Forem: Harini

Abstraction in Java

Harini — Thu, 14 May 2026 05:46:50 +0000

Abstraction in Java is a core concept of Object-Oriented Programming (OOP). It means hiding unnecessary implementation details and showing only the essential features of an object.

Abstraction focuses on what an object does instead of how it does it.

Example:
When you drive a car, you use the steering wheel, accelerator, and brakes — you don’t need to know how the engine works internally.

How Abstraction is Achieved in Java

In Java, abstraction is implemented using:

Abstract Classes
- Declared using the keyword abstract
- Can have:
- Abstract methods (no body)
- Concrete methods (with body)

abstract class Animal {
    abstract void sound(); // abstract method

    void eat() {
        System.out.println("This animal eats food");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Dog barks");
    }
}

2. Interfaces

Used for 100% abstraction (traditionally)
All methods are abstract by default (until Java 8 added default methods)

interface Vehicle {
    void start();
}

class Car implements Vehicle {
    public void start() {
        System.out.println("Car starts with a key");
    }
}

Key Points

You cannot create objects of abstract classes directly.
Interfaces support multiple inheritance, while classes do not.
Abstraction helps in:
- Reducing complexity
- Increasing code reusability
- Improving security (hiding data)

Difference from Encapsulation

Abstraction → Hides implementation details
Encapsulation → Hides data (using private variables and getters/setters)

Final Keyword in java

Harini — Thu, 14 May 2026 05:30:02 +0000

In Java, the final keyword is used to restrict changes. Once something is marked as final, it cannot be modified further. It can be applied in three main places: variables, methods, and classes.

1. Final Variable

A final variable acts like a constant — its value cannot be changed once assigned.

public class GRT{
    final int price = 15000;

    public void display(){
        GRT grt = new GRT();
        grt.price = 200;

        System.out.println(grt.price);
    }
}

Key point:

You must assign value once
After that, no reassignment allowed

2. Final Method
A final method cannot be overridden by subclasses.

class Parent {
    final void show() {
        System.out.println("This is final method");
    }
}

class Child extends Parent {
    // void show() { } ❌ Error: cannot override final method
}

Key point:

Used when you want to prevent method overriding

3. Final Class
A final class cannot be extended (inherited).

final class A {
    void display() {
        System.out.println("Final class");
    }
}

// class B extends A { } ❌ Error: cannot inherit final class

Key point:

Used when you want to stop inheritance

POJO in Java

Harini — Tue, 12 May 2026 05:09:47 +0000

What is a POJO?

POJO stands for Plain Old Java Object. It is a term used to describe a simple Java class that follows basic coding conventions and does not depend on any specific frameworks or external libraries.
In simple words, a POJO is just a normal Java object created to hold and manage data in a clean and straightforward way.

Key Features of a POJO

A typical POJO usually includes:

Private variables (fields) to store data
Public getter and setter methods to access and update those fields
Optional methods for additional behavior (like toString())

It may also have:

A no-argument constructor
A parameterized constructor

Properties of a POJO
A class is considered a POJO if it does not:

Extend any predefined class (e.g., HttpServlet, EntityBean, etc.)
Implement any predefined interface (e.g., javax.ejb.EntityBean)
Contain framework-specific annotations (e.g., @entity, @Component)

Example

public class Employee {

    private int id;
    private String name;

    // Default constructor
    public Employee() {}

    // Parameterized constructor
    public Employee(int id, String name) {
        this.id = id;
        this.name = name;
    }

    // Getter
    public int getId() {
        return id;
    }

    // Setter
    public void setId(int id) {
        this.id = id;
    }

    // Getter
    public String getName() {
        return name;
    }

    // Setter
    public void setName(String name) {
        this.name = name;
    }
}

What is a JavaBean?

A JavaBean is a special type of POJO that follows a set of strict rules (conventions). It is mainly used to encapsulate data and make it reusable across applications.

Every JavaBean is a POJO, but not every POJO is a JavaBean.

Rules for a JavaBean

Must implement the Serializable interface.
Must have a no-argument constructor.
All fields must be private.
Each property should have corresponding getter and setter methods.
Fields must be accessed only through these methods (not directly).

This design provides encapsulation and controlled access to object properties.

Method Overriding in Java

Harini — Wed, 06 May 2026 05:09:12 +0000

Method Overriding in Java

Overriding is a feature that allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its super-classes or parent classes.
When a method in a subclass has the same name, same parameters, and same return type(or sub-type) as a method in its super-class, then the method in the subclass is said to override the method in the super-class.
Method overriding is one of the way by which java achieve Run Time Polymorphism.

