Using UML in Class Design

When designing a class it is often helpful to draw a UML diagram.
UML
stands for
Unified
Modeling Language
. The UML provides a set of standard diagrams for graphically
depicting object-oriented systems. Figure E-1 shows the general layout of a UML diagram
for a class. Notice that the diagram is a box that is divided into three sections. The top
section is where you write the name of the class. The middle section holds a list of the
class’s member variables. The bottom section holds a list of the class’s member functions.
For example, in Chapter 13, Introduction to Classes, you studied a
Rectangle
class that
could be used in a program that works with rectangles. The class has the following member
variables:
•
width
•
length
The class also has the following member functions:
•
setWidth
•
setLength
•
getWidth
•
getLength
•
getArea
Figure E-1
Class name goes here
Member variables are listed here
Member functions are listed here
2
Appendix E: Using UML in Class Design
From this information alone we can construct a simple UML diagram for the class, as
shown in Figure E-2.
The UML diagram in Figure E-2 tells us the name of the class, the names of the member
variables, and the names of the member functions. Compare this diagram to the actual
C++ class declaration, which follows.
class Rectangle
{
private:
double width;
double length;
public:
void setWidth(double);
void setLength(double);
double getWidth() const;
double getLength() const;
double getArea() const;
};
The UML diagram in Figure E-2 does not convey many of the class details, such as access
specification, member variable data types, parameter data types, and function return
types. The UML provides optional notation for these types of details.
Showing Access Specification in UML Diagrams
The UML diagram in Figure E-2 lists all of the members of the
Rectangle
class but does
not indicate which members are private and which are public. In a UML diagram you may
optionally place a
-
character before a member name to indicate that it is private, or a
+
character to indicate that it is public. Figure E-3 shows the UML diagram modified to
include this notation.
Figure E-2
Rectangle
width
length
setWidth()
setLength()
getWidth()
getLength()
getArea()
Appendix E: Using UML in Class Design
3
Data Type and Parameter Notation in UML Diagrams
The Unified Modeling Language also provides notation that you may use to indicate the
data types of member variables, member functions, and parameters. To indicate the data
type of a member variable, place a colon followed by the name of the data type after the
name of the variable. For example, the
width
variable in the
Rectangle
class is a
double
.
It could be listed as follows in the UML diagram:
- width : double
The return type of a member function can be listed in the same manner: After the function’s
name, place a colon followed by the return type. The
Rectangle
class’s
getLength
function returns a
double
, so it could be listed as follows in the UML diagram:
+ getLength() : double
Parameter variables and their data types may be listed inside a member function’s parentheses.
For example, the
Rectangle
class’s
setLength
function has a
double
parameter
named
len
, so it could be listed as follows in the UML diagram:
+ setLength(len : double) : void
Figure E-4 shows a UML diagram for the
Rectangle
class with parameter and data type
notation.
Figure E-3
NOTE:
In UML notation the variable name is listed first, then the data type. This is
opposite of C++ syntax, which requires the data type to appear first.
Rectangle
- width
- length
+ setWidth()
+ setLength()
+ getWidth()
+ getLength()
+ getArea()
4
Appendix E: Using UML in Class Design
Showing Constructors and Destructors in a UML Diagram
There is more than one accepted way of showing a class’s constructor in a UML diagram.
In this text we show a constructor just as any other method, except we list no return type.
Figure E-5 shows a UML diagram for the first version of the
InventoryItem
class, which
was discussed in Chapter 13. This version of the class has one constructor, which accepts
three arguments.
Figure E-6 shows a UML diagram for the second version of the
InventoryItem
class,
which has three overloaded constructors, as well as some additional member functions.
Figure E-4
Figure E-5
Rectangle
- width : double
- length : double
+ setWidth(w : double) : void
+ setLength(len : double) : void
+ getWidth() : double
+ getLength() : double
+ getArea() : double
InventoryItem
- description : char*
- cost : double
- units : int
+ InventoryItem(desc : char*,
c : double, u : int) :
+ ~Inventory() :
+ getDescription() : char*
+ getCost() : double
+ getUnits() : int
Appendix E: Using UML in Class Design
5
Aggregation UML Diagrams
Aggregation occurs when a class contains an instance of another class as a member. You show
aggregation in a UML diagram by connecting two classes with a line that has an open diamond
at one end. The diamond is closest to the class that contains instances of other classes.
