How to Use Command-Line Arguments in Java

You can pass values to a program using command-line arguments. To run the MySecondApp program and pass the values "value1" and "value2" as input, use the following command:

java com.company.lesson2.MySecondApp value1 value2

These values need to be received within the program. The main method helps us achieve this:

public static void main(String[] args)

This method takes an array of type String as a parameter: String[] args. This variable will contain the values we passed: "value1" and "value2".

In the following example, the main() method uses a for loop to iterate over the args array and print the command-line arguments. We will discuss for loops and arrays in more detail in future lessons.

package com.company.lesson2;

public class MySecondApp {
    public static void main(String[] args) {
        for (String str : args) {
            System.out.println("Argument = " + str);
        }
    }
}

Run the program using the following command:

java com.company.lesson2.MySecondApp value1 value2

Console output:

Argument = value1
Argument = value2

System properties: the -D flag

Command-line arguments are not the only way to pass data into a program at startup. There are also system properties, set with the -D flag in the form -Dname=value. It is important not to confuse them with arguments:

• Arguments come after the class name and land in the args array.
• System properties (-D) come before the class name and are read with System.getProperty(), not from args.

For example, let's run the program with one system property and one argument:

java -DmyProp=hello com.company.lesson2.MySecondApp value1

Here myProp=hello is a system property, and value1 is a command-line argument. You can read them in code like this:

public class MySecondApp {
    public static void main(String[] args) {
        System.out.println(System.getProperty("myProp")); // hello
        System.out.println(args[0]);                      // value1
    }
}

To get all system properties at once, use System.getProperties() — it returns a Properties object with every JVM property, including your own -D values and standard ones such as java.version and os.name.