Welcome!

IBM Cloud Authors: Zakia Bouachraoui, Carmen Gonzalez, Elizabeth White, Pat Romanski, Liz McMillan

Related Topics: IBM Cloud, Java IoT, Adobe Flex, Open Source Cloud, Eclipse, Machine Learning

IBM Cloud: Article

Reflections on Java Command Line Options

There are many different types of command line options that programs need to recognize

Abstract
There are many different types of command line options that programs need to recognize. Many languages (e.g.: bash and perl) has built-in processing of command line options; Java does not. The Java Command Line Options (JCLO) package performs this task for a variety of option styles. It also uses Java's reflection capability to automatically assign values to variables in a specified class.

Introduction
Even in these days of sophisticated graphical user interfaces, many programs have a wide variety of command line options that help specify their behavior. It is also the case that command line only programs continue to enjoy wide use. It is also the case the command line arguments can become quite complicated, e.g.: -Djava.util.logging.config.file=All.finest -1 --list --this=that Some languages have built-in parsers for command line options; perl and bash are two obvious examples of this. Java has no such parser built in. The JCLO package provides the capability to parse several different command line option formats, and uses Java's reflecion mechanizm to both drive the parsing and assign the values provided by command line options to the variable in a specified class.

Brief overview of command line option formats
UNIX command line options started with a simple "dash and letter" format. "ls -l" is a classic example. If the option specified an additional option, the dash and letter were followed by a space and the additional option. "sort -t separator" is and example. UNIX also allowed the use of numbers, "ls -1", as options. As the number of possible option proliferated, and newer "GNU" style was developed. These have a "double dash/long name" style: "sort --version" for example. When additional options are needed for this style, it is typically provided with a "equals value" style: "gcc --std=c89". Finally, Java has its own style of "single dash/name with dots/equals value" style: "-Djava.util.logging.config.file=logging.props".

Specifying non-Java variable name options
JCLO uses Java reflection to extract variable names as the basis for parsing command line options. Not all styles just described are valid Java variable names and therefore JCLO uses several conventions to allow them specified. Java variables must begin with an alphabetic character or an underscore, numbers are not allowed to start a variable name. Variable names cannot contain dashes, as dashes specify subtraction or negation. They cannot contain dots as dots specify class references or decimal literals. Therefore, JCLO uses the convention of prefixing number options with an underscore (_1). For dashes embedded in options (e.g.: "--font-size=10"), JCLO uses two underscores ("font__size") in the variable name. For the embedded dots, JCLO uses a convention of an underscore followed by a dollar sign "_$".

Brief review of Java reflection
Java, as with other object oriented languages, has the ability to query and modify an object's internal information. One can retrieve a Class's constructors, methods, fields, etc. JCLO uses the getDeclaredFields()" method on a class to find the names it will accept and set the value for. One can either have a single class devoted to command line options or specify a prefix for the variables JCLO's will examine.

From class variables to command line options
First let's look at an example to see how JCLO works in practice. We have the obligatory import:

import edu.mscd.cs.jclo.JCLO;

and then we create a class whose class varibles will become the command line options:

class ExampleArgs
{
    private int a;
    private boolean b;
    private float c;
    private String d;
    private String[] additional;
}

A simple main will be used to illustrate JCLO's operation:

public class Example
{
    public static void main (String args[])
    {
        ExampleArgs ea = new ExampleArgs();
        JCLO jclo = new JCLO (ea);
        jclo.parse (args);
	System.out.println ("a = " + ea.a);
	System.out.println ("b = " + ea.b);
	System.out.println ("c = " + ea.c);
	System.out.println ("d = " + ea.d);
	System.out.println ("additional = " + 
	    java.util.Arrays.toString (ea.additional));
    }
}

First, an object that contains the variables whose values will be assigned by the command line options is created. This object is then used to create a JCLO object. The parse method is then called with the command line options to do the actual work Running this simple program with the following command line options:

java -cp .:JCLO-1.3.4.jar Example -a 5 --b=true --c=9.0 -d Example one two three

produces:

a = 5
b = true
c = 9.0
d = Example
additional = [one, two, three]

Here, each of the class varibles where assigned values from the associated command line option. The additional String array is assigned any options beyond the last dash option. JCLO can also use only certain fields of an object by the programmer specifying a string prefix that those fields begin with.

