Command Line Flags in Java

Here’s the translation of the Go code to Java, 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 package for parsing command-line flags, but we can use a third-party library like Apache Commons CLI. For this example, we’ll use a simple approach with args array parsing.

import java.util.ArrayList;
import java.util.List;

public class CommandLineFlags {
    public static void main(String[] args) {
        // We'll use these variables to store our flag values
        String word = "foo";
        int numb = 42;
        boolean fork = false;
        String svar = "bar";
        List<String> tail = new ArrayList<>();

        // Parse command line arguments
        for (int i = 0; i < args.length; i++) {
            String arg = args[i];
            if (arg.startsWith("--")) {
                String[] parts = arg.substring(2).split("=", 2);
                if (parts.length == 2) {
                    switch (parts[0]) {
                        case "word":
                            word = parts[1];
                            break;
                        case "numb":
                            numb = Integer.parseInt(parts[1]);
                            break;
                        case "fork":
                            fork = Boolean.parseBoolean(parts[1]);
                            break;
                        case "svar":
                            svar = parts[1];
                            break;
                    }
                }
            } else {
                tail.add(arg);
            }
        }

        // Print 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: " + tail);
    }
}

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=true --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
...
tail: [a1, a2, a3]

This simple implementation doesn’t provide automatic help text generation or strict flag ordering. For more robust command-line parsing in Java, consider using libraries like Apache Commons CLI or JCommander.

Note that unlike the flag package in Go, this simple Java implementation doesn’t enforce a specific order for flags and positional arguments. All non-flag arguments are treated as positional and added to the tail list.

For a more feature-complete command-line parsing experience in Java, similar to Go’s flag package, you would typically use a third-party library. These libraries often provide features like automatic help text generation, strict flag ordering, and more robust parsing options.

Command Line Arguments in Java

Context in Java

Learn X By Example

Title here

Command Line Flags in Java