Usage of Java Method Overriding

Method overriding is used for achieving run-time polymorphism.
Method overriding is used for writing specific definition of a subclass method (this method is known as the overridden method).

Rules for Overriding

Method name must be the same
Parameters must be the same
Return type must be the same (or compatible)
It must be in a parent-child (inheritance) relationship
Java picks which method to run at run time, based on the actual object type, not just the reference variable type.

Example

class Vehicle{  
    void run(){
        System.out.println("Vehicle is running");
    }  
}  

class Bike extends Vehicle{  
    void run(){
        System.out.println("Bike is running safely");
    }  

    public static void main(String args[]){  
        Bike obj = new Bike();  
        obj.run(); 
    }  
}

Output
Bike is running safely

String program

Harini — Wed, 22 Apr 2026 04:09:08 +0000

1. Remove extra space

name = "Kill     them      with   your    success"
result = ""
i = 0
while i < len(name):
    if name[i] != " ":
        result = result + name[i]
    else:
        if name[i-1] != " ":
            result = result + name[i]
    i += 1

print(result)

Output

2. Front space remove

name = "     kill them with your success"
i = 0
result = ""

while i < len(name):
    if name[i] != " ":
        break   
    i += 1

while i < len(name):
    result += name[i]
    i += 1

print(result)

Output

3. Remove last space

name = "Kill them with your success        "
result = ""

i = len(name)-1

while i >= 0:

    if name[i] == " ":
        i -= 1
        continue

    else:
        j = 0

        while j <= i:
            result += name[j]
            j += 1

        break

print(result)

Output

4. Reverse a word

name = input("Enter word:")
result = ""

i = len(name)-1

while i >= 0:
    result = result + name[i]
    i -= 1

print(result)

Output

String Program

Harini — Tue, 21 Apr 2026 09:07:01 +0000

1. Given string to uppercase

text=input("Enter some words :")
i=0
upper=""
while i<len(text):
    ch=text[i]
    if ch>="a" and ch<="z":
        upper=upper+chr(ord(ch) - 32)
    else:
        upper=upper+ch
    i+=1
print(upper)

Output

2. First letter to Uppercase

text=input("Enter some words :")
i=0
upper=""
while i<len(text):
    ch=text[i]
    if (ch>="a" and ch<="z") and (text[i-1]==" " or i==0):
        upper=upper+chr(ord(ch) - 32)
    else:
        upper=upper+ch
    i+=1
print(upper)

Output

3. Letter Count

text=input("Enter some words :")
i=0
count=0
while i<len(text):
    ch=text[i]
    if (ch>="a" and ch<="z") or (ch>="A" and ch<="Z"):
        count+=1
    i+=1
print(count)

Output

4. Word count

text=input("Enter some words :")
i=0
count=1
while i<len(text):
    if text[i]!=" " and text[i-1]==" ":
        count+=1
    i+=1
print(count)

Output

5. Password Validator

text=input("Enter password : ")

i=0
upper=False
lower=False
number=False
special=False
others=True

while i<len(text):
    ch=text[i]

    if ch>="A" and ch<="Z":
        upper=True

    elif ch>="a" and ch<="z":
        lower=True

    elif ch>="0" and ch<="9":
        number=True

    elif ch=="!" or ch=="@" or ch=="#" or ch=="$" or ch=="&":
        special=True

    else:
        others=False

    i+=1

if len(text)>=8 and upper and lower and number and special and others:
    print("Valid Password")
else:
    print("Invalid Password")

Output

6. Password Generator

import random

upper="ABCDEFGHIJKLMNOPQRSTUVWXYZ"
lower="abcdefghijklmnopqrstuvwxyz"
numbers="0123456789"
special="!@#$&"

all_chars=upper+lower+numbers+special

password=""

password=password+random.choice(upper)
password=password+random.choice(lower)
password=password+random.choice(numbers)
password=password+random.choice(special)

i=4

while i<8:
    password=password+random.choice(all_chars)
    i+=1

print("Generated Password :",password)