There are times when command line options have aliases; for example a long and short version. JCLO has the ability to deal with these directly. Adding

String aliases[][] ={{"boolean", "b"}};

and modifying

JCLO jclo = new JCLO (ea, aliases);

to the above example allows for --boolean to set the value for b

:

java -cp .:JCLO-1.3.4.jar Example --boolean
a = 0
b = true
c = 0.0
d = null
additional = null

JCLO also has a simple usage() method that returns a String of possible options and the type of values they require.

Conclusion
JCLO allows one to easily parse command line options and set the values inside a class based on those options. It is very flexible in its parsing, allowing intermixed single- and double-dashed options, along with aliases that allow long and short versions of an option. JCLO is available from http://jclo.sourceforge.net/.

More Stories By Steve Beaty

Steve has an extensive background in both the theoretic and pragmatic aspects of computer science. He wrote compilers at Cray Computer, and both managed a large group of developers and was a software test architect at HP. He has a number of active open-source projects and is a professor of computer science at the Metropolitan State College of Denver.

IoT & Smart Cities Stories
Every organization is facing their own Digital Transformation as they attempt to stay ahead of the competition, or worse, just keep up. Each new opportunity, whether embracing machine learning, IoT, or a cloud migration, seems to bring new development, deployment, and management models. The results are more diverse and federated computing models than any time in our history.
At CloudEXPO Silicon Valley, June 24-26, 2019, Digital Transformation (DX) is a major focus with expanded DevOpsSUMMIT and FinTechEXPO programs within the DXWorldEXPO agenda. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of business. Only 12% still survive. Similar percentages are found throug...
At CloudEXPO Silicon Valley, June 24-26, 2019, Digital Transformation (DX) is a major focus with expanded DevOpsSUMMIT and FinTechEXPO programs within the DXWorldEXPO agenda. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of business. Only 12% still survive. Similar percentages are found throug...
Where many organizations get into trouble, however, is that they try to have a broad and deep knowledge in each of these areas. This is a huge blow to an organization's productivity. By automating or outsourcing some of these pieces, such as databases, infrastructure, and networks, your team can instead focus on development, testing, and deployment. Further, organizations that focus their attention on these areas can eventually move to a test-driven development structure that condenses several l...
The term "digital transformation" (DX) is being used by everyone for just about any company initiative that involves technology, the web, ecommerce, software, or even customer experience. While the term has certainly turned into a buzzword with a lot of hype, the transition to a more connected, digital world is real and comes with real challenges. In his opening keynote, Four Essentials To Become DX Hero Status Now, Jonathan Hoppe, Co-Founder and CTO of Total Uptime Technologies, shared that ...
The graph represents a network of 1,329 Twitter users whose recent tweets contained "#DevOps", or who were replied to or mentioned in those tweets, taken from a data set limited to a maximum of 18,000 tweets. The network was obtained from Twitter on Thursday, 10 January 2019 at 23:50 UTC. The tweets in the network were tweeted over the 7-hour, 6-minute period from Thursday, 10 January 2019 at 16:29 UTC to Thursday, 10 January 2019 at 23:36 UTC. Additional tweets that were mentioned in this...
Over the course of two days, in addition to insightful conversations and presentations delving into the industry's current pressing challenges, there was considerable buzz about digital transformation and how it is enabling global enterprises to accelerate business growth. Blockchain has been a term that people hear but don't quite understand. The most common myths about blockchain include the assumption that it is private, or that there is only one blockchain, and the idea that blockchain is...
Never mind that we might not know what the future holds for cryptocurrencies and how much values will fluctuate or even how the process of mining a coin could cost as much as the value of the coin itself - cryptocurrency mining is a hot industry and shows no signs of slowing down. However, energy consumption to mine cryptocurrency is one of the biggest issues facing this industry. Burning huge amounts of electricity isn't incidental to cryptocurrency, it's basically embedded in the core of "mini...
Japan DX Pavilion at @CloudEXPO Silicon Valley
In his general session at 19th Cloud Expo, Manish Dixit, VP of Product and Engineering at Dice, discussed how Dice leverages data insights and tools to help both tech professionals and recruiters better understand how skills relate to each other and which skills are in high demand using interactive visualizations and salary indicator tools to maximize earning potential. Manish Dixit is VP of Product and Engineering at Dice. As the leader of the Product, Engineering and Data Sciences team at D...