Zargs

Add zero-dependency argument parsing to a Java CLI application using $ARGUMENTS. Apply all rules below.

What is zargs

zargs is a zero-dependency, enum-based argument parsing pattern for Java CLI applications. It is not a library or dependency — it is a set of conventions and code patterns to apply directly when generating argument parsing code.

• Reference: https://github.com/AdamBien/zeeds/blob/main/zargs
• Requires: Java 21+

Argument Enum Pattern

Define an enum where each constant represents a CLI option. The enum provides matching, lookup, and usage generation:

import java.util.stream.Collectors;
import java.util.stream.Stream;

public enum Arg {
    HELP("-help", "Show this help"),
    VERBOSE("-verbose", "Enable verbose output"),
    OUTPUT("-output", "Specify output file");

    final String option;
    final String description;

    Arg(String option, String description) {
        this.option = option;
        this.description = description;
    }

    boolean matches(String value) {
        return this.option.equals(value)
            || value.startsWith(this.option + ":");
    }

    static Arg from(String value) {
        return Stream.of(values())
            .filter(a -> a.matches(value))
            .findFirst()
            .orElse(null);
    }

    static String usage() {
        return Stream.of(values())
            .map(a -> "  %-12s %s".formatted(a.option, a.description))
            .collect(Collectors.joining("\n"));
    }
}

Parsed Arguments Record

Define a record matching the application's parsed arguments:

import java.util.List;

public record Args(boolean verbose, String output, List<String> files) {}

Argument Syntax

zargs supports two value styles for options that take a value:

Boolean flags (like -verbose) require no value.

Parsing Pattern

Use pattern matching with switch and case null to parse arguments:

Args parseArgs(String[] args) {
    var verbose = false;
    String output = null;
    var files = new ArrayList<String>();

    for (int i = 0; i < args.length; i++) {
        var arg = args[i];
        switch (Arg.from(arg)) {
            case HELP -> printUsage();
            case VERBOSE -> verbose = true;
            case OUTPUT -> output = extractValue(arg, args, i++);
            case null -> {
                if (arg.startsWith("-")) Log.stop("Unknown option: " + arg);
                files.add(arg);
            }
        }
    }
    return new Args(verbose, output, List.copyOf(files));
}

String extractValue(String arg, String[] args, int index) {
    if (arg.contains(":")) {
        return arg.substring(arg.indexOf(":") + 1);
    }
    if (index + 1 >= args.length) {
        Log.stop("Missing value for " + arg);
    }
    return args[index + 1];
}

void printUsage() {
    Log.user("""
        Usage: appname [options] <files...>

        Options:
        %s
        """.formatted(Arg.usage()));
    System.exit(0);
}

Key Design Principles

1. Enum constants define all options — each option is a constant with its flag string and description
2. Arg.from() returns null for unknown args — enables case null in switch for positional arguments and unknown option detection
3. matches() handles both syntaxes — exact match (-output) or colon-prefixed (-output:value)
4. extractValue() extracts values — from colon syntax or next argument, with missing-value error handling
5. usage() auto-generates help text — no manual formatting needed; derived from enum constants
6. Args record is the parse result — immutable, typed container for all parsed values

Applying the Pattern

When generating argument parsing code, adapt these parts for the specific application:

1. Enum constants — define constants matching the application's specific options
2. Args record fields — match the fields to the application's parsed result shape
3. parseArgs switch cases — add a case for each enum constant
4. Usage text — update the program name and description in printUsage()

Rules

1. Generate the enum and record directly in the application's source — there are no files to copy
2. Add package declarations matching the target package
3. Customize enum constants for the application's specific options
4. Use Log.stop() from zcl for fatal errors (missing values, unknown options)
5. Use Log.user() from zcl for usage/help output
6. Use List.copyOf() for immutable positional argument lists in the Args record
7. Prefer single-dash options (-verbose, not --verbose) for consistency