Object Classes
In UML
Object Concepts
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What is an object?
How do objects communicate?
How is an object’s interface defined?
What have objects to do with components?
Object
• Thing
– Which of the following are objects: hammer, sphere,
chemical process, fog, river, anger, cat, grayness, gray
cat?
• State
– Encapsulated in the data in its attributes
• Behaviour
– Understands and reacts to certain messages - can
receive messages and act on them.
• Identity
– Can be a name, but shouldn’t be dependent on the
values of its attributes.
Example myClock
• Understands the messages
reportTime
resetTimeTo(07:43),
resetTimeTo(12:30)
I.e. resetTimeTo (newTime)
for any suitable value of newTime.
– The object’s interface says it accepts messages of the
form reportTime() and
resetTimeto(newTime:Time)where Time is a
type whose elements are the sensible values of time.
Messages
• A message contains a keyword called a
selector - e.g. ResetTimeTo or
ReportTime
• A message can have zero or more
arguments, as specified by the definition of
the message in the interface.
Interfaces
• The object’s public interface defines which messages it
will accept regardless of where they come from.
• An object can send itself a message.
• The private interface is usable by itself and other
privileged parts of the system - this specifies the
attributes of the objects - not viewable to the public.
E.g. a clock will have an attribute Time.
Classes
• A class of objects is a group of objects that have
the same attribute types and behavior
• Rather than specifying each object, a class
definition shows how the object is defined and
behaves.
• A Clock class could contain an object
myClock. The Clock class has an
operation resetTimeTo(newTime:Time);
i.e. all of the clocks can reset time.
• myClock is an instance of the Clock class.
Interfaces
• The object’s public interface defines which messages it
will accept regardless of where they come from.
– A public interface is a method that can be operated on the
class.
• An object can send itself a message.
• The private interface is usable by itself and other
privileged parts of the system - this specifies the
attributes of the objects - not viewable to the public.
E.g. a clock will have an attribute Time.
– A private interface is an attribute that is held about the class.
What is a class?
• A business class is a group of objects that
have similar attributes and operations.
• Objects are nouns.
• Only objects that need to have data about
them recorded are suitable candidates.
• A user may be a business object, but
probably is not.
Starting a class diagram
• Choose your candidate objects from the
system description.
• For each candidate:
– Is there more than one object in this class?
– Is this object a system user / location?
– Can you describe the type of information you
need to know about each member of this group
of objects?
What is an attribute?
• An attribute is the type of information you
need to know about each object in a class.
• An attribute is an attribute when:– it has a finite length
– it has a single value for each object
• An attribute is not an attribute when:– it in turn has several attributes
– it has multiple values
What is an association?
• An association is a relationship between
object classes.
• An association is used to implement a link
between objects – to send messages or
instructions from one class to another.
– An object from one class can invoke a method
on an object from another class, thereby
accessing it through its public interface.
Operations
• What sort of operations are there in a class?
– Constructor operations create new objects in
the class.
– Selector operations get information about and
from an object in a class.
– Mutator operations set information about and
to an object in a class.
– Destructor operations destroy objects of the
class.
Starting class diagrams
• Rather than trying to define the entire class
– concentrate on the data that is required for
business classes
– set up a data model, that can be converted into a
relational database
– later, loosen the structure, to give objectoriented advantages
– add operations on the data.
Object oriented modelling and design
• Introduction
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The world as seen by the object
object instances and classes
object identifiers
Super and sub classes
Interaction between classes
function
three models
time
data
• Object model
• Static structure of objects in a system and their
relationships
• Contains class diagrams
• functional model
• data value transformations within a system - Use case
and object interaction diagrams
• dynamic model
• describes aspects that change over time: state transition
diagrams
Object modelling
• Object - Class (v instance)
• Attributes (v values)
• Abstraction and encapsulation
– Abstraction is when a client of a module doesn’t need
to know more than is in the interface.
– Encapsulation is when a client of a module isn’t able to
know more than is in the interface.
• Method - implementation of an operation for class
• Method signature - number and types of arguments; type of
result value
• Notation of object instances and classes
Sample object classes
«business»
::Person
«business»
::Elevator
age
name
social security number
Capacity
Highest floor
Location
Lowest floor
Model type
Speed
Go down
Go up
stop
«business»
::Telephone call
Hang up time
Number
Start time
date
Hang up
Play message
Sound dial tone
dial
find connection
get cost
play fast busy tone
ring bell
slow busy tone
sound loud beep
valid number
1..*
makes
1
«business»
::Caller
These are examples of object
classes; some have operations
associated with them. All have
attributes associated with them.
Adding relationships
• Links
• Physical or conceptual connection between object
instances
• Associations
• Groups of links with common structure and common
semantics
• bi-directional; forward and inverse
• direction implied by the name is forward
Link Notation
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class associations
one to one
zero or many
one to many
many to many
1
1
*
1
1..*
1..*
1..*
Sample associations
«business»
::Country
1
has capital
name
«business»
::Workstation
1
«business»
::City
name
«business»
::Window
0..1
1
console
Close
Display
Hide
Open
Examples
• Identify some of the classes that you would
expect to find in each of the following
systems:
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A system for a library
A system to manage hotel bookings
A mail-order clothes system
An airline booking system
A system for an X-ray clinic
Class Person
• List the attributes that a person would have
in each of the following systems:
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A system for a library
A payroll system
A voting registration system
A sporting club registration system
A dentist’s system