UML
or how to outline/describe your
well-engineered software
Communicating Your Code
Consider the band example. To
describe the code we had to write it
out.
 Darned inefficient.
 Hard to take in “what the code does” at a
glance.
Is There A Better Way?
1. Hard to communicate code!
2. Is this a good design?
1.
2.
3.
4.
Am I headed in the right direction?
How do I know?
High connectivity?
Low Cohesion?
Help!
Unified Modeling Language
Used to create diagrams of programs.
Show relationships between objects.


shows the classes/objects
shows messaging between the objects
Organizes thoughts when doing OO design
Communicates your design to others

last example was hard to understand because just
a bunch of code!
UML Class Diagrams
Represent a class as a rectangle.
Put name in Rectangle (italic if abstract)
Put methods and variables in lower
rectangles as needed (+ if public, - if private)
Meal
Meal
+getDessert() : String
+getMainDish() : String
+setMainDish(c: String)
Meal
-dessertType: String
+getDessert() : String
+getMainDish() : String
surely we could pick a better variable name!
Class Diagrams: State and
Behavior
Meal
-dessertType: String
+getDessert() : String
+getMainDish() : String
put STATE attributes here
(remember state?)
put BEHAVIOR here
(i.e., response to messages)
What about IDENTITY? Nope.
This is a class diagram, not an object diagram.
Class Diagrams: Association
Draw a line to indicate a relationship.
Library
contains many
Book
line indicates “association” between classes
frequently add a phrase to clarify the association
Class Diagrams: Association 2
When do you draw an association?
 When an object of class A sends a message to an object
of class B
 When class A creates or contains an object of class B
 if “B myObject = new B()” happens inside of class A
 When class A receives a message containing an object of
class B as an argument.
 When it is not confusing (don’t draw one every bloody
time you use a String or Character or Boolean…!)
Class Diagrams: Association 2
In other words, draw an association
when some object of class A has to
know about an object of class B.
Library
works in
Librarian
Book
contains many
?
Paper
necessary
in a library
checkout
system?
?
Binding
?
Zebra
Class Diagrams: Association
Example
Library
Book
contains many
+isOverdue() : boolean
uses
Patron
+isDonor() : boolean
borrows
Class Diagrams: Multiplicity
Can be more specific about the association.
1. How many objects will be instantiated from each
class!
2. And/or how many of each are associated.
Library
There is only
1 library
there are 0 to
arbitrarily many
Patrons
1
contains many
1..*
1
0..*
uses
Patron
0..*
borrows
Book
0..5
more than one
patron can check
out a single ebook
each patron may borrow
0 to 5 books
Class Diagrams: Inheritance
(or Generalization)
lines
indicate an
association
VegiMeal
abstract class (italics)
Meal
MeatMeal
open arrow means
VegiMeal “is a type of”
Meal. (i.e., VegiMeal and
MeatMeal extend Meal.)
regular classes (not italics)
I can see the relationships between classes.
Class Diagrams: Inheritance
Example
Suppose I give you the following classes:







Student
Person
Masochist
Undergrad,
GradStudent
Teacher
Classroom
Can you tell me how these are related?
 What inherits from what? Is there more than one
possibility?
 Which ones are abstract?
Use UML!
You can
only
guess
at my
intent!
Class Diagrams: Inheritance
Example (cont.)
Person
Classroom
uses
1..*
+age : int
1..*
+getName() : String
+setName(n : String)
Student
Can Teacher extend
the not abstract
Masochist? Sure…
Masochist
Teacher
+grade: double
Undergrad
GradStudent
What does this diagram tell
you about the class structure?
Handy, huh?
Class Diagrams: Multiple
Inheritance
Why are you punishing yourself?
 Ok, if you insist…
Mammal
Animal
Furry
Class Diagrams: Interfaces
An interface is generally shown in one of
three ways.
 You may see them all.
 Use your favorite.
Car
<<interface>>
Car
Driveable
Car
Driveable
Driveable
Class Diagrams: Aggregation
Aggregation is when a class is part of
another class.
 A Building contains People
 An Airport contains Aircraft
 A Library contains Books
But it could contain none at all (empty
building).
AND People, Aircraft, and Books don’t
disappear if we delete the Building, Airport,
and Library. (We say the airport doesn’t own the aircraft.)
Class Diagrams: Aggregation
Aggregation is usually a collection of
things.
 ResidentList
 Forest
(aggregates trees)
Not required to show aggregation in
UML – only if it make an association
clearer!
Class Diagrams: Aggregation
(Example)
ResidentList
1
open diamonds indicate “aggregation.”
A Resident “is a part of” a ResidentList.
(Or there are many Residents in a
ResidentList.)
1..*
Resident
lines indicate “association”
means there are anywhere from
1 to an infinite number (*) of Residents
means there is just one (1) ResidentList
Class Diagrams: Aggregation
Code Example 1
Lunch contains a Sandwich.
public class Lunch
{
private Sandwich sandwich;
}
public Lunch(Sandwich sandwich)
{
this.sandwich = sandwich;
}
The sandwich is created outside the Lunch.
So it does not go away when the Lunch goes
away. Hence aggregation!
Class Diagrams: Aggregation
Code Example 2
Lunch contains a list of meal items.
public class Lunch
{
private List meal = new ArrayList();
public void addLunchItem(Object lunchItem)
{
meal.add(lunchItem);
}
}
public void removeLunchItem(Object lunchItem)
{
meal.remove(lunchItem);
}
The lunchItems are created outside the Lunch.
So lunchItems do not go away when the Lunch
goes away. Hence aggregation!
By the way, can you tell me why the uses of
List and Object are good engineering? (Casting!)
Class Diagrams: Composition
Very similar to aggregation, but in
composition the parts disappear if we erase
the whole.
 Book contains Pages
 Deleting the Book class would eliminate the instances of
the Pages as well.

