The main method and args

Key points

• The JVM starts at public static void main(String[] args). Any other signature compiles but will not launch.
• static because no object exists yet, public so the launcher can reach it, void because the exit code is separate.
• args holds whatever followed the class name on the command line. When nothing was passed it is empty, never null.
• Every argument arrives as text, so "7" + 1 is "71". Parse it first with Integer.parseInt.
• Returning from main exits with code 0, which means success. System.exit(1) signals failure.
• Any class may declare a main, even when it is not the entry point — handy for quick experiments.

Example

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        System.out.println("real args.length = " + args.length + " -> " + Arrays.toString(args));

        String[] demo = args.length > 0 ? args : new String[]{"report", "--verbose", "sales.csv"};
        System.out.println("pretending you ran: java Main " + String.join(" ", demo));

        String command = demo[0];
        boolean verbose = false;
        String file = null;

        for (String arg : demo) {
            if (arg.equals("--verbose")) {
                verbose = true;
            } else if (arg.endsWith(".csv")) {
                file = arg;
            }
        }

        System.out.println("command = " + command + ", verbose = " + verbose + ", file = " + file);
        System.out.println("every argument arrives as text: \"7\" + 1 = " + ("7" + 1));
        System.out.println("so numbers need parsing: 7 + 1 = " + (Integer.parseInt("7") + 1));
        System.out.println("exit code would be " + (file == null ? 1 : 0) + "; 0 means success");
    }
}

main is an ordinary method with the one signature the launcher recognises.