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Classes and Class Diagrams
for
Requirements Analysis
(continued…)
Lecture 21
Finding Attributes
Associations:
Implementation point of view
n Two classes A and B are associated, if:
1. An instance of class A sends a message to
an instance of class B
2. An instance of class A creates an instance
of class B
3. An instance of class A has one or more
attributes whose values are instances of
class B
4. An instance of class A receives a message
with an instance of class B as an
argument
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Implementation of Point 3
n An instance of class A has one or more
attributes whose values are instances
of class B
class A {
B b; // declare ‘b’
void somethingACanDo() {
b = anotherInstanceOfClassB;
UML Representation of Point 3
A
A B
A B
more precisely
Conversely…
A
A B
B
A
B
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Example
RetailShop Item
sells
What if information about a particular sales has to be
preserved? For example:
- Shop Number
- Item Number
- Date of Sale
- Sale Value
- Warranty Type
- Warranty Expiry Date
The Association Class
RetailShop Item
sells
Sale
Also known as LINK ELEMENT in Rational Rose
Another example
Student Course
takes
Grade
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Implementation of Student
#include "Grade.h"
class Student
public:
Grade* get_Grade();
void set_Grade(Grade* ptr);
private:
Grade* grade;
Grade* Student::get_Grade()
return grade;
void Student::set_Grade (Grade*
ptr)
grade = ptr;
Additional Comments
The problem in the Student class is that a particular
Student has several links (and an equal no. of
Grade instances) with Course objects. The
get_Grade and set_Grade methods however do not
specify which Grade instance to get or set.
After you understand the problem, here are few tips to
solve the problem:
1. #include “Course.h”
2. Grade* Student::getGrade(Course* c);
3. Grade* Student::setGrade(Grade* ptr, Course* c);
Implementation of Course
#include "Grade.h"
class Course
public:
Grade* get_Grade();
void set_Grade(Grade* ptr);
private:
Grade* grade;
Grade* Course::get_Grade()
return grade;
void Course::set_Grade (Grade*
ptr)
grade = ptr;
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Format of a CRC Card
Responsibilites of a class are listed in this section
Collaborations with other classes are listed here, together with a brief description of the purpose of the collaboration
Responsibilities Collaborations
Class Name
15 cm
8 cm
Process of using CRC Cards
n Identify classes/ objects
n Allocate each object to a team member who will role
play the object
n Role play a scenario of the use case exploring and
recording responsibilities and collaborations
n Identify and record any missing or redundant objects
Library Member borrows a
Copy of a Book
Maintain data about one book
Maintain a count of borrowable copies
Responsibilities Collaborations
Book
Maintain data about the number of books borrowed
Make request to borrow copy
Make request to return copy
Copy
Copy
Responsibilities Collaborations
LibraryMember
Maintain data about a particular copy of a book Inform corresponding Book when borrowed
Inform corresponding Book when returned
Book
Book
Responsibilities Collaborations
Copy
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Summary
n Review of Finding class attributes and
behaviors
n Association Class (Link Element)
n CRC Cards
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