This document provides an introduction to object-oriented analysis and class diagrams. It discusses key object-oriented concepts like classes, objects, attributes, and behaviors. It explains how class diagrams can be used to represent these concepts and relationships between classes. Specifically, class diagrams allow developers to visualize coupling, cohesion, and the potential impact of changes, helping ensure high-quality object-oriented design. The document emphasizes that class diagrams are a useful tool for communication between developers during the analysis and design of object-oriented systems.
Unified Modeling Language ( UML) is one of the most important tool in C++ programming language.
This presentation will help you alot to understand the UML constructs and Object Oriented Programming.
Unified Modeling Language ( UML) is one of the most important tool in C++ programming language.
This presentation will help you alot to understand the UML constructs and Object Oriented Programming.
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2. +
Introduction
In this lecture we’re going to begin discussing how we actually
go about developing a complex program.
And we’ll do it through the medium of object orientation.
This is likely to be a somewhat alien way to approach a
problem for many of you.
One of the great lies of object orientation is that it is ‘easy because it
mirrors the way people think’
However, it is the conceptual understanding that will enable you
to build very complex programs as time goes by.
It’s a very powerful way to build functionality.
3. +
An Object Oriented System
A properly designed object oriented system consists of:
One or more objects.
These objects are defined by classes.
Each object consists of:
Attributes
Behaviours
Attributes contain information about the object.
Data
Behaviours are instructions for acting upon that data.
4. +
The Structure of an Object
First, we must define what we mean by an object.
In terms of object oriented programming.
An object is an instance of a class.
A class is like a blueprint telling the object what structure it has.
A class defines:
Attributes
Behaviours
The object defines
The state of attributes.
The values that each piece of data has.
5. +
Eh?
Let’s look at in the abstract.
There exists, somewhere, a blueprint for a chair.
Blueprints for the chairs on which you are sitting.
The blueprint defines what the chair looks like.
It defines the structure of the chair
It defines the relationship of the legs to the seat
This blueprint would be the class.
The specific chairs on which you are sitting would be objects.
They were made (instantiated) from that blueprint.
6. +
Uh…
The blueprint tells us how the chair is supposed to behave and
what information about that chair may be mutable.
The colour of the chair
The material of the chair
The size of the chair
The class says:
A chair has a colour, material, and size
The object says:
I am blue, made of leather, and am medium sized.
Different chairs can have the same basic structure, but different
states.
I am pink, made of suede, and am large.
7. +
Okay!
We’ll come back to this subject later.
Because regardless of what people may tell you, object orientation
does not come naturally to most people.
This however is going to be the fundamental bedrock of the systems
we analyse in the coming weeks.
The programming languages you use throughout this course
are object oriented languages.
Visual Basic .NET
Java
C#
The objects may not be emphasized to begin with.
But they’re coming.
8. +
Attributes
The class defines what attributes will be present in an object.
It has these.
We don’t know what they are yet.
Attributes are things that an object will have.
Usually things that are mutable (they can change).
Consider a human face.
It has eyes
It has a nose
It has a mouth
These are modeled in a computer program as variables.
A class is thus a collection of variables.
9. +
Behaviours
As well as these variables, a class contains behaviours for
acting upon those variables.
If the class is a human face:
Attributes: Eyes, Mouth, Nose
Behaviours: blink, smile, sniff
These are modeled in a computer program as functions.
Also called methods.
Two names for the same thing, we’ll use these interchangeably.
Behaviours / Functions / Methods
Different words, all for the same basic concept.
10. +
Behaviours
Behaviours can be broken down into further parts.
Behaviours may have variables of their own.
Temporary variables that only exist as long as the method is
executing.
These are known as ‘local variables’ in programming.
They have a limited scope within which they can be accessed.
Behaviours will usually incorporate programming structures.
Some structures allow the programmer to choose between
courses of actions.
Some structures allow the programmer to repeat blocks of code.
