`args` in `public static void main(String[] args)`

Every Java program starts running from the main method. You’ve seen it many times:

public static void main(String[] args)

But what’s the purpose of `String[] args`?

It allows your program to receive input from the command line.

When you run your program from the terminal, you can pass extra data directly to it. Those pieces of data are stored in an array called args.

What exactly is `args`?

args is a String array.
When you run a java program, each space-separated item typed after the program name becomes one element in that array.

For example, if you run:

java MyProgram hello world 1

Then inside your program:

args[0] is "hello"
args[1] is "world"
args[2] is "1"
args.length is 3

Even though "1" looks like a number, it is still a String, because all command-line arguments start as strings.

How do you use these arguments?

Here’s a example that prints whatever the user typed:

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

If you run:

java Fruits apple banana cherry 1

Output:

Argument 0: apple
Argument 1: banana
Argument 2: cherry
Argument 3: 1

Converting the arguments

Since every element of args is a String, you must convert values to numbers if needed:

int x = Integer.parseInt(args[3]);
double y = Double.parseDouble(args[3]);

Exercise

Add two numbers from the command line

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

        int a = Integer.parseInt(args[0]);
        int b = Integer.parseInt(args[1]);
        int sum = a + b;

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

Running:

java AddNums 8 12

Output:

The sum is: 20

Classwork

Lesson 27 - args main method

args in public static void main(String[] args)