Unit-2
Classes and
Objects
Class
A class is a user defined data type with a
template that serves to define its
properties.
java is a true object-oriented language
and therefore anything we wish to
represent in a java program must be
encapsulated in a class that defines the
state and behavior of the basic program
components known as object.
Class
Once the class type has been defined, we
can create variables of that type, called
as instance of classes, which are the
actual object.
The basic form of a class definition
is
class classname [extend superclassname]
{
[filed declaration;]
[Methods declaration];
}
Field and Method Declaration
Syntax:
class classname
{
type instancevariable;
[also known as member variable
it is called instance variable because
they are created whenever an object of
the class is instantiated.]
type methodname (parameter list)
{
method-body;
}
}
Example
class rectangle
{
int length, width;
void getdata (int x, int y)
{
}
}
Creating Object
Obtaining objects of a class is a two-
step process.
First, you must declare a variable of
the class type. This variable does not
define object. Instead, it is simply a
variable that can refer to an object.
Second, you must acquire an actual
physical copy of the object and assign it
to that variable.
Creating Object
We can do this using the new
operator, it’ll dynamically allocates
memory for an object and returns a
reference to that object.
Creating Object
Syntax:
classname objectname;
// declare the object
objectname = new classname();
//instantiate the object
OR
classname objectname = new
classname();
Creating Object
Rectangle rect1;
//declare a variable to hold the object
reference
rectl1 = new Rectangle();
// actually assign the object reference to the
variable. now it is an object of rectangle
class.
OR
Rectangle rect1 = new Rectangle();
Statement Effect
Rectangle rect1; rect1
null
rect1=new Rectangle(); rect1 rect1 Object
length
Width
Accessing Class Members:
If we are outside the class then to access
the instance variable and the methods,
we must use the concerned object and
the dot operator as shown below.
objectname.variablename = value;
objectname.methodname(paramater.list);
Introducing Methods:
A Java method is a set of Java
statements which can be included inside
a Java class.
Java methods are similar to
functions or procedures in other
programming languages.
General Form:
type name(parameter-list){
// body of method
}
Classification of Mehod
1. Based on nature of creation
1. User Defined
2. Predefined
2. Based on return type
1. Return a value
2. Return void
3. Based on method call
1. Nonrecursive
2. recursive
Constructors:
Constructor are special type of
method that enables an object to
initialize itself when it is created.
It have the same name as the class
itself, and do’t specify the return type
not even void because it return the
instance of the class itself.
Getter and Setter Method
• Define a specific methods for updating
or accessing too many items.
• A getter method “gets” the value of a
variable.
• A setter method that “sets” the value
of a variable.
Garbage Collection
• Technique which destroy object and
release it’s memory is called garbage
collection.
• How it work?
– When no reference to an object
exist, that object is assumed to be
no longer needed and the memory
occupied by the object can be
reclaimed(regain).
• Garbage collection only occurs
periodically during the execution of
your program.
The Finalize Method
• Sometimes an object will need to
perform some action when it is
destroy, to handle such situations, java
provide a mechanism called
finalization.
• For Example, if an object is holding
some non-java resource such as file
handle or character font, then you
might want to make sure these
resources are freed before an object is
destroyed.
The Finalize Method
• By using finalization, you can define
specific actions that will occur when an
object is just about to be reclaimed by
the garbage collector.
• Syntax:
– Protected void finalize()
{
// finalization code.
}
The Finalize Method
• finalize() is only called just prior to
garbage collection. It is not called
when an object goes out-of scope.
Final
There are three use of final.
1. A variable can be declared as final. It
will prevent its content from being
modified.
This mean that you must initialize a
final variable when it is declared.
For Example:
final int x=1;
Final
2. Using final to prevent overriding
To disallow a method from beign
overridden, specify final as a modifier at
the start of its declaration.
Method declared as a final can not
be overridden.
Syntax:
final return-type method-name()
{
}
Final
3. Using final to prevent Inheritance
To disallow a class from being
inherited, declare a class as a final
implicitly declared all of its methods as a
final, too.
Syntax:
final class class-name
{
}
WHAT IS STATIC FIELDS??..
• Static fields are also know as class
fields.
• They are simply fields that have the
static modifier in their declarations.
• Static field is associated with the
class rather than an object.
FEATURES OF STATIC FIELD
• A Static field is shared by all
objects of its class and thus relates
to the class itself.
• A Static field declared by using the
Static keyword as a modifier.
For Example:
static int x;
FEATURES OF STATIC FIELD
• Static variables are initialized to
default values when the class is
loaded into memory.
WHAT IS STATIC
METHOD??..
• A static method is a simple method
having static keyword as a modifier.
• Syntax:
static ReturnType
MethodName(parameters)
FEATURES OF STATIC METHOD
• static methods are created to access it
without any object instance.
• A static method can be accessed
without creating an instance of the
class.
• Static method can call only other static
methods and static variables defined
in the class.
FEATURES OF STATIC METHOD
• A static method cannot access non-
static/instance variables, because a
static method is never associated with
any instance.
• To call non static method from static
method user have to use class object
associated with that method.
