Java Hello World Program is the first program commonly used to learn Java. It introduces the basic structure of a Java program, including a class, the main() method, and the statement used to display output on the console.

• It shows how System.out.println() is used to print text on the console.
• It helps beginners understand how Java source code is compiled and executed.

Prerequisites

• Download install JDK.
• Download install intelliJ

Implementation of Java Hello World

The below-given program is the most simple program of Java printing "Hello World" to the screen. Let us try to understand every bit of code step by step.

public class HelloWorld {
    
    // Your program begins with a call to main()
    
    public static void main(String[] args)
    {
        // Prints "Hello, World" to the terminal window.
        
        System.out.println("Hello, World");
    }
}

Hello, World

Explanation

• public class HelloWorld declares a public class named HelloWorld.
• public allows the class to be accessed from other classes.
• static allows the main() method to be invoked without creating an object of HelloWorld.
• void indicates that the main() method does not return a value.
• main() is the method that the Java launcher uses to start the application.
• String[] args stores command-line arguments passed to the program.
• System.out.println() prints the specified text and moves the cursor to the next line.
• "Hello, World!" is the string displayed on the console.

Understanding the Java Hello World Program

1. Class Definition

Every Java program must have at least one class. Here, the class is defined using the class keyword:

public class HelloWorld {
// Statements go here
}

Note: If the class is public, the filename must match the class name HelloWorld.java

2. main Method

In the Java programming language, every application must contain a main method as it is the entry point of the application:

public static void main(String[] args)

• public: Allows JVM to access the method from anywhere.
• static: Method can run without creating an object.
• void: It doesn’t return any value.
• String[] args: Accepts command-line arguments.

3. System.out.println()

This prints output to the console.

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

• System: Built-in class from java.lang package.
• out: Static member (PrintStream object) of System.
• println(): Method that prints to console and moves to the next line.

Steps to Implement a Java Program

Java is a platform-independent language that follows a two-step execution process:

• Compilation (source code -> bytecode)
• Execution (bytecode -> machine code via JVM)

1. Compilation in Java

Java source code (.java files) is compiled by the Java Compiler (javac) into Bytecode, stored in .class files. This bytecode is platform-independent and ready to run on any system with a JVM.

• Internally, compilation involves:
• Parsing: Converts code into syntax trees (AST).
• Entering: Populates symbol tables.
• Annotation Processing: Handles annotations.
• Attribution: Performs type checking and name resolution.
• Flow Analysis: Checks for variable use and reachability.
• Desugaring: Removes syntactic shortcuts.
• Generation: Produces .class files.

2. Execution via JVM

The .class files are executed by the Java Virtual Machine (JVM), which includes the following stages:

Class Loader

Loads the main class and other dependencies into memory.

• Primordial Loader: Default system loader.
• Non-primordial Loader: Custom loaders for advanced control.

Class r = loadClass(String className, boolean resolveIt);

Bytecode Verifier

Checks that the loaded bytecode is safe to execute. It ensures:

• Variables are initialized
• Method signatures are correct
• Private access rules aren’t violated
• No stack overflows

Just-In-Time (JIT) Compiler

Converts bytecode into native machine code at runtime for faster execution.

How Java Hello World Program Works

Java source code goes through compilation and execution before the output is displayed.

• The programmer writes the source code in a .java file.
• The Java compiler (javac) compiles the source code into bytecode.
• The bytecode is stored in a .class file.
• The Java launcher starts the application.
• The JVM loads and executes the required class and its main() method.
• System.out.println() displays the output on the console.

The bytecode generated by the Java compiler is designed to run on any compatible JVM, which is a key part of Java's platform-independent execution model.

Java Compilation and Execution Example

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

Steps to Run

1. Create File: Save the file as HelloWorld.java.
2. Open Terminal: Navigate to the folder containing the file.
3. Compile:

javac HelloWorld.java

4. Run:

java HelloWorld

Output

Hello, World

1. In Windows

2. In Linux

Note: If you get ClassNotFoundException, ensure the .class file is in the correct directory or check your CLASSPATH.