Java Command-Line Arguments

Java allows us to pass arguments to the main() method when running a program from the command line. These arguments are received as a String array (String[] args).

Why Use Command-Line Arguments?#

• Dynamic input without modifying the code
• Used in real-world applications to provide configuration settings
• Helpful in automation and scripting

How Command-Line Arguments Work?#

When we run a Java program from the terminal, we can pass arguments after the class name:

java MyClass arg1 arg2 arg3

• These arguments are stored in the String[] args array.
• args[0] holds the first argument, args[1] holds the second, and so on.

Example: Printing Command-Line Arguments#

class CommandLineExample {
  public static void main(String[] args) {
    System.out.println("Number of arguments: " + args.length);

    // Print all arguments
    for (int i = 0; i < args.length; i++) {
      System.out.println("Argument " + (i + 1) + ": " + args[i]);
    }
  }
}

How to Run in Terminal:#

java CommandLineExample Hello World 123

Expected Output:#

Number of arguments: 3
Argument 1: Hello
Argument 2: World
Argument 3: 123

Explanation:

• The args array stores the command-line arguments.
• args.length gives the number of arguments passed.

Converting Command-Line Arguments to Other Data Types#

Since command-line arguments are always strings, we may need to convert them.

Example: Adding Two Numbers from Command-Line Arguments#

class SumCalculator {
  public static void main(String[] args) {
    if (args.length < 2) {
      System.out.println("Please provide two numbers.");
      return;
    }

    // Convert string arguments to integers
    int num1 = Integer.parseInt(args[0]);
    int num2 = Integer.parseInt(args[1]);

    // Calculate and print the sum
    System.out.println("Sum: " + (num1 + num2));
  }
}

How to Run in Terminal:#

java SumCalculator 10 20

Expected Output:#

Sum: 30

Explanation:

• Integer.parseInt(args[0]) converts string arguments to integers.
• The program calculates and prints their sum.
• If less than two arguments are provided, it prints an error message.

Handling Errors in Command-Line Arguments#

To avoid errors, always validate input before converting it.

Example: Handling Incorrect Input#

class SafeConverter {
  public static void main(String[] args) {
    try {
      if (args.length < 2) {
        System.out.println("Please provide two numbers.");
        return;
      }

      int num1 = Integer.parseInt(args[0]);
      int num2 = Integer.parseInt(args[1]);

      System.out.println("Sum: " + (num1 + num2));
    } catch (NumberFormatException e) {
      System.out.println("Invalid number format. Please enter valid integers.");
    }
  }
}

How to Run in Terminal:#

java SafeConverter 10 abc

Expected Output:#

Invalid number format. Please enter valid integers.

Key Takeaways#

✅ Command-line arguments are passed to main(String[] args).

✅ Arguments are stored as Strings and need to be converted if required.

✅ Always validate inputs to prevent errors.

✅ Useful for passing dynamic inputs without modifying code.

Conclusion#

• We learned how to use command-line arguments in Java.
• We saw how to convert arguments to integers and perform calculations.
• We also explored error handling when incorrect inputs are passed.

Command-line arguments make Java programs flexible and dynamic, widely used in real-world applications