Tutorials : Using Program Parameters in Java :

Using Program Parameters in Java

by Keld H. Hansen

Introduction

Every programmer will encounter a situation where a choice must be made between various alternative solutions. Should tabular data be sorted in one way or another? What exactly is the graphical layout of your HTML pages? How often should you poll a queue? Most often you'll know the path to take based on the requirements specifications, a design manual or some other kind of documentation, but now and then you'll have to choose without help. Instead of just selecting one solution and hoping the customer will like it when he or she starts using your code, you could make it a "program parameter" or "option".

This means that the actual choice is given to your program in some way at startup time, for example from a configuration file. Programs like command line utilities, where there's no GUI and no database, simply take their data as "program parameters".

In this article we'll look into the various ways you may enter parameters to a Java program:

1. Command line parameters
2. Property files
3. A combination of 1 and 2
4. System properties
5. XML files
6. Databases 

Finally I'll show how you can hide the implementation--at least to some degree.

Command line parameters

Let's start with the most basic and simplest way to enter parameters to a Java program: the command line. We all should know that a Java main program takes a String array as its parameter, and it contains the program arguments given to the Java VM. I deliberately used the word "arguments", since the "parameters" given on the command line are meant for the VM. Entering "java -?" gives the syntax for the Java command:

C:\>java -?
Usage: java [-options] class [args...
            (to execute a class)
   or java [-options] -jar jarfile [
            (to execute a jar file)
. . .

If you have a fixed list of parameters for your program then you simply add them after the class name, but if you'd like to work with optional parameters, you need some way to identify the different parameters.

It's common practice to give the parameters descriptive names, and use a hyphen in front of the name of the parameter. The parameter value(s) then follow after the name. This syntax is, for example, used in the Java command shown above.

It might be fun to write a command line parser in Java once and maybe twice, but the third time it gets rather boring. The Jakarta Commons project has a command line parser called CLI (Command Line Interface), which is simple and efficient to use. There are 3 steps involved when using CLI: define, parse, interrogate

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