Presents your 
JAVA E-NEWSLETTER for August 29, 2002

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MAKE COMMAND-LINE ARGUMENTS THE EASY WAY

You can easily get a simple command-line program with a lot of power by
using the Java standard way of passing flags. In fact, the flags passed
are available from more than just the main method.

The java command will pass any value set with the -D flag straight into
the System class' Properties object. This allows the flag to be checked
by calling System.getProperty.

Running the following code with java -Dflag=jack will output "jack" to
the screen.

public class ShowFlag {

    static public void main(String[] args) {
        String flag = System.getProperty("flag");
        System.err.println(flag);
    }

}

To see all the properties, you can run the following:

import java.util.Properties;

public class ShowAll {

    static public void main(String[] args) {
        Properties sysprops = System.getProperties();
        sysprops.list(System.err);
    }

}

When passing in new system properties, it is common to style them in
reversed domain name, such as:

java -Dcom.generationjava.someproduct.color=green
rather than:
java -Dcolor=green

This technique prevents your flag from clashing with another passed
system property.

While this is a nice trick for simple classes, a full-on Java
command-line program really needs to use a proper argument parser. The 
argument
parser can help verify whether particular flags are allowed. It also 
allows
for simple one character flags and automatic conversion of values.

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