Parsing Command-line Arguments in Java

Parsing Command-line Arguments in Java | Generated by AI

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To use net.sourceforge.argparse4j, a Java library for parsing command-line arguments inspired by Python’s argparse, follow these steps:

1. Add the Dependency

To include argparse4j in your project, add it as a dependency. If you’re using Maven, update your pom.xml with the following:

<dependency>
    <groupId>net.sourceforge.argparse4j</groupId>
    <artifactId>argparse4j</artifactId>
    <version>0.9.0</version>
</dependency>

For other build tools like Gradle, check the Maven Central Repository for the equivalent configuration.

2. Create an `ArgumentParser` Object

Start by creating an ArgumentParser instance using ArgumentParsers.newFor("prog").build(), where "prog" is the name of your program. You can also add a description and enable automatic help generation.

Example:

import net.sourceforge.argparse4j.ArgumentParsers;
import net.sourceforge.argparse4j.inf.ArgumentParser;

ArgumentParser parser = ArgumentParsers.newFor("Checksum").build()
    .defaultHelp(true)  // Enables -h/--help option
    .description("Calculate checksum of given files.");

3. Add Arguments

Define the command-line arguments your program will accept using parser.addArgument(). You can specify:

Optional arguments (e.g., -t, --type) with flags, choices, defaults, and help text.
Positional arguments (e.g., file) with optional variable-length support using .nargs("*").

Example:

parser.addArgument("-t", "--type")
    .choices("SHA-256", "SHA-512", "SHA1")  // Restrict to these options
    .setDefault("SHA-256")                  // Default value if not provided
    .help("Specify hash function to use");  // Description for help message

parser.addArgument("file")
    .nargs("*")                             // Accepts zero or more arguments
    .help("File to calculate checksum");    // Description for help message

4. Parse Command-Line Arguments

Parse the command-line arguments (typically passed as String[] args from your main method) using parser.parseArgs(). Wrap this in a try-catch block to handle parsing errors gracefully.

Example:

import net.sourceforge.argparse4j.inf.Namespace;
import net.sourceforge.argparse4j.inf.ArgumentParserException;

public class Checksum {
    public static void main(String[] args) {
        ArgumentParser parser = ArgumentParsers.newFor("Checksum").build()
            .defaultHelp(true)
            .description("Calculate checksum of given files.");
        
        parser.addArgument("-t", "--type")
            .choices("SHA-256", "SHA-512", "SHA1").setDefault("SHA-256")
            .help("Specify hash function to use");
        parser.addArgument("file").nargs("*")
            .help("File to calculate checksum");

        Namespace ns = null;
        try {
            ns = parser.parseArgs(args);  // Parse the arguments
        } catch (ArgumentParserException e) {
            parser.handleError(e);        // Print error and help message
            System.exit(1);               // Exit on error
        }
    }
}

5. Access Parsed Values

The parseArgs() method returns a Namespace object containing the parsed argument values. Use methods like getString() or getList() to retrieve them.

Example:

String hashType = ns.getString("type");  // e.g., "SHA-256"
List<String> files = ns.getList("file"); // List of file names

System.out.println("Hash Type: " + hashType);
System.out.println("Files: " + files);

Putting It All Together

Here’s a complete example:

import net.sourceforge.argparse4j.ArgumentParsers;
import net.sourceforge.argparse4j.inf.ArgumentParser;
import net.sourceforge.argparse4j.inf.ArgumentParserException;
import net.sourceforge.argparse4j.inf.Namespace;
import java.util.List;

public class Checksum {
    public static void main(String[] args) {
        // Step 2: Create parser
        ArgumentParser parser = ArgumentParsers.newFor("Checksum").build()
            .defaultHelp(true)
            .description("Calculate checksum of given files.");

        // Step 3: Add arguments
        parser.addArgument("-t", "--type")
            .choices("SHA-256", "SHA-512", "SHA1").setDefault("SHA-256")
            .help("Specify hash function to use");
        parser.addArgument("file").nargs("*")
            .help("File to calculate checksum");

        // Step 4: Parse arguments
        Namespace ns = null;
        try {
            ns = parser.parseArgs(args);
        } catch (ArgumentParserException e) {
            parser.handleError(e);
            System.exit(1);
        }

        // Step 5: Access parsed values
        String hashType = ns.getString("type");
        List<String> files = ns.getList("file");

        // Example output
        System.out.println("Hash Type: " + hashType);
        System.out.println("Files: " + files);
    }
}

Additional Features

Automatic Help: Run your program with -h or --help to see a generated help message.
Subcommands: Use parser.addSubparsers() for complex commands (e.g., git commit, git push).
Annotations: Bind arguments to class fields using annotations (see the user manual for details).

For more advanced usage, consult the Argparse4j User Manual.

This workflow lets you easily parse and handle command-line arguments in your Java application using net.sourceforge.argparse4j.

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