Module1 elementary concepts of objects and classes

QUICK REVISION GUIDE – THEORY PORTION
MODULE

ELEMENTARY CONCEPTS OF
OBJECTS AND CLASSES

Prepared by Merson Mathew, St. Joseph’s School Bhagalpur for ICSE Computer

TOPICS TO REVISE







class
object
abstraction
encapsulation
inheritance
polymorphism

Applications Students

 Definition of a class in OOP

 Definition of a class in the real world
 Example of a classes and objects in the

real world
 Format of defining a class
 Attributes required for defining a class
 members of a class
i) instance variable/member
variable/data member / fields
ii) class variable / static variable
iii) constructor

OBJECT











definition of object in the real world
Definition of object in OOP
Example of an object in the real world with
characteristics and behaviour
Characteristics / state of an object
Behaviiour of an object
format of object declaration
Format of object creation
memory allocation of an object
referencing members of a class using an object
representing real world objects in object
oriented programming


ABSTRACTION
 Abstraction is the act of representing




essential features of a system without
including the background details or
explanations
Example of abstraction in the real world
Example of abstraction in OOP


ENCAPSULATION
 The wrapping up of data and the related


functions into a single unit called class is
known as encapsulation.
Data hiding /Information hiding
preventing unauthorized accessing of data
from outside the class using the principle of
encapsulation and private access modifier is
called data hiding


INHERITANCE







Definition
Need for Inheritance
base class and derived class
Different forms of inheritance
- types of inheritance supported by
java
Use of key words extends and super
Function overriding


Definition
 Inheritance is the process by which
a class acquires the properties of
another class.


Advantages of inheritance

 help us to express inheritance



relationship which ensures
closeness with real-world
models
reusability of existing classes


Base class and Derived class

class A

base class
class B

derived class

 base class is the class from which
properties are acquired. It is also called
super class or parent class

Base class and derived class
class A

base class
class B



derived class

derived class is the class that acquire
properties from another class. It is also
called sub class or child class or
extended class


single inheritance
class A

class B

A sub class inherits only from a base class


multilevel inheritance
class A

class B

a sub class
inherits from a
class that itself
inherits from
another class

class C


multiple inheritance
class A

class B

class C

a class is derived
from two or more
base classes

hybrid inheritance
class A

class B

class C

class D
a class acquires
properties from
another class
through two or more
paths

hierarchical inheritance
class A

class B

class C

class D

a class has
several sub
classes

Use of access modifiers in
Inheritance

 private members of a class can



be used only with in the class
members of a class with default
access can be used with in the
class as well as in all the classes
with in the same package


Use of access modifiers in
Inheritance

 protected members of a class can



be used with in the class, in a
class of the same package as
well as in a sub class. The sub
class may be in
the same package or in a
different package
public members of a class can be
used everywhere


private access
modifier
yes
no
no
no
no

default access
modifier
yes
yes
yes
no
no

protected
access
modifier
public access
modifier
yes
yes
yes
yes
no

yes
yes
yes
yes
yes

19

can be
accessed with
in the class
can be
accessed in a
sub class of
same package
can be
accessed in a
class of same
package
can be
accessed in a
sub class of a
different
can be
accessed in a
class of
another

access
modifier of
member
variable /
member
function

POLYMORPHISM



Polymorphism comes from Greek
meaning “many forms.”
Polymorphism is the process by which
the same message can be given to
objects of different classes and each
object responds to this message in a
different manner depending on its class.


POLYMORPHISM





passing a message to an object is same as
calling a function using an object.
for e.g: String s1=s.substring(5,8);
In the above statement we invoke the
substring function of class string using the
object s1.
We can also say that we pass a message
substring to the object s1 with the values 5
and 8


POLYMORPHISM

 Function overloading can be considered



as an example of polymorphism
Binding
- dynamic binding or late binding
- static binding or early binding


Module1 elementary concepts of objects and classes

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