Java 命令行编译 获得参数

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 JavaDrink
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.