Java Hello World Program

Java Hello World Program is the first and simplest program that beginners learn. It demonstrates the basic structure of a Java program and how to display output on the console using Java syntax.

• It helps understand the basic structure of a Java program (class, main method)
• It is the starting point for learning Java programming

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:

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.

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.