Introduction to Java

1. What is Java?

Java ek high-level, object-oriented programming language hai jise Sun Microsystems ne 1995 mein banaya tha (ab ise Oracle own karta hai). Ye "Write Once, Run Anywhere" principle pe kaam karti hai, matlab ek baar code likho aur kisi bhi device (Windows, Linux, Mac, Mobile) pe run karo!

Java Ke Features

• Simple & Easy to Learn – Syntax C/C++ jaisa hai, but easier.
• Platform Independent – Bytecode ke wajah se kahi bhi run ho sakti hai.
• Object-Oriented – Sab kuch objects aur classes ke around revolve karta hai.
• Secure – No direct memory access, no pointers (hacker attacks kam hote hai).
• Multithreading Support – Multiple tasks ek saath handle kar sakti hai.

2. Java Editions

Java ko different platforms ke liye alag editions mein divide kiya gaya hai:

1. Java SE (Standard Edition) – Core Java, desktop & server applications ke liye.
2. Java EE (Enterprise Edition) – Large-scale web & enterprise apps ke liye.
3. Java ME (Micro Edition) – Mobile & embedded devices ke liye (ab ye kam use hota hai).

3. JDK, JRE, JVM – Architecture

JVM (Java Virtual Machine)

• Kya karta hai: Java program ka Bytecode ko leke usse Machine code mein convert karta hai (jo aapka computer samajh sake).
• Special baat: JVM har OS ke liye alag hota hai, isliye JVM platform-dependent hota hai.
• Example: Windows ke liye alag JVM, Linux ke liye alag.

Java ka code platform-independent isliye hota hai kyunki JVM OS ke according handle karta hai sab kuch.

JRE (Java Runtime Environment)

• Kya hota hai:JVM + Jar files / Libraries jo program ko run karne mein madad karti hain
• Use kab hota hai: Agar aapko sirf program run karna hai, aur coding ya compile nahi karni, to JRE enough hai.

JDK (Java Development Kit)

• Kya hota hai: JRE (toh JVM bhi included) + Tools jaise javac (compiler), debugger, etc.
• Use kab hota hai: Jab aapko Java program likhna, compile karna aur run karna hai, to JDK chahiye.

4. Java Setup – Install & Path Set Karo

Step 1: JDK Download Karo

• Oracle se latest version download karo jo tumhare system ke according ho:

 Oracle JDK

Step 2: Install JDK

• Downloaded file ko run karo aur default settings pe install karo.

Step 3: Path Set Karo (Windows)

1. Control Panel > System > Advanced Settings > Environment Variables
2. "Path" variable mein JDK ka "bin" folder ka location add karo.
3. Example: C:Program FilesJavajdk-17in

Step 4: Check Installation

Command Prompt pe ye type karo:

java -version
javac -version

Agar version dikhe, toh setup sahi hai!

5. Your First Java Program - "Hello, World"

Step 1: File Banao

Ek file banao HelloWorld.java aur ye code likho:

Note: Ye code tum vscode ya kisi bhi editor mein likh sakte ho.

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

Step 2: Compile Karo

CMD/Terminal pe iss file ke folder mein jaake ye likho:

javac HelloWorld.java

Isse .class file generate hogi.

Step 3: Run Karo

Ab ye command run enter karo:

java HelloWorld

Output:

Hello, World!

Code Explanation:

public class HelloWorld {

public class HelloWorld:

Yeh ek class ka declaration hai jiska naam HelloWorld hai. Java mein har program ek class ke andar likha jaata hai.

public ka matlab hai yeh class sab jagah access ho sakti hai (accessible from anywhere).

public static void main(String[] args) {

public static void main(String[] args):

Yeh Java program ka entry point hai — matlab jab program run hota hai, yeh method sabse pehle chalta hai.

• public: method sab jagah se accessible hota hai.
• static: bina object banaye is method ko run kar sakte hain.
• void: yeh method kuch return nahi karta.
• main: yeh method ka naam hai — Java isi ko run karti hai sabse pehle.
• String[] args: command-line arguments receive karne ke liye hota hai (filhaal ignore kar sakte ho).

 System.out.println("Hello, World!");

System.out.println("Hello, World!");

Yeh line console pe "Hello, World!" print karti hai.

• System: System Java ki ek built-in class hai.
• out: output stream (console ke liye).
• println(): ek line print karta hai aur next line mein cursor le jaata hai.

    }
}

Yeh closing brackets hain jo main method aur HelloWorld class ko close karte hain.

6. Java Kaise Kaam Karti Hai?

1. Compilation: .java file ko .javac compiler bytecode (.class file) mein convert karta hai.
2. Execution: JVM bytecode ko machine code mein convert karke run karta hai.

Diagram:

Java Code (.java) → Compiler → Bytecode (.class) → JVM → Machine Code → Output