Forem: Divya bharathi G

Constructor in java

Divya bharathi G — Tue, 10 Feb 2026 15:21:20 +0000

What is Constructors?

A constructor is a special method in Java that is automatically executed when an object of a class is created. It is mainly used to initialize data members of a class.

Unlike normal methods, a constructor:

It has the same name as the class.
It does not have a return type.
It is called automatically using the new keyword.

Why do we need Constructors?

Constructors are used because they:

Initialize object data at the time of creation.
Assign default or user-defined values.
Ensure the object starts in a valid state.

Types of Constructors in Java

Default Constructor

A constructor with no parameters is called a default constructor.

class Student 
{
    Student() 
    {
        System.out.println("Default constructor called");
    }
}

class Main 
{
    public static void main(String[] args) 
    {
        Student s = new Student();
    }
}

Parameterized Constructor

A constructor that accepts parameters is called a parameterized constructor.

class Student
 {
    String name;
    int age;

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

    void display()
    {
        System.out.println(name + " " + age);
    }
}

Constructor Overloading

When a class has multiple constructors with different parameters, it is called constructor overloading.

class Student 
{
    Student() 
    {
        System.out.println("Default constructor");
    }

    Student(String name) 
    {
        System.out.println("Student name: " + name);
    }
}

Rules of Constructors in Java

Constructor name must be the same as the class name.
Constructors do not have a return type.
Constructors can be overloaded.
Constructors cannot be inherited.
If no constructor is defined, Java provides a default constructor.

this Keyword in Constructors

The this keyword refers to the current object of the class.
It is commonly used when constructor parameters have the same name as instance variables.

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 acc1 = new Bank(101, "Kumar", 1000);
        Bank acc2 = new Bank(102, "Hari", 1500);

        System.out.println(acc1.accountNo);
        System.out.println(acc2.accountNo);
    }
}

Control Flow Statements in Java

Divya bharathi G — Tue, 20 Jan 2026 14:59:31 +0000

Control Flow Statements:

In Java, control flow statements decide the order in which statements are executed in a program.
They help your program make decisions, repeat tasks, and jump to different parts of code.

Types of control flow statements in java:

Decision making statement.
Looping statement.
Jump statement.

1.Decision making statement.

Decision making statement take decision based on the value.

if statement:

if is a java keyword.
Execute code only if condition is true.

Syntax:

if (condition)
{
    // statements
}

Else statement:

else is a java keyword.
Execute code only if condition is false.

Syntax:

if (condition)
{
    // statements if true
} 
else 
{
    // statements if false
}

Else-if statement:

else if is a java keyword.
If we want to check multiple condition,we go for else if statement.

Syntax:

if (condition1)
{
    // statements if condition1 is true.
} 
else if (condition2)
{
    // statements if condition2 is true
} 
else
{
    // Statements if none of the condition is true.
}

Example:

package project1;
public class elseif
{

    public static void main(String[] args) 
    {
        int mark = 60;

        if(mark >= 75)
        {
            System.out.println("Distinction");
        }
        else if(mark >= 60)
        {
            System.out.println("First class");
        }
        else if(mark >= 50)
        {
            System.out.println("Second class");
        }
        else
        {
            System.out.println("Fail");
        }

    }

}

Switch statement:

Used when there are multiple fixed choices.

switch (expression)
{
    case value1:
        // statements
        break;
    case value2:
        // statements
        break;
    default:
        // statements
}

Example:

ackage project1;

public class Switch {

    public static void main(String[] args)
    {
        int day = 2;

        switch(day)
        {
        case 1: //1==2
            System.out.println("Monday working day");
            break;

        case 2:
            System.out.println("Tuesday working day");
            break;

        case 3:
            System.out.println("Wednesday working day");
            break;

        case 4:
            System.out.println("Thursday working day");
            break;

        case 5:
            System.out.println("Friday working day");
            break;

        default:
            System.out.println("Holiday");
        }

    }

2.Looping statement:

Looping statements are used to execute a block of code repeatedly as long as a condition is satisfied.

While loop:

The while loop executes a block of code as long as the condition is true.

Syntax:

while (condition)
{
    // statements
}

Example:

public static void main(String[] args)
 {
        int i =1;
        while(i <=10)
        {
            System.out.print(i);
            i++;
        }
}

Explanation

The variable i is initialized before the loop starts.
The condition i <= 10 is checked before each iteration.
The value of i is incremented inside the loop body, ensuring the loop eventually terminates.

For loop:

It combines initialization, condition checking, and increment/decrement in a single line.

Syntax:

for (initialization; condition; increment/decrement) 
{
    // statements
}

Example:

for(int i =1;i <=5;i++)
{
System.out.println(i); 
}

Explanation

Initialization runs once (int i = 1)
Condition is checked before every iteration (i <= 5)
Increment happens after each iteration (i++)

Operators in Java

Divya bharathi G — Thu, 08 Jan 2026 17:11:46 +0000

Opeartors:

Operators are special symbols used to perform operations on variables and values.

Types of Operators:
1.Assignment Operators:

Used to assign values. Eg:

package project1;
public class assignment_operator 
{
    public static void main(String[] args)
    {
        int a=10 , b=2;
        System.out.println(a=b);
    }

}

Output:
2

2.Arithmetic Opeartors:

Used for mathematical calculations.