For example, suppose we have the
PersonalInfo
class shown in Figure E-7, which holds
information about a person.
Figure E-6
Figure E-7
InventoryItem
- description : char*
- cost : double
- units : int
+ InventoryItem():
+ InventoryItem(desc : char*) :
+ InventoryItem(desc : char*,
c : double, u : int) :
+ ~Inventory() :
+ setDescription(d : char*) : void
+ setCost(c : double) : void
+ setUnits(u : int) : void
+ getDescription() : char*
+ getCost() : double
+ getUnits() : int
- customerName : char*
- customerAddress : char*
- customerCity : char*
- customerState : char*
- customerZip : char*
+ PersonalInfo(name : char*,
address : char*,
city : char*,
state : char*,
zip : char*) :
+ getName() : char*
+ getAddress() : char*
+ getCity() : char*
+ getState() : char*
+ getZip() : char*
PersonalInfo
6
Appendix E: Using UML in Class Design
Suppose we also have the
BankAccount
class shown in Figure E-8, which holds the balance
of a bank account, and can perform operations such as making deposits and withdrawals.
Figure E-9 shows a UML diagram for another class,
BankCustomer
, which contains
instances of the
PersonalInfo
and
BankAccount
classes as members. The relationship
between the classes is shown by the connecting lines with the open diamond. The open
diamond is closest to the
BankCustomer
class because it contains instances of the other
classes as members.
Inheritance in UML Diagrams
You show inheritance in a UML diagram by connecting two classes with a line that has an
open arrowhead at one end. The arrowhead points to the base class. For example,
Figure E-10 shows a UML diagram depicting the relationship between the
GradedActivity
and
FinalExam
classes that you studied in Chapter 15. The arrowhead points toward the
GradedActivity
class, which is the base class.
Showing Protected Members
Protected class members may be denoted in a UML diagram with the
#
symbol. In the second
version of the
GradedActivity
class, in Chapter 15, the
score
member variable was
declared
protected
. Figure E-11 shows a UML diagram for the class.
Figure E-8
BankAccount
- balance : double
- interestRate : double
- interest : double
+ BankAccount(startBalance : double,
intRate : double) :
+ deposit(amount : double) : void
+ withdraw(amount : double) : void
+ addInterest() : void
+ getBalance() : double
+ getInterest() : double
Appendix E: Using UML in Class Design
7
Figure E-9
- info : PersonalInfo
- checking : BankAccount
- savings : BankAccount
+ BankCustomer(i : PersonalInfo,
c : BankAccount,
s : BankAccount) :
+ getCheckingBalance() : double
+ getSavingsBalance() : double
+ checkingDeposit(amt : double)
: void
+ checkingWithdrawal(amt : double)
: void
+ savingsDeposit(amt: double) : void
+ savingsWithdrawal(amt : double)
: void
BankCustomer
BankAccount
- balance : double
- interestRate : double
- interest : double
+ BankAccount(startBalance : double,
intRate : double) :
+ deposit(amount : double) : void
+ withdraw(amount : double) : void
+ addInterest() : void
+ getBalance() : double
+ getInterest() : double
- customerName : char*
- customerAddress : char*
- customerCity : char*
- customerState : char*
- customerZip : char*
+ PersonalInfo(name : char*,
address : char*,
city : char*,
state : char*,
zip : char*) :
+ getName() : char*
+ getAddress() : char*
+ getCity() : char*
+ getState() : char*
+ getZip() : char*
PersonalInfo
8
Appendix E: Using UML in Class Design
Figure E-10
Figure E-11
- score : double
+ GradedActivity() :
+ GradedActivity(s : double) :
+ setScore(s : double) : void
+ getScore() : double
+ getLetterGrade() : char
GradedActivity
- numQuestions : int
- pointsEach : double
- numMissed : int
+ FinalExam() :
+ FinalExam(questions : int,
missed : int) :
+ set(questions : int,
missed : int) : void
+ getNumQuestions() : double
+ getPointsEach() : double
+ getNumMissed() : int
FinalExam
# score : double
+ GradedActivity() :
+ GradedActivity(s : double) :
+ setScore(s : double) : void
+ getScore() : double
+ getLetterGrade() : char
GradedActivity