Command Line Flags in Mercury

Here’s the translation of the Go code 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 package 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 System.getProperty() to demonstrate the concept.

import java.util.Arrays;

public class CommandLineFlags {
    public static void main(String[] args) {
        // Basic flag declarations are available as system properties.
        // We'll use default values if the property is not set.
        String word = System.getProperty("word", "foo");
        int numb = Integer.parseInt(System.getProperty("numb", "42"));
        boolean fork = Boolean.parseBoolean(System.getProperty("fork", "false"));
        String svar = System.getProperty("svar", "bar");

        // Here we'll just dump out the parsed options and
        // any trailing positional arguments.
        System.out.println("word: " + word);
        System.out.println("numb: " + numb);
        System.out.println("fork: " + fork);
        System.out.println("svar: " + svar);
        System.out.println("tail: " + Arrays.toString(args));
    }
}

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

$ javac CommandLineFlags.java

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

$ java -Dword=opt -Dnumb=7 -Dfork=true -Dsvar=flag CommandLineFlags
word: opt
numb: 7
fork: true
svar: flag
tail: []

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

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

Trailing positional arguments can be provided after any flags:

$ java -Dword=opt CommandLineFlags a1 a2 a3
word: opt
...
tail: [a1, a2, a3]

In this Java implementation, we’re using system properties to simulate command-line flags. This approach is not as robust as using a dedicated command-line parsing library, but it demonstrates the concept.

To get help text or handle undefined flags, you would need to implement this functionality yourself or use a more comprehensive command-line parsing library like Apache Commons CLI.

This Java implementation provides a basic demonstration of command-line flag functionality, but for more complex use cases, consider using a dedicated library that offers features like automatic help text generation and undefined flag handling.