Java Hello World Program

Java is one of the most popular and widely used programming languages and platforms. In this article, we will learn how to write a simple Java Program. This article will guide you on how to write, compile and run your first Java program. With the help of Java, we can develop web and mobile applications.

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

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:

2.1 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);

2.2 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

2.3 Just-In-Time (JIT) Compiler

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

Java Compilation & Execution Example

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

Steps:

1. Create file: Save above code as GFG.java

2. Open terminal and navigate to file location

3. Compile the program:

javac HelloWorld.java

4. Run the compiled file:

java HelloWorld

Output

Welcome to Geeks

1. In Windows

2. In Linux

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

Comments

They can either be multiline or single-line comments.

// Simple Java program
// FileName: "HelloWorld.java"

This is a single-line comment. This type of comment must begin with // as in C/C++. For multiline comments, they must begin from /* and end with */.

/*
This is a
multi-line comment
*/