Command Line Arguments

Command-line arguments let you pass information into a Java program at the moment it starts, without hardcoding values or prompting for input. They arrive as the String[] args parameter of the main method.

Why it matters

Command-line arguments are how scripts, schedulers and deployment tools hand settings to a Java program: an input file, an environment name, a port. Handling them carefully — checking how many arrived and converting them safely — is the difference between a tool that reports "usage: ..." and one that crashes with a stack trace.

How Arguments Are Passed

When you run java ProgramName arg1 arg2 arg3, the JVM places "arg1", "arg2", and "arg3" into the args array in order, as Strings — args[0] is "arg1", args[1] is "arg2", and so on. If no arguments are given, args is an empty array (length 0), not null.

Converting Arguments to Other Types

Because every argument arrives as a String, you must explicitly convert it if you need a number or other type — for example, Integer.parseInt(args[0]) for an int, or Double.parseDouble(args[1]) for a double. Always validate the array length before accessing an index, since accessing a missing index throws an ArrayIndexOutOfBoundsException.

Example

public class ArgsDemo {
    public static void main(String[] args) {
        if (args.length < 2) {
            System.out.println("Usage: java ArgsDemo <name> <age>");
            return;
        }
        String name = args[0];
        int age = Integer.parseInt(args[1]);
        System.out.println(name + " is " + age + " years old.");
    }
}

// Run with: java ArgsDemo Asha 28

Asha is 28 years old.

Exercise

Write a program that adds up all the numbers passed on the command line. With no arguments it should print a usage message instead of failing.

Running java SumArgs 4 5 6 should print Sum: 15. Running java SumArgs should print Usage: java SumArgs <number> ...

public class SumArgs {
    public static void main(String[] args) {
        // TODO: if there are no arguments, print the usage message and return

        int sum = 0;
        // TODO: convert each argument to an int and add it to sum

        System.out.println("Sum: " + sum);
    }
}

public class SumArgs {
    public static void main(String[] args) {
        if (args.length == 0) {
            System.out.println("Usage: java SumArgs <number> ...");
            return;
        }

        int sum = 0;
        for (String arg : args) {
            sum += Integer.parseInt(arg);
        }

        System.out.println("Sum: " + sum);
    }
}

Quick quiz

Interview questions