Java Programs

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Introduction to Java

Hello World - Java Program

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

 
OUTPUT :
Hello, World!

How to compile and run

1. Save the code above into a file named HelloWorld.java.
2. Open a terminal (or command prompt) and go to the folder with the file.
3. Compile:

javac HelloWorld.java

This produces HelloWorld.class (the bytecode).

1. Run:

java HelloWorld

Expected output

Hello, World!

Detailed explanation (line by line)

• // HelloWorld.java
  A single-line comment — ignored by the compiler. Useful to label the file.
• public class HelloWorld {
  Declares a public class named HelloWorld. In Java, every application must have at least one class. The source filename must match the public class name (HelloWorld.java).
• public static void main(String[] args) {
  This is the entry point of the program — the method the Java Virtual Machine (JVM) calls to start your program.
• public — accessible to the JVM from outside the class.
• static — no object of the class is needed; JVM can call this method without instantiating HelloWorld.
• void — the method does not return any value.
• main — the method name the JVM looks for.
• String[] args — an array of command-line arguments passed to the program (can be empty).
• System.out.println("Hello, World!");
  Prints the text Hello, World! to standard output (console), then moves to the next line.
• System — a core Java class that provides access to system resources.
• out — a public static field of System (a PrintStream) representing standard output.
• println(...) — prints the argument, then prints a newline.
• }
  Closes the main method.
• }
  Closes the HelloWorld class.

Variations & quick notes

• Use System.out.print("Hello, World!"); if you don’t want a newline after the text.
• You can print multiple items: System.out.println("Hello, " + "World!"); — + concatenates strings.
• Command-line arguments: run java HelloWorld John and inside main args[0] would be "John" (guard against empty args before accessing).