Forem: Arul .A

Why useEffect Causes Infinite Loops in React?

Arul .A — Tue, 26 May 2026 13:00:49 +0000

React’s useEffect() Hook is used for side effects like API calls, timers, and event listeners. One common beginner mistake is creating infinite loops inside useEffect().

Example of wrong code:

useEffect(() => {
   setCount(count + 1);
});

This creates an infinite loop because:

Component renders
useEffect() runs
State updates
Component re-renders
useEffect() runs again

This process repeats forever.

The correct way is:

useEffect(() => {
   setCount(count + 1);
}, []);

The empty dependency array [] tells React to run the effect only once after the first render.

Dependency array behavior:

No dependency array → runs after every render
Empty array [] → runs once
[value] → runs when value changes

Another common mistake is using objects or functions in dependency arrays because React compares references, not actual values.

Best practices:

Use dependency arrays correctly
Avoid unnecessary state updates
Keep useEffect() simple
Understand React rendering flow

Understanding useEffect() properly helps developers avoid performance issues and debugging problems in React applications.

Difference Between JavaScript (.js) and JSX (.jsx)

Arul .A — Sat, 23 May 2026 17:04:56 +0000

JavaScript and JSX are closely related technologies used in modern web development, especially in React applications.

-What is JavaScript (.js)?

JavaScript is a programming language used to create dynamic and interactive web applications. Files with the .js extension contain pure JavaScript code.
JavaScript is commonly used for:

1.Variables and functions
2.Loops and conditions
3.API calls
4.Event handling
5.DOM manipulation
6.Business logic

Example :

function add(a, b) {
  return a + b;
}

console.log(add(5, 3));

What is JSX (.jsx)?
JSX stands for JavaScript XML. It is a syntax extension for JavaScript that allows developers to write HTML-like code inside JavaScript.
JSX is mainly used in React to create user interface components.

Example :

function App() {
  return (
    <div>
      <h1>Hello React</h1>
    </div>
  );
}

export default App;

Difference Between @PathVariable and @RequestParam

Arul .A — Fri, 22 May 2026 17:37:37 +0000

Both @PathVariable and @RequestParam are used to receive data from the client request in Spring Boot.But they work in completely different ways.

What is @PathVariable?

@PathVariable is used to get values directly from the URL path.

Example:

@GetMapping("/product/{id}")
public String getProduct(@PathVariable int id) {
    return "Product ID: " + id;
}

URL:

/product/10

Here:

id = 10 => The value 10 becomes part of the URL path itself.

Usually used for:

Specific resource IDs
Mandatory values
Unique objects
What is @RequestParam?

@RequestParam gets values from query parameters.

Example:

@GetMapping("/products")
public String search(@RequestParam String category) {
    return category;
}

URL:

/products?category=mobile

Here:

category = "mobile"

Usually used for:

Filtering
Searching
Sorting
Pagination
Optional data

Examples:

/products?category=mobile
/products?page=2
/products?sort=price
/products?search=iphone

These are not unique resources.

They are conditions or filters.

What is Maven?And Pom.xml?

Arul .A — Wed, 06 May 2026 04:58:18 +0000

What is Maven in Spring Boot?

Maven is a build automation and project management tool used in Java projects. In Spring Boot, Maven handles:
Downloading dependencies (libraries/jars) automatically
Compiling your Java code
Running tests
Packaging your app into a .jar or .war file
Deploying your application

What Maven Actually Does

Maven mainly handles 3 things:

Dependency Management

It downloads required libraries automatically.

Example:
You want to use MySQL in Spring Boot.

Without Maven:

search MySQL connector online
download jar
add to project manually

With Maven:

just add this in pom.xml

<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <version>8.3.0</version>
</dependency>

Maven downloads it automatically from the Maven repository.

Build Automation

Maven can:

compile code
run tests
package project
create .jar or .war

Instead of: javac Main.java

You simply run: mvn compile

To create a JAR: mvn package

Standard Project Structure

Maven forces a clean structure.

Example:

project-name/
│
├── src/
│   ├── main/
│   │   ├── java/
│   │   └── resources/
│   │
│   └── test/
│       └── java/
│
├── pom.xml

What is pom.xml ?

This is the heart of Maven.

POM = Project Object Model

It contains:

project name
dependencies
plugins
Java version
build settings

Example:

<project xmlns="http://maven.apache.org/POM/4.0.0">

    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>
    <artifactId>demo</artifactId>
    <version>1.0</version>

    <dependencies>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <version>3.2.5</version>
        </dependency>

    </dependencies>

</project>

Important Maven Commands :

Command	Purpose
`mvn compile`	Compiles code
`mvn test`	Runs tests
`mvn package`	Creates JAR/WAR
`mvn clean`	Deletes old build files
`mvn install`	Installs into local repository

Spring Boot MVC Explained: Build Smarter Web Apps with Model-View-Controller

Arul .A — Tue, 05 May 2026 07:19:52 +0000

Introduction

When building applications with Spring Boot, one of the most important architectural patterns you’ll encounter is MVC (Model-View-Controller). Many developers claim to “use MVC,” but in reality, they only use annotations like @RestController without understanding the separation of responsibilities.

This article breaks MVC down properly—what it is, how Spring Boot implements it, and how to use it correctly in real-world applications.

What is MVC?

MVC stands for:

Model
View
Controller

It is a design pattern that separates an application into three layers, each with a distinct responsibility. The goal is to reduce code complexity and improve maintainability.

Why MVC Matters (No-Nonsense Explanation)

If you don’t follow MVC:

Your controller becomes a dumping ground
Business logic gets duplicated everywhere
Debugging becomes painful
Scaling the application becomes difficult

MVC in Spring Boot

Spring Boot uses Spring MVC, which provides built-in support for this architecture using annotations and conventions.

Typical layers in a Spring Boot MVC application:

Controller → Service → Repository → Database

1. Model Layer (Data + Business Logic)

The Model represents:

Application data
Business rules
Database interaction

Components:

Entity classes (@Entity)
Repository interfaces (JpaRepository)
Sometimes DTOs

Example:

@Entity
public class User {

    @Id
    @GeneratedValue
    private Long id;

    private String name;
    private String email;

    // getters and setters
}

Repository:

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
}

2. View Layer (What the User Sees)

The View is responsible for presenting data.

Two common types in Spring Boot:

1. Traditional MVC

Thymeleaf
JSP

2. REST APIs (Most common today)

JSON responses
Used with frontend frameworks (React, Angular)

Example (JSON View):

{
  "id": 1,
  "name": "Arul",
  "email": "arul@example.com"
}

3. Controller Layer (Request Handling)

The Controller:

Receives HTTP requests
Validates input
Calls the service layer
Returns response

Example:

@RestController
@RequestMapping("/users")
public class UserController {

    @Autowired
    private UserService service;

    @GetMapping("/{id}")
    public User getUser(@PathVariable Long id) {
        return service.getUserById(id);
    }
}

4. Service Layer (The Most Ignored Layer)

This is where most beginners fail.

The Service layer:

Contains business logic
Acts as a bridge between Controller and Repository
Keeps controllers clean

Example:

@Service
public class UserService {

    @Autowired
    private UserRepository repo;

    public User getUserById(Long id) {
        return repo.findById(id)
                   .orElseThrow(() -> new RuntimeException("User not found"));
    }
}

How MVC Flow Works (Step-by-Step)

Let’s say a client requests:

GET /users/1

Flow:

Request hits Controller
Controller calls Service
Service calls Repository
Repository fetches data from Database
Data returns to Service
Service returns to Controller
Controller sends response to client (View)

Real Project Structure

A clean Spring Boot project should look like this:

com.example.project
│
├── controller
├── service
├── repository
├── model
└── dto

If your project doesn’t look like this (or similar), you’re probably mixing responsibilities.

Advantages of MVC

Clear separation of concerns
Easier debugging
Scalable architecture
Better team collaboration
Reusable components

Understanding the Map Interface in Java

Arul .A — Sat, 02 May 2026 04:25:45 +0000

Map:

Map is an interface in Java used to store data in key-value pairs.
It is part of Java Collections Framework, but technically Map does not extend Collection.
Map is an interface in Java used to store data in the form of:

key -> value

Each key points to one value.

Example:

101 -> Arul
102 -> Raj
103 -> Vijay

Here:

101, 102, 103 are keys
Arul, Raj, Vijay are values

Main Features of Map:

Stores Key and Value:

key -> value

Example:

1 -> Apple
2 -> Mango
3 -> Banana

Key Must Be Unique

Duplicate key not allowed.
Example:

map.put(1,"Arul");
map.put(1,"Raj");

1 = Raj  //key and values are replaced.

Values Can Duplicate:

1 = Arul
2 = Arul

Allowed.

One Null Key (depends on class)

In HashMap, one null key allowed.

Example:

null = Test

Methods in map:

Method - Use

put(k,v) - Add data
get(k) - Get value by key
remove(k) - Delete by key
containsKey(k) - Check key
containsValue(v - Check value
size() - Number of pairs
clear() - Remove all
keySet() - Get all keys
values() - Get all values
entrySet() - Get key-value pairs

Example:

import java.util.*;

public class Main {
    public static void main(String[] args) {

        Map<Integer,String> map = new HashMap<>();

        map.put(101,"Arul");
        map.put(102,"Raj");
        map.put(103,"Vijay");

        System.out.println(map);

        System.out.println(map.get(102));

        map.remove(101);

        System.out.println(map);
    }
}

Classes that Implement Map

HashMap:

Fast
No order guarantee

LinkedHashMap:

Maintains insertion order

TreeMap:

Sorted by key

Java Sets: HashSet, TreeSet, and LinkedHashSet Explained with Examples

Arul .A — Thu, 30 Apr 2026 15:14:20 +0000

What is the Set interface?

The Set interface in Java is your go-to when you need a collection that stores unique elements only. No duplicates, no index-based access.

-Set extends the Collection interface and lives in java.util. Its defining contract: it will never contain duplicate elements.

-Internally, "duplicate" is determined by equals() and hashCode() — so you must override both when using custom objects.

1.HashSet:

Use HashSet when you only care about uniqueness and O(1) average-case performance, and don't need a predictable iteration order.
O(1) avg -Backed by a HashMap. Fastest for add/remove/contains. No ordering guarantee whatsoever.

Example:

Set<String> fruits = new HashSet<>();
fruits.add("Apple"); 
fruits.add("Mango"); 
fruits.add("Apple"); // duplicate — silently ignored System.out.println(fruits.size()); // 2 System.out.println(fruits.contains("Mango")); // true

2.LinkedHashSet — insertion order preserved :

When you need deduplication but iteration must reflect the order elements were added, LinkedHashSet is the answer.
O(1) avg
-HashSet + doubly-linked list. Preserves insertion order. Slightly more memory than HashSet.

Example:

Set<String> steps = new LinkedHashSet<>();
steps.add("Login");
steps.add("Dashboard"); 
steps.add("Login"); // duplicate dropped 
steps.add("Logout"); 
steps.forEach(System.out::println); // Login → Dashboard → Logout

3.TreeSet — sorted order, always:

TreeSet keeps elements in ascending natural order (or your custom Comparator).
O(log n)
-Backed by a Red-Black tree. Elements stored in sorted (natural or custom Comparator) order.

Example:

Set<Integer> scores = new TreeSet<>();
scores.add(42); 
scores.add(7); 
scores.add(19); 
System.out.println(scores); // [7, 19, 42] — always sorted // NavigableSet methods (cast required) 
TreeSet<Integer> nav = (TreeSet<Integer>) scores; System.out.println(nav.floor(20)); // 19 System.out.println(nav.ceiling(20)); // 42

Difference between HashSet and LinkedHashSet and TreeSet:

Understanding String Immutability Through Practical Python Examples

Arul .A — Tue, 21 Apr 2026 16:00:49 +0000

1.Remove extra space :

name="practice makes    perfect"
i=0
result=""
while i<len(name):
  if name[i]!=" ":
    result=result+name[i]
  else:
    if i>0 and name[i-1]!=" ":
      result=result+name[i]
  i+=1
print(result)

Output:

2.Remove unwanted space in front of the sentence:

name = "practice makes perfect        "
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 unwanted space in back of the sentence:

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.Reversing a word:

name = "Arul"
result = ""

i = len(name)-1

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

print(result)

Output:

Password generator

Arul .A — Tue, 21 Apr 2026 08:58:59 +0000

1.Password Generator :

Algorithm:

1.Define character sets: uppercase, lowercase, digits, and special characters.

2.Combine all character sets into one common pool.

3.Initialize an empty password string.

4.Add one random character from each character set to ensure strength.

5.Use a loop to add remaining random characters from the common pool until length is reached.

6.Display the final generated password.1.

Example:

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 :

Password Validation ?

Arul .A — Tue, 21 Apr 2026 08:46:46 +0000

1.Lowercase to Uppercase converter:

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)

2.First letter Only 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)

3.Letters 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)

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)

5.Password Validation :

text = input("Create your password: ")

upper = False
lower = False
number = False
special = False
valid = True

i = 0
while i < len(text):
    ch = text[i]

    if "A" <= ch <= "Z":
        upper = True
    elif "a" <= ch <= "z":
        lower = True
    elif "0" <= ch <= "9":
        number = True
    elif ch in "!@#$&":
        special = True
    else:
        valid = False

    i += 1

if len(text) < 8:
    print("Password must be at least 8 characters")

else:
    if not valid:
        print("Only allowed special characters: ! @ # $ &")

    if not upper:
        print("Add at least one uppercase letter")

    if not lower:
        print("Add at least one lowercase letter")

    if not number:
        print("Add at least one number")

    if not special:
        print("Add at least one special character: ! @ # $ &")

    if upper and lower and number and special and valid:
        print("Password successfully created")

Sorting Custom Objects in Java: A Beginner's Guide to Comparable

Arul .A — Tue, 21 Apr 2026 04:56:57 +0000

Comparable Interface in Java Collections :

What is it?

Comparable is a Java interface in java.lang with a single method:

Ex:

public interface Comparable<T> {
    int compareTo(T other);
}

It defines the natural ordering of objects of a class. When a class implements Comparable, it's saying "I know how to compare myself to another object of my type."

compareTo return values:

Return Value - Meaning
negative - this < other
0 - this== other
positive - this > other

Example:

Product pen  = new Product("Pen",  10.0);
Product bag  = new Product("Bag",  50.0);
Product book = new Product("Book", 10.0); // same price as pen

System.out.println(pen.compareTo(bag));  // negative → pen < bag
System.out.println(bag.compareTo(pen));  // positive → bag > pen
System.out.println(pen.compareTo(book)); // 0        → equal price

Result - Meaning
negative - pen is cheaper than bag
0 - pen and book cost the same
positive - bag is more expensive than pen

Why use it?

Without Comparable, Java's sorting and ordering mechanisms have no idea how to rank your custom objects.

Implementing it lets you use:

Collections.sort(list)
Arrays.sort(array)
TreeSet / TreeMap (automatically sorted)
PriorityQueue (min-heap by default)

When to use it?

Use Comparable when your class has one obvious, default ordering that makes sense intrinsically — like a Student sorted by roll number, or a Product by price.

class Student implements Comparable<Student> {
    String name;
    int rollNo;

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

    @Override
    public int compareTo(Student other) {
        return this.rollNo - other.rollNo; // Natural order: ascending by rollNo
    }

    @Override
    public String toString() {
        return name + "(" + rollNo + ")";
    }
}

public class Main {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<>();
        students.add(new Student("Alice", 3));
        students.add(new Student("Bob", 1));
        students.add(new Student("Carol", 2));

        Collections.sort(students); // Uses compareTo internally

        System.out.println(students); // [Bob(1), Carol(2), Alice(3)]

        // Works with TreeSet too
        TreeSet<Student> set = new TreeSet<>(students);
        System.out.println(set); // [Bob(1), Carol(2), Alice(3)]
    }
}

Decode Any Indian Pincode Using Python?

Arul .A — Mon, 20 Apr 2026 08:42:31 +0000

We take a 6-digit pincode as input from the user and validate it before processing. By splitting the first digit, first two digits, and first three digits, we identify the Region, Sub-region, and District. This way, anyone can find the exact location details of any pincode.

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 Pincode: ").strip()
get_pincode_details(pincode)