We say that “the Book wholly owns the Pages”

The Pages have no reason to exist without the book.
Class Diagrams: Composition
Example
Book
1..*
Solid diamond indicates composition.
0..*
Paper
So what’s this diagram mean?
Class Diagrams: Solid or Open
Diamond?
A mnemonic:
 When you erase a solid diamond, you have to
erase the border and the interior.

just as when you erase the composite class you
have to erase the parts as well.
 When you erase an open diamond, you only
have to erase the border.

just as when you erase the aggregate class, the
insides don’t have to be erased.
Class Diagrams: Composition
Code Example
public class Ocean
{
private Water water;
}
public Ocean()
{
water = new Water();
}
The Water is created inside the Ocean.
So it goes away when the Ocean goes
away. Hence composition!
Barn Example
Draw a class diagram for a barn that
contains some animals such as chickens,
horses, cows, and calfs. A farmer milks
any cows in the barn, and the animals
eat hay bales that are stored in the barn.
The barn is made of wood planks.
Barn Example: Classes
Potential classes
 Barn, Animal, Chicken, Horse, Cow, Calf, HayBale,
WoodPlanks, Farmer


Calf and Cow same class? Calf extends Cow (implies has
additional behavior)? Just set youth attribute in class
Cow? (Depends on intent of the customer that ordered
the “barnyard” product.)
Chicken, Horse, Cow extend Animal
Potential methods
 milks, eats
Barn Example: Class
Associations (And UML)
WoodPlank
Chicken
is a
made of
Barn
live in
Animal
is a
Horse
+eats()
stored in
is a
Cow
HayBale
milks
Farmer
+milks()
is a
Calf
+doPrance()
Barn Example: Class
Aggregation
Barn contains the Animals and the
HayBales. That’s a clue to the UML
structure.
Aggregation! The Animals and
HayBales won’t disappear if we get rid
of the Barn.
Barn Example: Revised UML
WoodPlank
Chicken
is a
made of
Barn
live in
Animal
is a
Horse
+eats()
stored in
is a
Cow
HayBale
milks
Farmer
+milks()
is a
Calf
+doPrance()
Barn Example: Class
Composition
The WoodPlanks serve no purpose
except to be part of the structure of the
Barn. That’s a clue to the UML
structure!
Composition! These instances of the
WoodPlanks will disappear if the Barn is
removed.
Barn Example: Revised UML
WoodPlank
Chicken
is a
made of
Barn
live in
Animal
is a
Horse
+eats()
stored in
is a
Cow
HayBale
milks
Farmer
+milks()
is a
Calf
+doPrance()
Barn Example: Class
Multiplicities
There is only one Barn (1), but there are an
unspecified number of Animals (0..*) and
HayBales (0..*).
The barn is made of at least one and
probably many planks (1..*).
There is only one Farmer, but he may interact
(by sending “milking messages”) with many
Cows (0..*).
Barn Example: Revised UML
WoodPlank
Chicken
1..*
1
is a
made of
Barn
live in
1
0..*
Animal
is a
Horse
+eats()
1
stored in
0..*
is a
Cow
HayBale
0..*
milks
1
Farmer
+milks()
is a
Calf
+doPrance()
Barnyard Changes!
What if the farmer now builds a chicken coop
made of 100 wood planks? And the chickens
live in the coop?
new class ChickenCoop.
Composition is WoodPlanks with
multiplicity 100.
Chicken association changes.
Did We Do a Good Job Engineering the
Original Barnyard?
WoodPlank
1..*
1
ChickenCoop
1
1..*
1
made of
Barn
0..*
is a
live in
1
Chicken
live in
0..*
Animal
is a
Horse
+eats()
1
stored in
is a
0..*
Cow
HayBale
0..*
Sorta’. Have to get rid of the Animal to Barn
association. (Not all animals live in the barn.)
Must replace with associations to Cow and
Horse. Higher coupling!
milks
1
Farmer
+milks()
is a
Calf
+doPrance()
Better Engineering of
WoodPlank
Barnyard
1..* made of
1..*
Barn
1
Building
ChickenCoop
1
Chicken
stored in
0..*
HayBale
live in
is a
0..*
Animal
The Barn
takes a list of
occupants
(e.g., from a
constructor)
is a
Horse
+eats()
Why is this better?
is a
Lower coupling! And, the
ChickenCoop class aggregates
Chickens, but not the other
animals. And Barn won’t
aggregate Chickens.
Cow
0..*
1
milks
Farmer
+milks()
is a
Calf
+doPrance()