Output

How to Validate Indian Pincode Using Python

Harini — Mon, 20 Apr 2026 08:38:55 +0000

How to Validate Indian Pincode Using Python (Region, Sub-Region and District Finder)

regions = {
    1: "Delhi",
    2: "Uttar Pradesh & Uttarakhand",
    3: "Rajasthan & Gujarat",
    4: "Maharashtra",
    5: "Andhra Pradesh & Telangana",
    6: "Tamil Nadu & Kerala",
    7: "West Bengal & North East",
    8: "Bihar & Jharkhand",
    9: "Army Postal Service"
}

sub_regions = {
    11: "Delhi Region",
    40: "Mumbai Region",
    60: "Chennai Region",
    70: "Kolkata Region"
}

districts = {
    110: "Delhi",       111: "Delhi (North)",      112: "Delhi (South)",
    400: "Mumbai",      401: "Mumbai (Suburban)",   402: "Mumbai (West)",
    600: "Chennai",     601: "Tiruvallur",          602: "Kanchipuram",
    700: "Kolkata",     701: "Kolkata (North)",     702: "Kolkata (South)"
}

def get_pincode_details(pincode):
    if not pincode.isdigit() or len(pincode) != 6:
        print("Invalid pincode. Enter exactly 6 digits.")
        return

    num         = int(pincode)
    first_digit = num // 100000
    first_two   = num // 10000
    first_three = num // 1000

    print("Region:     ", regions.get(first_digit,   "Unknown"))
    print("Sub-region: ", sub_regions.get(first_two, "Unknown"))
    print("District:   ", districts.get(first_three, "Unknown"))


pincode = input("Enter 6-digit Pincode: ").strip()
get_pincode_details(pincode)

Output

Supermart Billing Program

Harini — Mon, 20 Apr 2026 04:04:36 +0000

Write a Java program to create a supermarket billing system using classes and objects. Calculate the final price of a product after applying discount if available. Display product details, discount percentage, and final price.

public class Supermart {

    String productName;
    int price;
    boolean discountAvailable;
    int discountPercentage;

    public Supermart(String productName, int price, boolean discountAvailable, int discountPercentage) {

        this.productName = productName;
        this.price = price;
        this.discountAvailable = discountAvailable;
        this.discountPercentage = discountPercentage;
    }

    public Supermart(String productName, int price) {

        this.productName = productName;
        this.price = price;

    }
    public static void main(String[] args) {

        Supermart p1 = new Supermart("Rice",1000,true,20);
        Supermart p2 = new Supermart("Oil",500,true,10); 
        Supermart p3 = new Supermart("Sugar",300);

        p1.display();
        p3.display();
    }

    public void display() {

        int finalPrice = price;

        if(discountAvailable) {
            finalPrice = price - (price * discountPercentage / 100);
        }

        System.out.println("Product Name: " + productName);
        System.out.println("Price: " + price);
        System.out.println("Discount Available: " + discountAvailable);

        if(discountAvailable) {
            System.out.println("Discount Percentage: " + discountPercentage);
        }

        System.out.println("Final Price: " + finalPrice);
    }

}

Output

Pincode,Gmail and Mobile Number Validation in Python

Harini — Sat, 18 Apr 2026 09:18:37 +0000

1. Check whether a given number is a valid 6-digit pincode or not.

def pincode(pc):
    if pc//100000!=0 and pc//100000<=9:
        print("Yes")
    else:
        print("No")
pincode(int(input("Enter a pincode :")))

Output

2. Check whether a given email ID is a valid Gmail address or not.

def verify_Gmail(Gmail):
    if Gmail[-10:]=="@gmail.com":
        i=0
        gmailcopy=Gmail[0:len(Gmail)-10]
        while i<len(gmailcopy):
            current=gmailcopy[i]
            i+=1
            if current>="a" and current<="z":
                continue;
            elif current>="A" and current<="Z":
                continue;
            elif current>="0" and current<="9":
                continue;
            elif current=="." or current=="_":
                continue;
            else:
                return False
        return True
    else:
        return False

if verify_Gmail(input("Enter Your Gmail :")):
    print("valid")
else:
    print("invalid")

Output

3. Check whether a given input is a valid Indian mobile number or not.

def verify(mobnum):
    index=0
    while index<len(mobnum):
        if mobnum[index]>="0" and mobnum[index]<="9":
            index+=1
            continue;
        else:
            return False
    return True

def mobile(n):
    i=0
    while i<len(n):
        if(n[i]>="0" and n[i]<="9") or (n[i]=="+") or (n[i]==" "):
            i+=1
        else:
            return "characters and special charactors not allowed"

    if len(n)==10:
        if n[0]>="6" and n[0]<="9" and verify(n):
            return "valid";
        else:
            return "incorrect"

    if len(n)==13:
        if n[0:3]=="+91":
            if n[3]>="6" and n[3]<="9" and verify(n[3:]):
                return "valid"
            else:
                return "incorrect"
        return "First three number should like +91"

    if len(n)==14:
        if n[0:4]=="+91 ":
            if n[4]>="6" and n[4]<="9" and verify(n[4:]):
                return "valid"
            else:
                return "incorrect"
        return "First four number should like +91 "
    return "Please Enter Your valid Mobile Number"      
print(mobile(input("Enter Your Mobile  Number : ")));

Output

Constructor in Java

Harini — Fri, 17 Apr 2026 16:54:15 +0000

What is a Constructor?
When we make an object of a class in Java, a special method called a constructor runs automatically. The main job of this is to set the initial values for the variables inside the object.

