This document discusses classes, objects, and methods in Java. It defines a class as a user-defined data type that contains fields and methods. Objects are instances of classes that allocate memory at runtime. Methods define behaviors for objects and are declared within classes. The document covers defining classes, creating objects, accessing members, constructors, method overloading and overriding, static members, passing objects as parameters, recursion, and visibility control.

1. CLASSES, OBJECTS
&
METHODS
Module-2

2. TOPICS TO COVER
 Introduction.
 Defining a class.
 Creating Objects.
 Accessing Class Members.

3. INTRODUCTION
 Underlying structure of each JAVA programs is
CLASSES.
CREATE
CLASS
FIELDS
DATA ITEMS basic program
OBJECTS
METHODS
FUNCTIONS Components
CREATE
OBJECTS
METHODS

4. DEFINING A CLASS
 A class is a user-defined data type.
 Variables and Functions can be created within
class
SYNTAX EXAMPLE
INSATNCE VARIABLES
class classname class area
{ {
field declaration; Declaring variables int side;
Instance variables are declared
int length;
exactly as LOCAL variables }
method declaration;
}

5. METHOD DECLARATION
 Without methods class has NO LIFE.
 Since objects created by such class cannot respond to any
messages.
 Thus, methods are necessary for MANIPULATING DATA.
SYNTAX EXAMPLE
class area
type method-name(parameter list) {
{ int side;
int length;
void get(int s, int l)
}
{
side = s;
length = l;
Type of the value the method returns. It can be }
void, int, float, double }

6. EXAMPLE FOR CREATING
CLASSES
 Design a class Account that stores customer
name, account number, and type of account.
Include necessary methods to achieve
following tasks:-
 Deposit money.
 Display balance.
 Permit withdrawal and update balance.

7.  An object in JAVA is essentially a block of memory that contains
space to store all the instance variables.
 Creating an object also refers to INSTANTIATING AN OBJECT.
 Objects in JAVA are created using new. The new operator dynamically
allocates memory for an object an returns a reference to it.
Indicates that it does not point class area
null Allocates at run-time
to any object {
area a1; a1
SYNTAX:- int side;
classname objectname;
a1 = new area(); int length;
objectname = new classname();
void get(int s, int l)
combined {
a1 side = s;
area a1 = new area();
length = l;
}

8.  class area
Values are to be assigned to variables in order to use them in
{
our programs. int side;
int length;
 Since we are outside the class, we cannot access the instance
void get(int s, int l)
{
variables and methods directly.
side = s;
 length = l;
Object and dot operator are used to do this.
} area a1 = new area();
 SYNTAX:-
objectname.variablename = value; .
a1 side = 10;
.
objectname.methodname(parameter-list); a1 get (10, 15);

11.  Constructors
 Method Overloading
 Constructor Overloading
 Nesting of methods

12.  JAVA allows objects to initialize themselves when they are
created CONSTRUCTOR
PROPERTIES:-
• Initializes an object immediately upon creation.
• Same name as the class in which it resides and syntactically similar
to a method.
• Is called automatically after the object is created.
• Does not have return type.
EXAMPLE

13. NO RETURN TYPE
OUTPUT

15.  Methods have same name, but different parameter list .
 Is used when objects are required to perform similar tasks but
using different input parameters.
 Also known as POLYMORPHISM.
 Here, the aim is to provide several method definitions all with
same name, but different parameter lists.
 The difference may either in number or type of arguments
Method’s return type does not play
any role in this

16.  In addition to overloading methods, you can also
overload constructor method
EXAMPLE

17.  A method of a class can be called only by an
object of that class using dot operator.
A method can be called by using only its name by
another method of the same class
NESTING OF
METHODS

18. STATIC MEMBERS
 Is used to define a member that is common to all objects and accessed
without using a particular object.
 Thus member belongs to the class as a whole rather than the objects
created from the class.
 Used when we want to have a variable common to all instances of a
class.
SYNTAX:-
static int count; STATIC MEMBERS
Referred to as
static int max(int x, int
y) Class variables and Class methods

19. EXAMPLE

20. RESTRICTIONS FACED BY STATIC
METHODS:-
STATIC
METHODS ARE
CALLED USING
CLASS NAME
 They can only call other static methods.
 They can only access static data.
 They cannot refer to this or super in any way.

21. USING OBJECTS as PARAMETERS
 We know how to pass simple types as parameters to
methods.
 It is possible, correct and common to pass OBJECTS
to methods.
EXAMPLE

22. CALL by VALUE vs. CALL by REFERENCE
CALL BY VALUE CALL BY REFERANCE
This method copies the value In this method, reference to
of an argument into the an argument is passed to the
formal parameter of the parameter.
subroutine
Does not access actual This reference is used to
argument access the actual argument.
Thus, changes made to Thus, changes made to
parameter of the subroutine parameter will have an effect
have no effect on the on the argument.
argument.

23. EXAMPLE
REMEMBE
R
CALL by VALUE:- Simple
type arguments are passed
to methods.
CALL by REFERENCE:-
Objects are passed to
methods

24. RECURSION
 JAVA supports recursion.
 Recursion is the process of defining something in
terms of itself.
 A method that calls itself is said to be recursive.

25. VISIBILITY CONTROL
 Visibility modifiers areused to restrict the access to
certain variables and methods from outside the class.
 Also known as ACCESS MODIFIERS.
Visibility
Labels
PUBLIC PROTECTED
PRIVATE

26. VISIBILITY CONTROL (Contd..)
PUBLIC Visible to entire class in which it is defined and
All the class Outside
PRIVATE Enjoys highest degree of protection.
Accessible only with their own class.
Cannot be inherited, thus not accessible in sub-class.
Friendly When no access modifier is specified then the default
version of public accessibility is known as
“FRIENDLY”
PUBLIC
PROTECTED ItsFriendly between PUBLIC ACCESS & FRIENDLY
level lies
ACCESS.
Makes fields visible in all Makes the fields visibleonly only to all classes and
Makes fields visible not in
classes, regardless of their subclasses in same package but also to subclasses in
the same package.
packages other packages