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:
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:
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.