Also called Whole-part, Composition, or Embedding
Cardinality is associated with this relationship
Inheritance
Represented by Is-A relation
No cardinality is associated
Also referred to as generalization/specialization
Association
It is used when objects of two classes communicate or
there is a dependency that doesn’t fit the category of
aggregation or inheritance
Cardinality is associated

41. Grouping constructs: Module:
A logical construct for grouping classes, associations, and generalizations. It captures one perspective or view of a situation.
For example:
Electrical, Plumbing and ventilation modules are different views of a building.
SHEET:
A mechanism for breaking a large object model into a series of pages.

42. Revision - Relationships
1. Dependency
a semantic relationship between two things in which a change to one thing (independent) may affect the semantics of the other thing (dependent).
Directed is optional and label is optional.
2. Associations
a structural relationship that describes a set of links, a link being a connection between objects.
Can be directed labels Can have multiplicity & role names
employer
employee
0..1 *
Aggregation
a special kind of association. It represents a structural relationship
between the whole and its parts. Aggregation is inherently TRANSITIVE.
An aggregate has parts, which may inturn have parts.
Represented by a diamond.
3. Generalization
a specialization/generalization relationship in which objects of the specialized element (the child) are more specific than the objects of the generalized element.

43. Aggregation Vs Association
Aggregation relates instances.
If two objects are tightly bound by a part-whole relationship, it is aggregation. Vs
Association:
If two objects considered are independent, even though they may often be linked
Division
Department
Company
Works for
Person

44. How do you find classes?
Learn about problem domain: Observe your client
Apply general world knowledge and intuition
Take the flow of events and find participating objects in use cases
Apply design patterns
Try to establish a taxonomy ( classification, derivatives )
Do a textual analysis of scenario or flow of events (Abbott Textual Analysis, 1983)
Nouns are good candidates for classes

45. Mapping parts of speech to object model components [Abbot 1983]
Part of speech
Model component
Example
Proper noun
object
Jim Smith
Improper noun
class
Toy, doll
Doing verb
method
Buy, recommend
being verb
inheritance
is-a (kind-of)
having verb
aggregation
has an
modal verb
constraint
must be
adjective
attribute
3 years old
transitive verb
method
enter
intransitive verb
method (event)
depends on

46. Example: Scenario from Problem Statement
Jim Smith enters a store with the intention of buying a toy for his 3 year old child.
Help must be available within less than one minute.
The store owner gives advice to the customer. The advice depends on the age range of the child and the attributes of the toy.
Jim selects a dangerous toy which is unsuitable for the child.
The store owner recommends a more yellow doll.
( Is is-a (kind-of) and has-a present in the above text ? )

47. Object Modeling in Practice: Class Identification
Foo
Balance
CustomerId
Deposit()
Withdraw()
GetBalance()
Class Identification: Name of Class, Attributes and Methods

48. Object Modeling in Practice: Encourage Brainstorming
“Dada”
Balance
CustomerId
Deposit()
Withdraw()
GetBalance()
Foo
Balance
CustomerId
Deposit()
Withdraw()
GetBalance()
Account
Balance
CustomerId
Deposit()
Withdraw()
GetBalance()
Naming is important!

49. Object Modeling in Practice
Account
Balance
AccountId
Deposit()
Withdraw()
GetBalance()
Customer
Name
CustomerId
Bank
Name
Find New Objects
Iterate on Names, Attributes and Methods

50. Object Modeling in Practice: A Banking System
Account
Balance
AccountId
Deposit()
Withdraw()
GetBalance()
Customer
Name
CustomerId *
Bank
Name
Has
Find New Objects
Iterate on Names, Attributes and Methods
Find Associations between Objects
Label the associations
Determine the multiplicity of the associations

51. Object Modeling in Practice: Categorize
Object Modeling in Practice: Categorize! ( many accounts belong to 1 bank OR a bank holds many accounts – aggregation)
Account
Balance
AccountId
Deposit()
Withdraw()
GetBalance()
Bank
Customer
Name * *
Name
Has
CustomerId
Savings
Account
Checking
Account
Mortgage
Account
Withdraw()
Withdraw()
Withdraw()

52. Object Modeling in Practice: Heuristics
Explicitly schedule meetings for object identification
Try to differentiate between entity, boundary and control objects
Find associations and their multiplicity
Unusual multiplicities usually lead to new objects or categories
Identify Aggregation
Identify Inheritance: Look for a Taxonomy, Categorize
Allow time for brainstorming , Iterate, iterate

53. Summary In this lecture, we described:
Identification of objects and classes
Refinement of objects with attributes and operations
Generalization
Associations
Aggregation
Composition
Multiplicity

54. A static class model for ABC distributing company
Create the class model using the following class relations and the
business rules:
An Invoice is composed of one or more Entries
An Invoice is created for exactly one Customer
An Invoice is created from one or more CustomerOrders
An Invoice is used to update the AccountsReceivable account
An Inventory is composed of zero or more StockItems
A PackingOrder is initiated from one or more Invoices
A PackingOrder is created from Inventory items
A PackingOrder is used to update the Inventory
A StockOrder is received from a Supplier
A StockOrder is used to update the Inventory
A StockOrder is used to update the AccountsPayable account

55. Class Diagram (a rough sketch for the above problem)
Invoice
AccountsReceivable
CustomerOrder
Account
Customer
Entry
PackingOrder
Created from
updates
Inventory
Supplier
Received from
AccountsPayable
StockItem
StockOrder
updates
updates

56. Please refer to Design_Principles.doc, for details on: Coupling
Notes:
Please refer to Design_Principles.doc, for details on:
Coupling
Cohesion
References:
Notes derived from
Ms. Chandrakala .C.B.,
Dept. of Information & Communication Technology,
Manipal Institute of Technology, Manipal University, India: