BASIC JAVA

☕ Java Core & Advanced Programming Notes

JVM Architecture, Object-Oriented Principles, Exception Handling, Collections Framework, Multithreading aur JDBC ka complete reference guide.

Curriculum Index

1. Module 1: Architecture & Core Syntax
2. Module 2: Variables, Types & Operators
3. Module 3: Conditionals, Loops & Methods
4. Module 4: The 4 Pillars of OOPs & Interfaces
5. Module 5: Exception Handling Architecture
6. Module 6: Collections Framework (List, Set, Map)
7. Module 7: File I/O, Multithreading & JDBC

Module 01

1️⃣ Java Architecture & First Program

Definition: Java ek high-level, robust, strongly typed aur pure Object-Oriented Programming (OOP) language hai jise Sun Microsystems (ab Oracle) ne banaya tha.

WORA Principle (Write Once, Run Anywhere):
Java source code (.java) compile hokar direct machine code nahi banta, balki intermediate Bytecode (.class) banta hai. Yeh Bytecode kisi bhi OS par chal sakta hai jahan JVM (Java Virtual Machine) installed ho.

First Java Program Structure

public class HelloWorld {
    // Execution starting point
    public static void main(String[] args) {
        System.out.println("Hello, Java Enterprise World!");
    }
}

• public class HelloWorld: File ka naam aur class ka naam exact match hona mandatory hai (HelloWorld.java).
• static: JVM is method ko bina class ka object create kiye direct execute kar sakti hai.
• void: Yeh method execution complete hone par kuch return nahi karta.
• String[] args: Command-line arguments pass karne ke liye array.

Module 02

2️⃣ Variables, Primitive Data Types & Operators

Java ek strongly typed language hai; har variable ko use karne se pehle uska type explicitly declare karna padta hai.

Data Type	Memory Size	Default Value	Example
int	4 Bytes (32-bit)	0	int age = 20;
double	8 Bytes (64-bit)	0.0d	double pi = 3.14159;
char	2 Bytes (Unicode)	'\u0000'	char grade = 'A';
boolean	1 Bit (logical)	false	boolean isEnrolled = true;
String (Non-Primitive)	Object reference	null	String name = "Rahul";

Operators Overview

Category	Operators	Purpose
Arithmetic	+, -, *, /, %	Mathematical calculations (Remainder calculation with %).
Assignment	=, +=, -=, *=	Value calculate karke variable me assign karna.
Relational	==, !=, >, <, >=	Values compare karna (Returns boolean true/false).
Logical	&& (AND), || (OR), ! (NOT)	Multiple conditions ko combine karna.

Module 03

3️⃣ Control Statements & Methods

Code execution flow ko control karne ke liye branching aur loops ka upyog hota hai.

// 1. Conditionals (if-else)
int score = 85;
if (score >= 90) {
    System.out.println("Grade: A+");
} else if (score >= 75) {
    System.out.println("Grade: A");
} else {
    System.out.println("Grade: B");
}

// 2. For Loop
for (int i = 1; i <= 5; i++) {
    System.out.println("Iteration: " + i);
}

// 3. Reusable Static Method
public static int multiply(int a, int b) {
    return a * b;
}

Live Java JVM Execution Simulator

[JVM Console Output]: Click any button above to simulate executing Java bytecode...

Module 04

4️⃣ The 4 Pillars of OOPs & Interfaces

Object-Oriented Programming real-world entities ko code structure me map karta hai:

Pillar	Concept	Code Keyword
Encapsulation	Data (fields) aur methods ko single unit me band karna aur direct access hide karna.	private variables with getters/setters
Inheritance	Parent class ki properties aur methods ko child class me reuse karna.	extends
Polymorphism	Ek hi method ka multiple forms me kaam karna (Overloading & Overriding).	@Override, Method Overloading
Abstraction	Internal implementation hide karke sirf essential features show karna.	abstract class, interface

// Interface (Pure Contract)
interface Animal {
    void makeSound(); // abstract method
}

// Inheritance & Implementation
class Dog implements Animal {
    @Override
    public void makeSound() {
        System.out.println("Bark! Bark!");
    }
}

Module 05

5️⃣ Exception Handling Architecture

Runtime errors se application crash hone se bachane ke liye robust try-catch-finally blocks use hote hain:

try {
    int result = 100 / 0; // Throws ArithmeticException
} catch (ArithmeticException e) {
    System.err.println("Error: Zero se division allowed nahi hai! " + e.getMessage());
} catch (Exception e) {
    System.err.println("Generic Exception caught: " + e.getMessage());
} finally {
    // Har haal me execute hota hai (resource cleanup ke liye)
    System.out.println("Database/File connections closed safely.");
}

Module 06

6️⃣ Java Collections Framework

Dynamic objects ke groups ko store aur manipulate karne ka centralized framework:

Collection Interface	Concrete Class	Ordering & Duplicate Behavior
List	ArrayList, LinkedList	Ordered index, Duplicates allowed hote hain.
Set	HashSet, TreeSet	Unordered, No duplicate elements.
Map	HashMap, TreeMap	Key-Value pairs (Unique keys, Duplicate values).

// List Example
List<String> list = new ArrayList<>();
list.add("Java");
list.add("Spring Boot");

// Map Example (Key-Value)
Map<Integer, String> userMap = new HashMap<>();
userMap.put(101, "Rahul Sharma");
userMap.put(102, "Anita Verma");
System.out.println("User 101: " + userMap.get(101));

Module 07

7️⃣ File I/O, Multithreading & JDBC Database Connectivity

1. Multithreading (Concurrent Execution)

class WorkerThread extends Thread {
    public void run() {
        System.out.println("Thread ID: " + Thread.currentThread().getId() + " is processing task.");
    }
}

// Execution
WorkerThread t1 = new WorkerThread();
t1.start(); // Spawns a new independent thread

2. JDBC (Java Database Connectivity) Pipeline

import java.sql.*;

public class JdbcDemo {
    public static void main(String[] args) {
        String url = "jdbc:mysql://localhost:3306/company_db";
        String user = "root";
        String pass = "secret";

        // Try-with-resources automatically closes connections
        try (Connection con = DriverManager.getConnection(url, user, pass);
             Statement stmt = con.createStatement();
             ResultSet rs = stmt.executeQuery("SELECT id, name FROM employees")) {

            while (rs.next()) {
                System.out.println(rs.getInt("id") + " - " + rs.getString("name"));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}