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Trail: Essential Classes
Lesson: The Platform Environment
Section: Configuration Utilities
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
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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 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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