1. Operator   Meaning               Example
2. +         Addition                a + b
3. -         Subtraction         a - b
4. *         Multiplication          a * b
5. /         Division                a / b
6. %         Modulus (remainder)     a % b

Eg:

package project1;

public class arrithmetic_operator {

    public static void main(String[] args)
    {
        int a=10 , b=2;
        System.out.println(a+b);
        System.out.println(a-b);
        System.out.println(a*b);
        System.out.println(a/b);
        System.out.println(a%b);
    }

}

Output:

3.Unary Operators

Operate on single operand.

   1.++   -> Increment operator
   2.--   -> Decrement operator
   3.no++ --> post increment
   4.++no --> Pre increment
   5.no-- --> post Decrement
   6.--no --> Pre Decrement

Eg:

package project1;
public class unary_operator 
{
    public static void main(String[] args)
        {
        int i = 10;
        System.out.println(i++);// post Increment operator 10 + 1
        System.out.println(i);//11

        int j = 10;
        System.out.println(j--); //Post Decrement operator 10 -1
        System.out.println(j);

                int a = 10;
        System.out.println(++a);//Pre increment operator 10+1=11

        int b = 20;
        System.out.println(--b); //Pre Decrement operator 20-1=19
    }
}

Output:
10
11
10
9
11
19

4.Equality and Relational Operators :

Used to compare two values → result is boolean.

Operator Meaning
== Equal to
!= Not equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal

Eg:

package project1;
public class relational_operator 
{
    public static void main(String[] args) 
    {
        int tamil_mark = 70;
        int english_mark = 70;
        System.out.println(tamil_mark == english_mark); // boolean
        System.out.println(tamil_mark < english_mark); // Boolean
                System.out.println(tamil_mark > english_mark);
                System.out.println(tamil_mark != english_mark);
    }

}

Output:
true
false
false
false

5.Conditional Operators :

    - Used with boolean values.
          1. && = AND operator
          2.|| = OR operator

Variable in Java

Divya bharathi G — Mon, 05 Jan 2026 15:51:09 +0000

Variable:

- A variable is the name of a memory location.
- A variable is used to store a value.
- The value of a variable can change during program execution.

Rules for Variable Names in Java

-  Variable name should start with a lowercase letter.
- Variable name can start with $ or _.

Example

byte tamilMark = 60;

Explanation:

- byte → data type
- tamilMark → variable name
- = → assignment operator
- 60 → value
- ; → end of statement

Objects and Data types in Java

Divya bharathi G — Sun, 04 Jan 2026 17:12:32 +0000

Object:

An Object is a physical entity meaning that it will physically create a heap memory at runtime.
An object is a combination of state (data/variables-Color, RAM) and behavior(methods /functions-Call, SMS).

Syntax:

Classname referenceName = new Classname();

new is a Java keyword.
If we mention a new keyword in object,JVM will allcated one memory in heap at runtime.

Data Types:

Data types specify what type of data a variable can store.
Java has two main types of data types:
- Primitive data types.
- Non-primitive data types.

Primitive data type:
-Primitive data types store simple values.
-Memory management of primitive data type is stack.
-There are 8 primitive data types in java.
1.short
2.byte
3.int
4.long
5.float
6.double
7.char
8.boolean
-These 8 primitive data types are known as Keywords.

Non-Primitive Data Type:

-String is a Non-primitive data type.

Size and Range of data types:

Size of all primitive data types:
1.short = 2 byte.
2.byte = 1 byte.
3.int = 4 byte.
4.long = 8 byte.
5.float = 4 byte.
6.double = 8 byte.
7.char = 2 byte.
8.boolean = 1 bit.

To calulate range:

   -1 byte = 8 bit = 2*2*2*2*2*2*2*2.
                   = 256 (so 256/2=128)
   - To store the negative value the range starts from -128 to 127(The range ends at 127 because 0 i included).

Range:

 -short = -32,768 to +32,767.
 -byte = -128 to 127.

Class in Java

Divya bharathi G — Sun, 04 Jan 2026 06:51:04 +0000

Class:
->A class is a blueprint or template used to create objects.
->Class is a Java keyword.
->A class does not occupy memory until an object is created.

Syntax for class:

class Classname
{

}

Rules for Classname:
->The first letter of Classname should be in capital.
->Classname should allow special characters like $ and _ .
-> $ and _ can be used at the beginning, middle, or end of a class name, but no other special characters are allowed.
->Classname should allow numeric values that can be used in the middle or at the end of a class name.

2nd Level Translator(JRE,JVM,JIT)

Divya bharathi G — Sat, 03 Jan 2026 12:16:01 +0000

1.JRE(Java Runtime Environment)
->JRE(Java Runtime Environment) provides an environment to run Java
programs.
->It contains the JVM and standard class libraries.

2.JVM(Java Virtual Machine)
-> JVM(Java Virtual Machine) performs memory management operation.
-> It is responsible for converting bytecode into machine-specific
instructions.

3.JIT(Just intime Compiler)
-> It is a component of the JVM that improves the performance of Java
programs.
-> It converts bytecode into native machine code at runtime.
-> It compiles only the frequently executed code.

Introduction to Java

Divya bharathi G — Fri, 02 Jan 2026 12:48:14 +0000

Computers only understand binary language (1s and 0s), but humans write

programs in high-level languages like Java.

1.How does Java code written by humans get executed by machines?
It uses Translation system(JDK → JRE → JVM → JIT) to convert Programming language to Machine Language.

2.What is JDK?
-> JDK is Known as Java Development Kit.
-> We should save the java file as .java (Eg:Sample.java).
-> If there is no error in the .java file then it convert .java file into .class file (Eg:Sample.java will be converted into Sample.class file)
-> Where .java file is a readable file and .class is a unreadable file.
->If there is a error it will shown specifically like line 3.