Setting Program Attributes

Conventions for Command Line Arguments

The Java language follows UNIX conventions that define three different types of command line arguments:

• word arguments (also known as options)
• arguments that require arguments
• flags

In addition, your application should observe the following conventions that apply to Java command line arguments:

• The dash character ( - ) precedes options, flags, or series of flags.
• Arguments can be given in any order, except where an argument requires another argument.
• Flags can be listed in any order, separately or combined: -xn or -nx or -x -n.
• Filenames typically come last.
• The program prints a usage error when a command line argument is unrecognized. Usage statements usually take the form:

  usage: application_name [ optional_args ] required_args
  

Word Arguments

Arguments such as -verbose are word arguments and must be specified in their entirety on the command line. For example, -ver would not match -verbose.

You can use a statement such as the following to check for word arguments.

if (argument.equals("-verbose"))
    vflag = true;

The statement checks for the word argument -verbose and sets a flag within the program indicating that the program should run in verbose mode.

Arguments that Require Arguments

Some arguments require more information. For example, a command line argument such as -output might allow the user to redirect the output of the program. However, the -output option alone does not provide enough information to the application: how does the application know where to redirect its output? Thus, the user must also specify a filename. Typically, the next item on the command line provides the additional information for command line arguments that require it. You can use a statement such as the following to parse arguments that require arguments.

if (argument.equals("-output")) {
    if (nextarg < args.length)
        outputfile = args[nextarg++];
    else
        System.err.println("-output requires a filename");
}

Notice that the code snippet checks to make sure that the user actually specified a next argument before trying to use it.

Flags

Flags are single character codes that modify the behavior of the program in some way. For example, the -t flag provided to the UNIX ls command indicates that the output should be sorted by time stamp. Most applications allow users to specify flags separately in any order:

-x -n     or    -n -x

In addition, to make life easier for users, applications should also allow users to concatenate flags and specify them in any order:

-nx    or    -xn

The sample program described on the next page implements a simple algorithm to process flag arguments that can be specified in any order, separately or combined.

Setting Program Attributes