Behaviours will incorporate flow control.
11. +
Class Diagrams
One of the reasons why software development has such a
complex vocabulary of jargon is the need for experts to
communicate succinctly.
This is what all formal software design techniques are about.
In order for us to begin talking about how to discuss programs,
we need some kind of share vocabulary.
We don’t know how to really use them yet, but let’s talk about
how we can represent a class in a modeling language.
In this case, the language is UML
Unified Modelling Language
12. +
Object Oriented Programs
Object oriented programs introduce new complexities of
interaction.
Objects have a relationship to classes.
Objects may be composed of other objects.
Objects define their own behaviours and attributes.
Objects can offer several levels of indirection to other objects.
Gaining a clear perspective on how an object oriented program
fits together is key to developing and maintaining OO code.
Which is what our analysis and design eventually leads towards.
13. +
Object Relationships
Objects can contain references to other objects.
This is known as composition.
This is a has a relationship.
Composition relationships also imply a multiplicity (or
cardinality).
Where there is a 0 or 1 relationship, it is known as a composition.
Where it is more, it is an aggregation.
Formal UML notations have more precise differentiations of
composition.
That’s for Future Us to worry about.
14. +
Object Relationships
An object may be a specialisation of another object.
This is known as inheritance in object orientation.
This is a is a relationship.
More specialised forms of inheritance exist.
We’ll talk about these as we go along.
For now, we’ll just use the one kind.
The class from which another class inherits is called the parent.
Or Super class.
The class that does the inheriting is called the child.
Or sub class.
15. +
Examples
A Car:
Has an engine (composition)
Has four wheels (aggregation)
Has 2 or more doors (aggregation)
Is a vehicle (inheritance)
A Kitten:
Has four legs (aggregation)
Is an animal (inheritance)
Has a tail (composition)
Has the property of being as cute as the dickens (composition)
16. +
Object Oriented Programs
The relationship between objects and classes becomes very
complex in many computer programs.
We’ll see this before too long when we return to our case study.
Some means of representing this complexity ‘at a glance’ is
useful.
This is what a class diagram is used for.
It gives the static view of a program’s architecture.
It doesn’t change when the programming is running.
Other techniques exist too.
They vary in effectiveness.
17. +
Class Diagrams
At their simplest, class diagrams are just boxes with lines
connecting them to other boxes.
They show the relationship between classes only.
18. +
Class Diagrams
At the next level of usefulness, class diagrams also contain
information about attributes and methods.
Attributes in the second row
Methods in the third
At their best, class diagrams contain class relationships,
methods, attributes, and the visibility of each.
They also explicitly show multiplicity.
Class diagrams are the most fundamental unit of UML.
And also the most useful.
If you only ever take one diagraming notation to heart, let it be this
one.
20. +
Why Use A Class Diagram?
There are several reasons why using a class diagram leads to
a more complete view of a project.
It shows the relationship between classes in a way that allows you
to trace accessibility.
It shows at a glance the amount of coupling in a project.
It shows at a glance the amount of cohesion in a project.
It shows at a glance the impact of change.
These are elements that tell us ‘how good an OO design we
have’
That’s an incredibly difficult thing to assess at the moment, which is
one of the reasons why OOP is so hard to learn.
21. +
Coupling and Cohesion
One of the biggest problems OO developers have is ensuring a
good OO design.
What is a good OO design, you ask?
Certainly for people unfamiliar with how OO is best done, it’s
hard to objectively rate an object architecture.
‘Uh, sure – it’s ten good?’
Two objective measures exist for this though.
Coupling
Cohesion
As a rule, we are aiming for high cohesion and low coupling.
22. +
Coupling
Coupling relates to the amount of interconnectedness of
classes.
We want this to be as low as is reasonably possible.
There’s a reason why we don’t just link classes up in every possible
way.
A coupling relationship is any composition, aggregation, or
inheritance.
