Transcript Program Development

CS413
Object Oriented Methodology
Objects
An object is a bundle of variables and related methods.
A method is a procedure (code) to manipulate variables
Procedural Languages: written with very little reference to
the real world objects
Object Oriented is simulation of real-world objects
Object Oriented methodology: a program simulates the
states and activities of real world objects
An object variable is a single variable from an object's data
structure
A method is an operation which can modify an objects
behaviour. Change an object by manipulating it's
variables.
Only an object's methods can modify it's variables
cs413_OO.ppt
CS413
Object Oriented Methodology
encapsulation = information hiding:
Cannot directly access variables from
outside the object (other objects or
modules)
Only the objects methods (procedures)
and can access the objects variables
cs413_OO.ppt
CS413
Object Oriented Methodology
Classes:
Ignore anything not directly concerning the
behaviour or state of an object
A class is a blueprint for an object
A blueprint, or outline of an object
Whether we have 1 or 1,000 such objects
A class does not represent an object
A class represents
• all the information a typical object should have
• all the methods it should have
cs413_OO.ppt
CS413
Object Oriented Methodology
Java class definition for a counter example
class Counter extends Object
{
private int MyCounter;
Counter()
{
MyCounter = 0;
}
public void InitialiseCounter(int value)
MyCounter = value;
}
public void IncrementCounter(void)
{
MyCounter++;
}
public int GetCounterValue(void)
return (MyCounter);
}
}
cs413_OO.ppt
{
{
CS413
Object Oriented Methodology
JAVA
All new classes must be defined with the extension
extends Object
This defines the superclass
There are no public or private sections
All variables and methods are prefixed with the
appropriate qualifier
private :
These define the state of the object
They cannot be accessed from outside the
class declaration.
This is encapsulation
public :
Contains all the object's methods
Can be accessed outside the class
declaration
Methods are the only means of
communication with the object
cs413_OO.ppt
CS413
Object Oriented Methodology
JAVA
Class Constructor:
• a method with the same name as the class
definition
• should contain any start-up code for the object
Instantiating (creating) objects in Java
Counter i;
Variable ‘i’ is an instance of Counter object
i = new Counter();
• define a variable to reference the object in the first line
• create an instance of the object by a call to new in the
second line
cs413_OO.ppt
CS413
Object Oriented Methodology
Benefits of Object Oriented Programming:

Encapsulation - cannot alter the value of the counter
other than by incrementing it or setting it to a initial
value. This reduces possible bugs.

Modularity - different programmers or teams can
work
on different independent objects.
Inheritance – objects can have the same variables as
higher order objects
cs413_OO.ppt
CS413
Object Oriented Methodology
Subclass - a class definition which takes functionality from a
previous class definition
Only define the additional information
class ReverseCounter extends Counter
{
public void DecrementCounter(void)
{
MyCounter--;
}
}
The new class ReverseCounter is a subclass of
Counter
Instantiate a subclass ReverseCounter object, we can
use any method that the superclass Counter provides
i.IncrementCounter();
Superclass Counter
i.DecrementCounter ();
Private int MyCounter;
Subclasses cannot use MyCounter
protected int MyCounter;
cs413_OO.ppt
Subclass ReverseCounter
Allow subclasses to use MyCounter
CS413
Object Oriented Methodology
blueprint
Superclass
Variables
Methods
object
object
Subclass
Subclass
Variables
Variables
Methods
Methods
cs413_OO.ppt