Java “Hello, World!” Program

Code Example 

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

Explanation

  1. Class Definition

      public class HelloWorld {
    • public: An access modifier that makes the class accessible from any other class.
    • class: A keyword used to define a class in Java.
    • HelloWorld: The name of the class. It must match the file name (HelloWorld.java).

  2. Main Method

      public static void main(String[] args) {
    • public: An access modifier that makes the method accessible from any other class.
    • static: A keyword that allows the method to be called without creating an instance of the class.
    • void: The return type of the method, indicating it does not return any value.
    • main: The name of the method. It is the entry point of the program.
    • String[] args: The parameter of the main method. It is an array of String objects that can store command-line arguments.

  3. Print Statement

      System.out.println("Hello, World!");
    • System: A built-in class that contains several useful methods and fields, including out.
    • out: A static member of the System class, which is an instance of PrintStream. It is connected to the console.
    • println: A method of PrintStream that prints the argument passed to it, followed by a new line.

Steps to Run the Program

  1. Write the Code

    • Create a file named HelloWorld.java and write the code shown above.

  2. Compile the Code

    • Open a terminal or command prompt.
    • Navigate to the directory where the file is saved.
    • Run the command:
      javac HelloWorld.java
    • This will compile the Java file and create a HelloWorld.class file.

  3. Run the Program

    • In the same terminal or command prompt, run the command:
      java HelloWorld
    • This will execute the compiled bytecode and print Hello, World! to the console.

The Java Compilation Model

Java source compiles to bytecode (.class files), not native machine code. The JVM interprets or JIT-compiles bytecode at runtime:

  HelloWorld.java  →  javac  →  HelloWorld.class  →  java HelloWorld  →  output

This indirection enables platform independence — the same .class file runs on Windows, Linux, and macOS.

Command-Line Arguments

The args parameter receives values passed after the class name:

  public static void main(String[] args) {
    if (args.length > 0) {
        System.out.println("Hello, " + args[0] + "!");
    } else {
        System.out.println("Hello, World!");
    }
}

Run with: java HelloWorld Alice → Hello, World! Alice!

Package Declaration

Real projects declare a package as the first line:

  package com.example.app;

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

The file must live at com/example/app/HelloWorld.java.

Common Beginner Mistakes

Mistake Fix
Class name ≠ file name Rename file to match public class
Missing semicolon Add ; after statements
Main instead of main Method names are case-sensitive
Running .java instead of .class Compile first with javac

Exercise

  1. Modify the program to print your name from a variable.
  2. Add a second println that prints the sum of two integers.
  3. Pass a name as a command-line argument and greet that name.
  4. Add a package declaration and recompile from the project root.