Command Line Flags in Karel

Here’s the translation of the Go code example to Java, along with explanations in Markdown format suitable for Hugo:

Command-line flags are a common way to specify options for command-line programs. For example, in wc -l the -l is a command-line flag.

Java doesn’t have a built-in flag parsing library like Go’s flag package, but we can use a third-party library like Apache Commons CLI to achieve similar functionality. For this example, we’ll use a simple approach with args parsing to demonstrate the concept.

public class CommandLineFlags {
    public static void main(String[] args) {
        // Default values for our flags
        String word = "foo";
        int numb = 42;
        boolean fork = false;
        String svar = "bar";

        // Parse command line arguments
        for (int i = 0; i < args.length; i++) {
            String arg = args[i];
            if (arg.startsWith("--word=")) {
                word = arg.substring(7);
            } else if (arg.startsWith("--numb=")) {
                numb = Integer.parseInt(arg.substring(7));
            } else if (arg.equals("--fork")) {
                fork = true;
            } else if (arg.startsWith("--svar=")) {
                svar = arg.substring(7);
            }
        }

        // Print out the parsed options
        System.out.println("word: " + word);
        System.out.println("numb: " + numb);
        System.out.println("fork: " + fork);
        System.out.println("svar: " + svar);

        // Print any remaining arguments
        System.out.print("tail: ");
        for (String arg : args) {
            if (!arg.startsWith("--")) {
                System.out.print(arg + " ");
            }
        }
        System.out.println();
    }
}

To experiment with the command-line flags program, first compile it and then run the resulting class file directly.

$ javac CommandLineFlags.java

Try out the compiled program by giving it values for all flags:

$ java CommandLineFlags --word=opt --numb=7 --fork --svar=flag
word: opt
numb: 7
fork: true
svar: flag
tail:

Note that if you omit flags, they automatically take their default values:

$ java CommandLineFlags --word=opt
word: opt
numb: 42
fork: false
svar: bar
tail:

Trailing positional arguments can be provided after any flags:

$ java CommandLineFlags --word=opt a1 a2 a3
word: opt
numb: 42
fork: false
svar: bar
tail: a1 a2 a3

Unlike the flag package in Go, this simple implementation doesn’t automatically generate help text or handle errors for undefined flags. In a real-world scenario, you would likely use a more robust command-line parsing library like Apache Commons CLI or JCommander, which provide these features out of the box.

For a more feature-rich command-line parsing experience in Java, consider using a library like Apache Commons CLI. This would allow you to define options, automatically generate help text, and handle errors more robustly.