Tony's answer to the SF Offline Programming Challenge "Hello World" problem

HelloWorld.java

import java.util.regex.Pattern;

/**
 * A (very) simple program that takes one optional command-line argument <name>
 * and prints "Hello, <name>!" to STDOUT
 *
 * If no command line argument is provided, the program prints "Hello, world!" instead
 *
 * Can be compiled run using the following:
 * $ javac HelloWorld.java
 * $ jar cf HelloWorld.jar HelloWorld.class
 * $ java HelloWorld "Stellan Skarsgård"
 *
 * Notes: the minimalist unicode-friendly name-matching regex is from
 * https://stackoverflow.com/questions/15805555/java-regex-to-validate-full-name-allow-only-spaces-and-letters
 */
public class HelloWorld {

    public static void main(String[] args) {
        final String name = args.length > 0 ? args[0] : "world";
        // verify command line arguments
        if (args.length > 1) {
            throw new IllegalArgumentException("Please provide only one name, enclose multi-word names in quotes");
        }
        // validate the name to make sure there is no unsafe characters, this is probably overkill
        final Pattern nameValidator = Pattern.compile("^[\\p{L} .'-]+$");
        if (!nameValidator.matcher(name).matches()) {
            throw new IllegalArgumentException("Invalid characters found in provided name");
        }
        System.out.print(String.format("Hello, %s!\n", name));
    }
}

General greeting output

Hello, world!

Targeted greeting output

• java HelloWorld

Hello, world!

• java HelloWorld "Stellan Skarsgård"

Hello, Stellan Skarsgård!

Bonus output

• java HelloWorld too many cooks

Exception in thread "main" java.lang.IllegalArgumentException: Please provide only one name, enclose multi-word names in quotes
	at HelloWorld.main(HelloWorld.java:24)

• java HelloWorld "mr. Comma,man"

Exception in thread "main" java.lang.IllegalArgumentException: Invalid characters found in provided name
	at HelloWorld.main(HelloWorld.java:29)