Constructor in Java

In Java, a constructor is a special method that is used to initialize an object when it is created. Unlike regular methods, constructors are called automatically when you use the new keyword.

A constructor has some unique characteristics:

• Its name is the same as the class name
• It is called automatically when an object is created
• It is mainly used to initialize object variables

Simple Program using constructor:
Scenario 1:When a student is registered, roll number and name are mandatory.So they must be passed during object creation.

package Constructor;

public class Student {

    int roll_no;
    String name;
    Student(int roll_no,String name)
    {
    this.roll_no=roll_no;
    this.name = name;

    }
    Student(int roll_no){
        this.roll_no=roll_no;
    }
    Student(String name){
        this.name=name;
    }
        public static void main(String[] args)
    {
    Student user1 = new Student(1,"Kavitha");

    Student user2 = new Student(2,"Arun");
    System.out.println("Roll No: " + user1.roll_no);

    }
}

Why constructor here?
1.Student must have roll number
2.Prevents invalid objects like Student() with no data

Scenario 2:
When you open a bank account, details like account number, name, and balance must be set at the time of creation.

class Bank
{
int accountNo;
String name;
int balance;

Bank(int accountNo, String name , int balance)
{
this.accountNo = accountNo;
this.name = name;
this.balance = balance;
}

public static void main(String[] args)
{

Bank accholder = new Bank(101,"kumar",1000);
System.out.println(accholder.accountNo);//101

Bank accholder1 = new Bank(102,"hari",1000);
System.out.println(accholder1.accountNo); //102
}
}

Why constructor here?
1.Account cannot exist without details
2.Values are initialized immediately
3.No chance of creating an incomplete object

Scenario 3:
Imagine a training institute called Payilagam that manages student or user accounts.

class Payilagam
{
String name;
String email;
String password;

Payilagam(String name, String email, String password)
{
this.name = name;
this.email = email;
this.password = password;
}

Payilagam( String email, String password)
{

this.email = email;
this.password = password;
}
public static void main(String[] args)
{
Payilagam user1 = new Payilagam("kumar","abc.com","kumar@123");

Payilagam user2 = new Payilagam("abc.com","kumar@123");
}
}

Why constructor here?

• Initialize user details when an object is created
• Ensure user data is stored correctly at the start
• Support different user actions (Registration vs Login)

this keyword

• The this keyword in Java is used to refer to the current object of a class.
• It helps avoid confusion between instance variables and constructor/method parameters that have the same name.

Constructor overloading:
Constructor overloading allows creating objects in different ways by using multiple constructors with different parameters.

How to Achieve a Constructor in Java?

https://dev.to/kavitha_krish/how-to-achieve-a-constructor-in-java-1gi3