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Jb0260: Java OOP: Command-Line Arguments

Module by: R.G. (Dick) Baldwin. E-mail the author

Summary: Although the use of command-line arguments is rare is this time of Graphical User Interfaces (GUI), they are still useful for testing and debugging code. This module explains the use of command-line arguments in Java.

Preface

Although the use of command-line arguments is rare is this time of Graphical User Interfaces (GUI) , they are still useful for testing and debugging code. This module explains the use of command-line arguments in Java.

Viewing tip

I recommend that you open another copy of this module in a separate browser window and use the following link to easily find and view the listing while you are reading about it.

Listings

Discussion

Familiar example from DOS

Java programs can be written to accept command-line-arguments.

DOS users will be familiar with commands such as the following:

Note:

Familiar DOS command

copy fileA fileB

In this case, copy is the name of the program to be executed, while fileA and fileB are command-line arguments.

Java syntax for command-line arguments

The Java syntax for supporting command-line arguments is shown below (note the formal argument list for the main method) .

Note:

Java syntax for command-line arguments

public static void main(String[] args){
. . . 
}//end main method

In Java, the formal argument list for the main method must appear in the method signature whether or not the program is written to support the use of command-line arguments. If the argument isn't used, it is simply ignored.

Where the arguments are stored

The parameter args contains a reference to a one-dimensional array object of type String .

Each of the elements in the array (including the element at index zero) contains a reference to an object of type String . Each object of type String encapsulates one command-line argument.

The number of arguments entered by the user

Recall from an earlier module on arrays that the number of elements in a Java array can be obtained from the length property of the array. Therefore, the number of arguments entered by the user is equal to the value of the length property. If the user didn't enter any arguments, the value will be zero.

Command-line arguments are separated by the space character. If you need to enter an argument that contains a space, surround the entire argument with quotation mark characters as in "My command line argument" .

The first command-line argument is encapsulated in the String object referred to by the contents of the array element at index 0, the second argument is referred to by the element at index 1, etc.

Sample Java program

The sample program in Listing 1 illustrates the use of command-line arguments.

1
Listing 1: Illustration of command-line arguments.

/*File cmdlin01.java Copyright 1997, R.G.Baldwin
This Java application illustrates the use of Java 
command-line arguments.

When this program is run from the command line as follows:

java cmdlin01 My command line arguments

the program produces the following output:

My
command
line
arguments
********************************************************/
class cmdlin01 { //define the controlling class
  public static void main(String[] args){ //main method
    for(int i=0; i < args.length; i++)
    System.out.println( args[i] );
  }//end main
}//End cmdlin01 class.

The output from running this program for a specific input is shown in the comments at the beginning of the program.

Run the program

I encourage you to copy the code Listing 1 . Compile the code and execute it. Experiment with the code, making changes, and observing the results of your changes. Make certain that you can explain why your changes behave as they do.

Looking ahead

As you approach the end of this group of Programming Fundamentals modules, you should be preparing yourself for the more challenging ITSE 2321 OOP tracks identified below:

Miscellaneous

This section contains a variety of miscellaneous information.

Note:

Housekeeping material
  • Module name: Jb0260: Java OOP: Command-Line Arguments
  • File: Jb0260.htm
  • Originally published: 1997
  • Published at cnx.org: 11/27/12
  • Revised: 01/02/13

Note:

Disclaimers:

Financial : Although the Connexions site makes it possible for you to download a PDF file for this module at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should be aware that some of the HTML elements in this module may not translate well into PDF.

I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.

In the past, unknown individuals have copied my modules from cnx.org, converted them to Kindle books, and placed them for sale on Amazon.com showing me as the author. I neither receive compensation for those sales nor do I know who does receive compensation. If you purchase such a book, please be aware that it is a copy of a module that is freely available on cnx.org and that it was made and published without my prior knowledge.

Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.

-end-

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