1. Object-Oriented Programming
Introduction
What is an object
Collaboration Among Objects
Classes
Relationships Between Classes
“Use”, “Has”, “Is a”
Vocabulary

2. Introduction ...
Let's try for example to see how to implement Bethe-Bloch formula
where
β = v / c
v velocity of the particle
E energy of the particle
x distance travelled by the particle
c speed of light
ze particle charge
e charge of the electron
me rest mass of the electron
n electron density of the target
I mean excitation potential of the target
...

3. ... Introduction ...
In traditional (algorithmic) programming, we would implement a function that uses all parameters
CalculateDeDx(v, E, x, c, q, e, me, n, I, ...)
This would introduce in our program 10 (or more) independent variables which we would have to handle (initialize, update etc.)
In Object-Oriented programming, we first identify the entities with properties which belong together:
v velocity of the particle
E energy of the particle
x distance travelled by the particle
q particle charge
c speed of light
e charge of the electron
me rest mass of the electron
n electron density of the target
I mean excitation potential of the target
...
Particle properties
Physical constants
Material properties

4. ... Introduction
When we group the properties in the entities, we can simplify the CalculateDeDx() function:
CalculateDeDx(particle, material, constants)
We have introduced 3 objects to hold the related variables
We can now let the particle to execute CalculateDeDx() using material and constant parameters:
particle.CalculateDeDx(material, constants)
speed
e density
speed of light
energy
charge of e
Ion. potential
charge
mass of e
distance
Calculate
DeDX()
Particle
Material
Constants
We have intuitively introduced 3 objects
in order to describe the “Bethe-Bloch formula”

5. What is an object ? “Something you can do things to” (G. Booch)
An object = an entity which has its state and behaviour
The object retains its state within one or more variables
Variable = data appointed by an identifier
The object implements its behaviour by the methods
Method = function associated with an object
State
(variables)
variable
variable
Behaviour
(methods)
variable
An object is an entity comprised of variables and methods

6. What is an object ? Examples of objects: Particle Material speed
CalculateDeDx
speed
e density
energy
Move
charge
Ion. potential
distance
...
Particle
Material

7. Collaboration Among Objects
The object appears as a component of a program that contains many other objects
It is through the interaction of its objects that the program operates at a higher level of functionality and complexity
The objects interact with each other by sending messages
The message includes:
The message receiver to whom the message is sent (Particle)
The name of the method to be executed (CalculateDeDx)
The parameter (or parameters) that the method requires (material, constants)
CalculateDeDx(material, constants)
CalculateDeDx
You
Particle

8. Classes
In the real world we have a number of objects of the same type:
The proton and neutron are two of many particles in the world
Speaking OO (Object Oriented) - we say that the proton is an instance of the class of objects known as particles
A class is a prototype that declares properties common to a set of objects (variables and methods)
It also appears as a mold or a prototype from which it is possible to create objects
An instance is a case, a realization of an object
CalculateDeDx
CalculateDeDx
CalculateDeDx
2212
2112
PDG
PDG
PDG
Move
charge
Move 1
Move
charge
charge
energy
energy
energy
10 keV
15 keV
Proton
(An instance)
Neutron
(An instance)
The Particle class

9. Classes – UML notation
UML (Unified modelling language) = a graphical language for modelling data and their treatment
Particle
PDG
charge
energy
CalculateDeDx()
Move()
Class name
CalculateDeDx
PDG
Data members or attributes
Move
charge
energy
Function members or methods
The Particle class
If the details of the class are not important in the context, the class can be well represented by a simple rectangle
Particle
Class name

10. Encapsulation and Information Hiding ...
We differentiate the external from the internal – the interface from the implementation
We hide the inside from the clients
Don't need to understand the methods and internal variables
Unaffected by their nature
Unaffected by their changes
There is a "wall" around an object
No other object can become dependent on internal state and services
Public services
(Interface)
variable
variable
variable
Implementation
State
(variables)
Internal services
(not-accessible from outside)

11. Polymorphism ...
A class (A) can inherit the state (variables) and behaviour (methods) of another class (B)
The relationship "Is a" ( "Inherits from")
We say class A is a base class (or a generalization, or a super-type) of class B and class B is a class derived from Class A (or a specialization, or a subtype)
For example, leptons, hadrons, bosons are all particles, but belong to different groups, each with their special properties
Particle
Lepton
Hadron
Boson

12. ... Polymorphism
Derived classes share the same states and the same methods, but they can add their own data members and/or methods
Eg. depending on the particle subtype, they may have 2 quarks or 3 quarks
Derived classes may also redefine the method inherited from its super-class and provide their specialized implementation
Eg. Bethe-Bloch formula is not suitable for calculating DeDx for electrons, we may need to redefine CalculateDeDx()
Polymorphism
An ability to create an object that has more than one form
The objects belonging to different types can respond to method, field, or property calls of the same name, each one according to an appropriate type-specific behavior.
The programmer (and the program) does not have to know the exact type of the object in advance, and so the exact behavior is determined at run-time (this is called late binding or dynamic binding).
When processing a particle of electron type, its special implementation of CalculateDeDx() will be called

13. Vocabulary
Object = a computing entity comprised of variables and methods
Variable = data appointed by an identifier
Method = function associated with an object
Message = the means of interaction and communication among objects
Class = a prototype that declares properties common to a set of objects (variables and methods)
Instance = is a case, a realization of an object of a given class
Encapsulation = packaging of object variables behind its methods
Polymorphism = ability to handle different objects in a common manner