Important Points

The name of the constructor should be the same as the name of the class.
It doesn't have a return type, not even void.
Using constructor overloading, a class can have more than one constructor.
The constructor is called automatically every time you make an object.

Syntax

class Classname{
    Classname(){
        // constructor body
    }
}

Example

public class Shop {

  public Shop() {
    System.out.println("Constructor");
  }

  public static void main(String[] args) {
    System.out.println("Main Method");

    Shop product = new Shop();
  }
}

Types of Java Constructors

Default Constructor
No-Argument Constructor
Parameterized Constructor

1. Default Constructor

Java will automatically give us a constructor if we don't make one ourselves. This is known as the default constructor. It doesn't need any parameters and sets the variables to their default values.

No arguments are needed.
Variables get default values like this:
- int = 0
- String is null and boolean is false.

Example

public class Shops {
    String name;
    int price;

    public static void main(String[] args) {
        Shops obj = new Shops();

        System.out.println(obj.name);   
        System.out.println(obj.price);  
    }
}

Output

2. No-Argument Constructor

A constructor with no parameters is called a no-argument constructor. We mostly use it when we want to give an object some set or pre-defined values when we make it.

Example

public class Shops {

    public Shops() {
        System.out.println("No-argument constructor");
    }

    public static void main(String[] args) {
        Shops product = new Shops();
    }
}

Output

3. Parameterized Constructor

When we want to pass values while making an object, we use a parameterized constructor. Then, these values are used to set up the object's variables.

Example

public class Shops {
    String name;
    int price;

    public Shops(String str, int i) {
        name = str;
        price = i;
    }

    public static void main(String[] args) {

        Shops product1 = new Shops("Bag", 100);
        Shops product2 = new Shops("Note", 40);

        product1.display();
        product2.display();
    }

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

Output

Test Case

Harini — Wed, 15 Apr 2026 16:08:23 +0000

What is a Test Case?

A test case is a set of conditions, inputs, and actions used to validate whether a specific feature of an application works correctly.

It defines what to test, how to test, and expected results
It acts as a blueprint for testing
It helps ensure the system works in normal, error, and edge conditions

Components of a Test Case

A standard test case includes:

Test Case ID
Test Scenario / Name
Preconditions
Test Steps
Test Data
Expected Result
Actual Result
Status (Pass/Fail)

What is a Test Scenario?
A test scenario is a high-level idea of what you want to test in a product or application.

Test Case vs Test Scenario

Test Scenario → What to test (high-level)
Test Case → How to test (detailed steps)

How to Write Test Cases

Understand requirements
Identify test scenarios
Write clear test case name
Define preconditions
Write step-by-step actions
Add test data
Define expected result
Execute and record actual result

Example: Invalid Password

Test Case Table

Top SDLC Models Every Developer Should Know

Harini — Mon, 13 Apr 2026 04:32:44 +0000

What is SDLC?

SDLC (Software Development Life Cycle) is a step-by-step process used to develop high-quality software.

Phases of SDLC:

Requirement Gathering
Planning
Designing
Development
Testing
Implementation and Maintenance

What are SDLC Models?

SDLC Models are different approaches used to execute the SDLC phases.

Each model is chosen based on:

Project size
Requirements clarity
Risk level
Timeline

Types of SDLC Models
1. Waterfall Model
Definition:

A linear and sequential model where each phase is completed before moving to the next.

Advantages:

Simple and easy to understand
Well-structured
Good for small projects

Disadvantages:

No flexibility
Difficult to handle changes

2. Agile Model
Definition:

An iterative and flexible model where development is done in small parts called sprints.

Advantages:

Handles changing requirements
Continuous feedback
Faster delivery

Disadvantages:

Requires experienced team
Less documentation

3. V-Model (Validation & Verification)
Definition:

Each development phase has a corresponding testing phase.

Advantages:

Early testing
High quality

Disadvantages:

Rigid structure
Not flexible

Choosing the right SDLC model is important for project success.

Use Waterfall when requirements are fixed
Use Agile when requirements change frequently
Use V-Model when testing is critical