A class defines common attributes and behaviors of objects through properties and methods. An object is an instance of a class. Classes serve as blueprints for creating multiple objects that share the same structure and behavior. Each object has a unique identity even if they share the same state defined by their class. Static members like variables and methods belong to the class itself while instance members like non-static variables and methods are unique to each object instance.

1. Classes & Objects

2. Class & Object
• A Class is a description of a group of objects with common properties (attributes) & behavior
(operations)
• It serves like a blue print or prototype or template for creating objects
• An Object is an instance of a class.
• All objects have state and behaviour which is defined by its class
Marry
Jane Steffy
Student

3. Object Characteristics - Identity
• Every object has an unique identity. Two objects may have same state, but can not have same
identity.
• Even if states of two objects are same, their identities should be different. (Student Number. may
work as identity)
Marry is listening class Jane is listening class

4. Concept of Class
An object is an instance of a class
e.g. Mary is an object of Student class
Jane is an object of Student class
Mary 1001
name rollNo
Jane 1002
name rollNo

5. Example of class
class Student {
String name;
int rollNumber;
char gender;
void writing (){ }
void listening (){ }
}
• Here class is a keyword and Student is the name of the class having
attributes name, roll Number and gender and a method listening().
Data Members
(State)
Method
(Behavior)

6. Object
• An object is an instance of a class
• The new operator creates a object & returns a reference to it
• Memory allocation of objects happens in the heap area
• Reference returned can be stored in reference variables
Student s;
s = new Student();
or
Student s = new Student();
s is a reference
variable
new keyword creates
an object and
returns a reference
to it

7. Instance Members
• A variable declared within a class and outside the
method is an instance variable.
• Non static method is called an instance method.
• The runtime system creates a copy of each instance
variable for every object created for that class.
• An instance member is initialized to its default value.
public class Student{
String name; //instance variables
int rollNumber;
public void listening(){ } //instance method
}

8. Instance Members
Memory Heap
jane
public class Student{
String name;
int rollNumber;
char gender;
public void listening(){
}
}
Student mary = new Student();
Student jane = new Student();
name
rollNumber
gender
name
rollNumber
gender
mary
Object
Object
References
mary.listening();

9. Java Naming Conventions
public class Student{
static final String ADDRESS = “PINNACLE, 1310 S. Stemmons Freeway”;
private String name;
private int rollNumber;
public String getName(){
return name;
}
public void setName(String name){
this.name=name; }
public void listeningProgramme(){
// ...
}
}
Class names are nouns in
upper camel case.
Constants should
be in all
uppercase letters
Variable names in
lower camel case.
Methods should be verbs,
in lower camel case.

10. Class (or static) Members
• A class member is declared by using the static modifier
• The static members belong to the class not to a specific object and hence a
single copy is shared by all objects
• static keyword can be used in three scenarios:
• For class variables
• For methods
• For a block of code

11. Using static
• static variable
• Belongs to a class
• A single copy to be shared by all instances of the class
• Creation of instance not necessary for using static variables
• Accessed using <class-name>.<variable-name> unlike instance variables which are accessed
as <object-name>.<variable-name>
• static method
• It is a class method
• Accessed using <class name>.<method name>
• Creation of instance not necessary for using static methods
• A static method can access only other static data & methods, and not non-static members

12. Using static
• static block: A block of statement inside a Java class that is executed
only once when a class is loaded
• Used to initialize static variables of the class
• Static blocks cannot return. They are simply nameless blocks of code that
execute in the order they are specified.
class Test{
static{
//Code goes here
}
}

13. Static and Instance Members
• Static methods can access static variables and static methods.
• Instance methods can access static variables and static methods.
• Static methods cannot directly access instance variables or instance
methods.
• Both instance and class variable is assigned default value

14. this
• this can be used inside any method to refer to the current object.
• When a local variable has the same name as an instance variable, the
local variable hides the instance variable.