10-Writing your First Hello World Program in Java? Here is all you need to know.

👋 Writing Your First Hello World Program in Java

📌 Introduction

The Hello World program is traditionally the first program written in any programming language. It helps beginners understand the basic structure of a program, how code is written, compiled, and executed.

In Java, this program introduces several important concepts such as:

• Class structure
• The main method
• Keywords
• Compilation and execution process

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🧱 Structure of a Java Program

📌 Basic Building Blocks

A Java program is composed of:

• A class
• A main method
• Statements inside the method

Every Java application starts execution from the main method.

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🧪 Hello World Program in Java

📘 Program Code

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

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🔍 Line-by-Line Explanation (VERY IMPORTANT)

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🔹 class HelloWorld

📌 What is a Class?

A class is a blueprint that contains:

• Data
• Methods
• Instructions for the program

In Java, every program must have at least one class.

📌 Class Name Rules

• Must start with a letter
• Cannot contain spaces
• Usually written in PascalCase
• File name must match class name

✔ Correct:

class HelloWorld

❌ Incorrect:

class hello world

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🔹 public static void main(String[] args)

📌 What is the main Method?

The main method is the entry point of a Java program.
Execution always starts from main.

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🔹 Breakdown of main Method Keywords

🔸 public

• Makes the method accessible from anywhere
• JVM must be able to access main

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🔸 static

• Allows JVM to call main without creating an object
• Memory efficient for startup

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🔸 void

• Specifies that the method returns nothing

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🔸 main

• Special method name recognized by JVM
• Execution begins here

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🔸 String[] args

• Command-line arguments
• Used to pass data at runtime
• args is just a variable name

Example:

java HelloWorld Ajay Java

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

📌 Purpose

Prints output to the console.

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🧠 Explanation

• System → predefined class
• out → output stream
• println → prints text and moves cursor to next line

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⚙️ How Java Program Execution Works

📌 Two-Step Process

Java uses a two-step execution model:

1. Compilation
2. Execution

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🛠️ Step 1: Compiling the Program

📌 Save the File

File name must match class name:

HelloWorld.java

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📌 Compile Using javac

javac HelloWorld.java

✔ If no error → compilation successful
✔ Generates:

HelloWorld.class

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🧠 What Happens During Compilation?

• Source code is converted to bytecode
• Syntax errors are checked
• Bytecode is platform-independent

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▶️ Step 2: Running the Program

📌 Run Using java

java HelloWorld

✔ Output:

Hello, World!

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⚠️ Important Rule

Do not include .java or .class while running.

❌ Wrong:

java HelloWorld.java

✔ Correct:

java HelloWorld

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📊 Flow of Execution (Conceptual)

HelloWorld.java
   ↓ (javac)
HelloWorld.class
   ↓ (JVM)
Output on Screen

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⚠️ Common Mistakes Beginners Make

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❌ Mistake 1: File Name and Class Name Mismatch

class HelloWorld { }

Saved as:

Test.java

❌ Compilation error

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❌ Mistake 2: Missing main Method

class HelloWorld {
}

❌ Program compiles but does not run

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❌ Mistake 3: Wrong Case in Keywords

Public static void main(...)

❌ Java is case-sensitive

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❌ Mistake 4: Missing Semicolon

System.out.println("Hello World")

❌ Syntax error

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❌ Mistake 5: Using Quotes Incorrectly

System.out.println('Hello World');

❌ Single quotes are for characters, not strings

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🧠 Why Java Needs This Structure

📌 Reasons

• Supports Object-Oriented Programming
• Ensures uniform execution
• Enables platform independence
• Allows JVM to manage execution

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📘 Real-World Importance

Even though it is simple, the Hello World program:

• Introduces Java syntax
• Teaches compilation and execution
• Builds foundation for all future programs

Every complex Java application starts with this structure.

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📌 Summary

• Java programs start with a class
• Execution begins from main method
• javac compiles source code
• java executes bytecode
• File name must match class name
• Java follows a strict but powerful structure

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🏁 Conclusion

The Hello World program is more than just printing text—it introduces the core architecture of Java programming. Understanding each component of this program lays the foundation for learning variables, operators, control statements, object-oriented concepts, and advanced Java topics.