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The Platform Environment
Configuration Utilities
Properties
Command-Line Arguments
Environment Variables
Other Configuration Utilities
System Utilities
Command-Line I/O Objects
System Properties
The Security Manager
Miscellaneous Methods in System
PATH and CLASSPATH
Questions and Exercises
Trail: Essential Classes
Lesson: The Platform Environment
Section: Configuration Utilities
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# Command-Line Arguments

A Java application can accept any number of arguments from the command line. This allows the user to specify configuration information when the application is launched.

The user enters command-line arguments when invoking the application and specifies them after the name of the class to be run. For example, suppose a Java application called `Sort` sorts lines in a file. To sort the data in a file named `friends.txt`, a user would enter:

    java Sort friends.txt

When an application is launched, the runtime system passes the command-line arguments to the application's main method via an array of `String`s. In the previous example, the command-line arguments passed to the `Sort` application in an array that contains a single `String`: `"friends.txt"`.

## Echoing Command-Line Arguments

The ``Echo`` example displays each of its command-line arguments on a line by itself:

    public class Echo {
        public static void main (String[] args) {
            for (String s: args) {
                System.out.println(s);
            }
        }
    }

The following example shows how a user might run `Echo`. User input is in italics.

    java Echo Drink Hot Java
    Drink
    Hot
    Java

Note that the application displays each word — `Drink`, `Hot`, and `Java` — on a line by itself. This is because the space character separates command-line arguments. To have `Drink`, `Hot`, and `Java` interpreted as a single argument, the user would join them by enclosing them within quotation marks.

    java Echo "Drink Hot Java"
    Drink Hot Java

## Parsing Numeric Command-Line Arguments

If an application needs to support a numeric command-line argument, it must convert a `String` argument that represents a number, such as "34", to a numeric value. Here is a code snippet that converts a command-line argument to an `int`:

    int firstArg;
    if (args.length > 0) {
        try {
            firstArg = Integer.parseInt(args[0]);
        } catch (NumberFormatException e) {
            System.err.println("Argument" + args[0] + " must be an integer.");
            System.exit(1);
        }
    }

`parseInt` throws a `NumberFormatException` if the format of `args[0]` isn't valid. All of the `Number` classes — `Integer`, `Float`, `Double`, and so on — have `parseXXX` methods that convert a `String` representing a number to an object of their type.

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