Command-Line Arguments

1. Introduction

Command-line arguments allow users to pass information to a Java program at the time of execution, without modifying the code.

They are commonly used for:

• Configurations
• File paths
• User inputs in automated scripts
• Running programs with dynamic values
• Processing multiple inputs efficiently

In Java, command-line arguments are stored in the String array args of the main() method.

2. main() Method and args[]

The standard Java main method is:

public static void main(String[] args)

Here:

• args → array of Strings
• Each value passed from the command line becomes an element of this array

Example:

java ProgramName hello world 123

Inside main:

• args[0] → "hello"
• args[1] → "world"
• args[2] → "123"

3. Basic Example

Program: Print Command-Line Arguments

public class Main {
    public static void main(String[] args) {
        for (int i = 0; i < args.length; i++) {
            System.out.println("Argument " + i + ": " + args[i]);
        }
    }
}

Execution:

java Main Apple Banana Mango

Output:

Argument 0: Apple
Argument 1: Banana
Argument 2: Mango

4. Passing Numbers as Command-Line Arguments

Arguments are always Strings. To use them as numbers, convert them manually.

Example: Add Two Numbers

public class Add {
    public static void main(String[] args) {
        int a = Integer.parseInt(args[0]);
        int b = Integer.parseInt(args[1]);

        int sum = a + b;
        System.out.println("Sum: " + sum);
    }
}

Execution:

java Add 10 20

Output:

Sum: 30

5. Handling Errors (Missing Arguments)

If you run:

java Add 10

Then args[1] does not exist → runtime error:

ArrayIndexOutOfBoundsException

Fix using validation:

if (args.length < 2) {
    System.out.println("Please provide two numbers!");
    return;
}

6. Using args.length

args.length tells how many arguments were passed.

Example:

System.out.println("Total arguments: " + args.length);

7. real-world use cases

7.1 File input from command line

java FileReader data.txt

args[0] → "data.txt"

7.2 Running programs with configurations

java Server --port=8080 --mode=dev

7.3 Automating tasks in DevOps scripts

java Backup /home/user/files

8. Advanced Example

Example: Sum of all command-line arguments

public class SumArgs {
    public static void main(String[] args) {
        int sum = 0;

        for (String val : args) {
            sum += Integer.parseInt(val);
        }

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

Execution:

java SumArgs 5 10 15 20

Output:

Total Sum: 50

9. Common Mistakes

• Forgetting to check args.length
• Assuming command-line arguments are always numbers
• Attempting to modify args (avoid doing so)
• Mixing spaces incorrectly while passing input

Example mistake:

java Test "Hello World"

If quotes are not used:

java Test Hello World
# args[0] = Hello
# args[1] = World

10. Summary

• Command-line arguments allow passing dynamic values to a program.
• They are accessed through the args[] array in the main() method.
• All arguments are Strings; convert manually for numeric use.
• Always validate the number of arguments using args.length.
• Useful for automation, scripts, configurations, and flexible programs.

Written By: Shiva Srivastava