1. Introduction to Object-Oriented Software Development
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2. Object-oriented modelling involves classes functioning as types describing their instances which are called objects.
- You look around, and you see one particular chair, a table, a person, and you realize that the objects are related - the person is sitting on the chair by the table.
- A system developed by using the object-oriented technique can be seen as a system consisting of a number of objects which cooperate to solve a task.
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3. Object-oriented modelling includes description of: object properties links between objects object behaviour
- You look around, and you see one particular chair, a table, a person, and you realize that the objects are related - the person is sitting on the chair by the table.
- A system developed by using the object-oriented technique can be seen as a system consisting of a number of objects which cooperate to solve a task.
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4. Objects Icons representing “real world objects” of different types.
myChair:
:Bus
:Plane
:Truck
:Ship
PN62001:Car
Represented in the UML notation.
Anonymous object, only type is given
Object name and no type
Object name (id) and type
At this moment you have x number of busses, one of them is represented by the buss picture.
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5. Classify Objects = Decide Their Classes
Some objects share the same type of qualities so we may formulate a general concept – that is to define a class.
This is a form of abstraction - some of the differences between the objects are “abstracted away” (ignored).
Furniture
myChair:
yourChair:
theBrownSofa:
(a person has a name, the same number of chromosomes, ... )
A class describes a set of objects that have the same type of attributes, behaviour and relationships.
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6. Class and Objects in the UML Notation
Person
SSN
name
address
drive()
Run()
...
class
attributes
operations
(behaviour)
Class name
person1:Person
SSN =
name= Jane
address=Norway
person2:Person
SSN =
name = James
address = USA
objects
You when you declare the class you select which attributes and operations (behavior) are of interest for your system.
Objects of the given class will have individual values for the specified attributes.
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7. Plato versus Aristotle
Plato talked about the ideas (classes) “behind the objects”, e.g., you have the idée horse – which in his view existed - and you have concrete horses.
The programming language Self have only prototypes (objects). This is more in accordance with Aristotle: The classes have no existents by they own, the objects have an “immanent form” which makes it an object of a certain class.
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8. Does the ideas (/concepts/classes) “exist by themselves”?
Creating an object by copying a prototype is that different from creating an object by instantiating a class?
Copy prototype ~ copy operation is creating a copy of a section of memory.
Instantiating a class ~ would involve execution of some code and may be also some copying…
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9. Making a Model
You have a problem domain and you want to make a system that solves some problem in this domain.
You single out what is essential for your problem, and you also typically simplify reality.
You make a model of your system, ”objects from reality” are mapped ”directly onto objects in the model.
chair:
car:
Tom:
Seated in
owner
model
mapping
reality
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10. Subclass and Superclass
The class of human beings
The subclass of ”female objects”
People is superclass of Female
Jane:
James:
Person
Female
Male
The classes represented in the UML notation.
Person
Female
Male
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11. Class Hierarchy ~ Reuse by Inheritance
An employee is a special type of person, so if you already have a Person class and needs an Employee class, then inheritance allows you to reuse class Person.
Person
SSN
name
address
Employee
title
department
An object of type Employee
Jane:Employee
SSN =
name = Jane
address = Norway
title = accountant
department = accounting
inheritance or specialization
Employee is a specialisation
of Person
superclass
subclass
An object of type Employee is also a Person.
An will have all attributes that a Person has plus all of its own.
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12. Multiple Inheritance UML supports multiple inheritance.
Some object-oriented theoreticians claim that multiple inheritance should not be used! (e.g., Java).
Cassette
volume : Integer
Book
CassetteBook
Some times when we describe a new class we see that this new class should have characteristics from two (or more) existing classes. It is then possible to inherit from both classes.
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13. One Common Ancestor for All Classes
Object
hashCode
In Java all classes have a common ancestor called Object, e.g., in C++ this is not the case.
Person
Car
E.g., common operations: clone(), getClass()
Employee
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14. Linking Objects
Objects are typically connected in some way or another. For example: Jane is married to James, Jane owns a car.
ownership
Person
Car
married
association
Class Model
:ownership
Object Model (model instance
of class model)
Jane:Person
someWreck:Car
Draw associations…
:married
link
James:Person
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15. Part-of Relation Some objects can be seen as compost of other objects.
Head 1 1
Arm 1
Person 2 1 2
Leg
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16. There Are Typically Many Ways to Classify Objects
Jim might be classified as a person, a male person, an employee, a student and so on, the context decides.
the classes may overlap (or they may be disjoint)
xxx:
Big box could be Person small once could be Male and Employee…
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17. Different Models May be Possible!
Person
Gender
Person
gender : String
Gender
Person
Female
Male
Female
Male
Gender
Person ?
Androgyni
Female
Male
Undecided
Big box could be Person small once could be Male and Employee…
Gender
Person
Female
Male
Androgyni
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18. Objects Have Behaviour
operations ~ behaviour
time
sending a message
Synchronous and also asynchronous “message passing”…
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19. An Object-Oriented Program
Objects work together and solve problems.
Objects have state, the values of the attributes defines the state. The state change when the values of the attributes change. Objects have identity.
Some objects are linked together.
The objects communicate by sending messages to each other (messages follows the links). Sending a message is the same as asking an object to perform one of its operations.
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