What is a class?
Answer: -
class
§ A
class is a specification or blue print or template of an object.
§ Class
is a logical construct; an object has physical reality.
§ Class
is a structure
§ Binding
the data with its related and corresponding functions.
§ Class
is the base for encapsulation.
§ Class
is a user defined data type in java.
§ Class
will act as the base for encapsulation and implement the concept of
encapsulation through objects.
§ Any
java applications look like collection of classes but whereas c- application
looks
like collection of functions.
§ Class
contains variables and methods.
Object
§ Objects
are key to understanding object-oriented technology. Look around right now
and
you will find many examples of real-world objects: your dog, your desk, your
television set, your bicycle etc.
§ Real-world
objects share two characteristics. They all have state and behaviour.
§ Object is nothing but instance (dynamic memory
allocation) of a class.
§ The dynamic memory allocated at run time for the
members [non-static variables] of
the class is known as object.
Object: Object is an instance of class, object has state and behaviours.
An Object in java has three characteristics:
· State
· Behavior
· Identity
State: Represents data (value) of an object.
What is state of the Object?
ü The data present inside object of a class at that point of time is known as state ofthe object.
Behaviour:
Ø
Represents the behavior
(functionality) of an object such as deposit, withdraw etc.
What is behavior of the object?
ü The functionalities associated with the object => Behavior of the object.ü The state of the object changes from time-to-time depending up on the
functionalities that are executed on that object but whereas behavior of the object
would not change.
Identity:
Ø
Object identity is typically
implemented via a unique ID. The value of the ID is not visible to the external user. But it is used internally by the PVM to identify each
object uniquely.
Question: -
What Object Contains?
Answer: -
Ø Object
of any class contains only data.
Ø Apart
from the data object of a class would not contains anything else.
Ø Object
of a class would not contain any functionalities or logic.
Ø Thus
object of a class would be representing only data and not represent logic.
Question: -
What is state of the Object?
Answer: -
The data present inside object of a class at
that point of time is known as state of
the
object.
Question: -
What is behaviour of the object?
Answer: -
The
functionalities associated with the object => Behaviour of the object.
The
state of the object changes from time-to-time depending up on the functionalities that are executed on that
object but whereas behaviour of the
object would not change.
How to Define a class?
§
We
can write a class to represent properties (attributes) and actions (behaviour)
of object.
§
Properties can be represented by variables.
§
Actions can be represented by Methods.
§
We
can define a class by using class keyword.
class className:
''' documenttation string '''
variables:instance variables,static and local variables
§
Documentation
string represents description of the class. Within the class doc string
is
always optional. We can get doc string by using the following 2 ways.
print(classname.
__doc__)
help(classname)
What is a Reference variable?
§
Reference
variable is a variable which would be representing the address of the
object.
§
Reference
will act as a pointer and handler to the object.
§ Since
reference variable always points an object.
§ In
practice we call the reference variable also as an object.
§
By
using reference variable, we can access properties and methods of object.
§ Suppose there is a class named Person, then see how to create its object.
§
Suppose
there is a class named Customer, then see how to create its object.
c=Customer ()
class Car:
# class attribute
Type1 = "Four
wheeler"
# instance attribute
def __init__(self, name, old):
self.name = name
self.old = old
# instantiate the Car class
Maruti = Car("Maruti", 14)
Tata = Car("Tata", 13)
# access the class attributes
print("Maruti is a {}".format(Maruti.__class__.Type1))
print("Tata is also a {}".format(Tata.__class__.Type1))
# access the instance attributes
print("{} is {} years old".format( Maruti.name, Maruti.old))
print("{} is {} years old".format( Tata.name, Tata.old))
Output
Maruti is a Four wheeler
Tata is also a Four wheeler
Maruti is 14 years old
Tata is 13 years old
There are a few things to note when looking at the above example.
§
The
class is made up of attributes (data) and methods (functions)
§
Attributes
and methods are simply defined as normal variables and functions
§
As
noted in the corresponding docstring, the __init__()
method is called the initializer. It's equivalent to the constructor in other object-oriented
languages, and is the method that is first run when you create a new object, or
new instance of the class.
§
Attributes
that apply to the whole class are defined first, and are called class attributes.
§
Attributes
that apply to a specific instance of a class (an object) are called instance attributes.
They are generally defined inside __init__(); this is not necessary, but it is
recommended (since attributes defined outside of __init__() run the risk of
being accessed before they are defined).
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