pick of the day

10-Jan-2003

Lecture 2

Crash course on OO Analysis Models

January 10 2003 Lecture 2 2

Outline

Previous Business Survey Results

Constructor Challenge Results

Reading Questions

Crash course in OO modeling

Continuing Mini-Example

January 10 2003 Lecture 2 3

Survey Results

Topics of interest Primary

• OOD

Secondary• UML, OOP, Design Patterns, Software Architecture

Tertiary• Security, Swing, Transactions,

Quaternary• Persistence, Domain Analysis

January 10 2003 Lecture 2 4

Reading Questions

What is the “inherent identity” that Lau considers objects to have? [p. 1]

Book b = new Book ();Book b2 = b;Book b3 = new Book();

// identity checked by comparing object references

b == b2 // true

b2 == b3 // false

January 10 2003 Lecture 2 5

Equality tangent

What is difference between == and equals

Book b = new Book ();Book b2 = new Book();

// only Book class knows the answer

if (b.equals (b2)) {

}

equals must be reflexive, symmetric, transitive

January 10 2003 Lecture 2 6

Reading Questions

Object state is set by the object’s operations. Is this the only way to set an object’s state?

No: 1) constructor can set initial state

2) class attributes can be pre-set

upon construction

3) Thread of computation within object can alter values

January 10 2003 Lecture 2 7

Reading Questions

What is the difference between uni-directional and bi-directional associations?

Category Book1 *

The underlying link between the objects is one-way for unidirectional associations but two-way for bi-directional associations

Category

Books:Book[*]

Book

January 10 2003 Lecture 2 8

Reading Questions

What is the practical impact of maintaining a bidirectional association?

1) referential integrity

2) simultaneous creation

A B

A B

A

January 10 2003 Lecture 2 9

Binary Associations

Familiar to C programmers

Links are realizations of associations Unidirectional: object reference (pointer)

Bi-direction: more than doubly-linked list

What is accessscope of object?

What are core operations?

January 10 2003 Lecture 2 10

Binary Association Alternatives

Does Book know of its Category “parent”?

Does Category maintain “ownership” over the aggregate set?

Can a change to a Book impact/invalidate set?

Who is responsible for add/remove?

Does the relationship scale to 10,000 books? 10,000,000 books?

Category Book1 *

January 10 2003 Lecture 2 11

N-ary associations

How common are they?a complex set of binary associations can be greatly simplified through an n-ary association

Committee

Year

Person

post

<c,p>

<y,p>

<y,c>

0..2

1..4

3..5

Committee

Year

Person

Post

*

*

*

*

**

January 10 2003 Lecture 2 12

Aggregation and CompositionNormal AssociationAggregation

special binary association between aggregate (the set) and its elements“operations on whole are propagated to subparts”

Composition stronger than aggregation, implies that a sub-part is integral part of greater whole “whole cannot exist without subpart” but “subpart can exist without whole”

Differences are subtle and may be refined later.

January 10 2003 Lecture 2 13

Aggregate Composition Quiz

Category –––– Book

Frame –––– Button

Canvas –––– Font

Course –––– Student

Polygon –––– Point

Student –––– Identifier

January 10 2003 Lecture 2 14

Aggregation/Composition Summary

Characteristic Normal Association

Aggregation Composition

UML

Ownership None Weak Strong

Multiplicity Any Any One : Any

Transitivity No Yes Yes

Propagation of properties

Undefined Whole to Part Whole to Part

January 10 2003 Lecture 2 15

Servant Classes & Delegation

To reduce complexity and increase cohesion

create new class as servant to existing class

servant object has only one “master”

operations are delegated to servant object

Lau claims this leads to higher abstraction and better reuse. Why?

January 10 2003 Lecture 2 16

Inheritance vs. Servant

Point

real x, y

Circle

Circle uses (x,y) as center

But this can’t be right!

Point

real x, y

CirclePoint center

abstract

reuse

January 10 2003 Lecture 2 17

Monday Assignments

Read: TAOO: Chapter 2.1 – 2.4

Modeling exercisetry to model Person, Committee, Year as only binary relationships

try to model Post as a separate class to manage Person/Committee/Year relationships

Exercise 2-1, 2-2

January 10 2003 Lecture 2 18

References

none