1) A class is a logical entity that defines the structure and behavior of objects. It contains data members, constructors, methods and blocks but does not allocate memory itself. 2) An object is a physical entity that is instantiated from a class. Memory is allocated when an object is created using the 'new' keyword. An object is an instance of a class. 3) Constructors are used to initialize objects. They are called when an object is created and allocate memory for the object. They can be default, no-argument, or parameterized based on whether they take arguments or not.

1. Class and Object

2. Class
• Class is a logical entity because when we are creating a class then
memory will not be created for a class.
• Class is a collection of data members, constructors, methods and
blocks.

3. Cont…
• Class is a collection of objects of same type.

4. Objects
• Object is a physical entity because when we are creating object then
memory will be allocated for object.
• Object is created with the help of new keyword.

5. Cont…
• Object is the instance of a class.

6. Constructors in Java
• In Java, a constructor is a block of codes similar to the method.
• It is called when object is created.
• At the time of calling constructor, memory for the object is allocated
in the memory.

7. Cont…
• It will be used to initialize the object.

8. Rules for creating Java constructor
• Constructor name must be the same as its class name.
• A Java constructor cannot be abstract, static, final, and synchronized

9. Types of Java Constructors
• There are 3 Types of constructors.
• Default Constructor.
• No-argument Constructor.
• Parameterized Constructor.

10. 1. Default Constructor.
• If there is no constructor in a class, compiler automatically creates a
default constructor.

11. Cont…

12. Example
class Student
{
int id;
public static void main(String a[])
{
Student s1=new Student();
System.out.println(s1.id);
}
}
Output: 0

13. 2. No-argument Constructor.
• A constructor that has no parameter is known as no-argument
constructor.

14. Example
class Bike
{
Bike()
{
System.out.println("Bike is created");
}
public static void main(String a[])
{
Bike b=new Bike();
}
}
Output: Bike is created

15. 3. Parameterized Constructor
• A constructor which has a specific number of parameters is called a
parameterized constructor.

16. Example
class Student
{
int id;
String name;
Student(int i, String n)
{
id = i;
name = n;
}
void display()
{
System.out.println(id);
System.out.println(name);
}
public static void main(String a[])
{
Student s1 = new Student(111,"Karan");
s1.display();
}
}

17. Cont…
Output:
111
Karan

18. Difference between constructor and method
in Java
Constructor Method
A Constructor is called implicitly by the
system.
A Method is called by the programmer.
A Constructor is called when a object is
created using the keyword new.
A Method is called through method calls.
A Constructor’s name must be same as
the name of the class.
A Method’s name can be anything.
A Constructor doesn’t have a return type. A Method must have a return type.

19. Ways to initialize object:
• Initializing an object means storing data into the object.
• There are 3 ways to initialize object in Java.
• By reference variable
• By method
• By constructor

20. 1) Initializing an Object by reference variable

21. Example
class Student
{
int id;
public static void main(String a[])
{
Student s1= new Student();
s1.id=101;
System.out.println(s1.id);
}
}
Output
101

22. 2) Initializing an Object by method

23. Example
class Student
{
int id;
void display(int n)
{
id = n;
System.out.println(id);
}
public static void main(String a[])
{
Student s1=new Student();
s1.display(101);
}
}

24. Cont…
Output:
101

25. 3) Initializing an Object by a constructor

26. Example
class Student
{
int id;
Student(int i)
{
id = i;
}
void display()
{
System.out.println(id);
}
public static void main(String a[])
{
Student s1 = new Student(101);
s1.display();
}
}

27. Cont…
Output:
101