We can’t get away from these, but we can limit them.
The more connections made between objects and classes, the
harder it is to make any changes.
Because of the knock-on effect associated.
23. +
Coupling
If your program has high coupling, there are several
disadvantages built in:
Changes in one object may result in unexpected side-effects
elsewhere in a program.
Often the cause is not at all obvious.
Classes are difficult to understand at a glance.
They make use of so much external functionality.
Classes are difficult to test.
They cannot be easily extracted from their context.
We aim for low coupling, of the right kind of coupling.
More on that later.
24. +
Cohesion
Cohesion is the level to which an object adheres to a single set
of responsibilities.
We want this as high as is reasonably possible.
In a good object oriented program, each object has one firmly
defined responsibility.
A cat object would not be responsible for keeping track of its
owner’s employment.
A car object would not be responsible for storing the name of the
garage where it gets services.
Good OO design requires us to make classes that have unique,
specific responsibilities.
25. +
Cohesion
The dangers that come from low cohesion are the same as that
of high coupling.
They’re really just two ways of reflecting the same basic concept.
Low cohesion is an insidious problem.
Programs with low cohesion tend to become even less coherent
over time.
It is a consequence of badly analysed and badly designed
programs.
You’ll learn how to do this properly in third year.
26. +
The Tension of OO Development
Object Oriented programs are a tension between coupling and
cohesion.
We increase cohesion by increasing coupling.
We reduce coupling by decreasing cohesion.
As OO developers we have to find a happy medium between
the two.
In some cases, simple rationalization of a design can remove
problems.
That is a judgment call that comes from experience.
We’ll have plenty of opportunities to do this as we go through
the module.
27. +
Impact of Change
Both of these concepts are a way of objectively rating the
impact of change in a program.
What makes a program easy to work with?
Clearly written code.
Comments (seriously)
Changes do not have unexpected consequences.
The latter of these three is what the impact of change defines.
That when we change a piece of code, we know what is going to be
changed as a result of it.
28. +
Impact of Change
One of the reasons we restrict visibility of variables and
methods is to manage the impact of change.
If you are a Respectful Developer, you avoid making changes that
will impact on code you did not write.
Lest terrible vengeance be visited upon you and your kith and kin
The lower the visibility, the better.
Changing a local variable for example only impacts the method in
which it is defined.
Changing a class wide attribute will potentially impact on every
method in the class.
29. +
Impact of Change
In a multi-developer environment you have to assume that if
functionality is available, it has been used.
Someone saw your calculate_awesome() method and thought ‘hey,
that does exactly what I want’
If it can be used, you can’t idly make structural changes.
Refactoring is fine
Redesign is not
We’ll talk more about refactoring in a later lecture.
It basically means ‘improving code so well that nobody notices’
30. +
The Class Diagram
The best quality of class diagrams shows the impact of change
at a glance.
Where there are lots of arrows, there be coupling issues.
Where there are huge lists of methods and attributes in a class,
there be likely cohesion problems.
Where there are lots of public methods and attributes, there be likely
impact of change issues.
Doing the class diagram from the start gives us a good
opportunity to identify structural problems before we write a
single line of code.
31. +
The Class Diagram
In multi-developer projects, the class diagram represents the
‘best understanding’ of how a program is being developed.
It’s kind of an informal contract with your fellow developers.
One of the benefits we get from object orientation is an ease of
development and integration.
But only if everyone is communicating effectively.
It is important that this diagram remain up to date with the state
of the program.
It is a ‘living document’.
Some tools exist for automatically generating them.
And generating program code from class diagrams.
32. +
Conclusion
Class diagrams are awesome.
Seriously, you need to believe me on that.
They give us a neat way of representing a number of issues
with our programs.
How classes relate
The degree of coupling
The degree of cohesion
The likely impact of change.
We can tell at a glance when our systems are going off the
rails.
Huzzah.