How to parse command line arguments in Java

Sometimes we need a simple parser of command line options. Here is a simple example how such a parser can be implemented in Java:

	import java.util.HashMap;
	import java.util.Map;
	public class CommandLine {
	public static Parameters parse(String[] args) {
	Parameters params = new Parameters();
	int i = 0;
	while (i < args.length) {
	switch(args[i]) {
	case "--host":
	if (i == args.length - 1) {
	throw new RuntimeException("No path specified for --host option");
	}
	params.add("host", args[++i]);
	break;
	case "--port":
	if (i == args.length - 1) {
	throw new RuntimeException("No path specified for --port option");
	}
	params.add("port", Integer.parseInt(args[++i]));
	break;
	case "--verbose":
	params.add("verbose", Boolean.TRUE);
	break;
	default:
	throw new RuntimeException("Unexpected parameter: " + args[i]);
	}
	i++;
	}
	return params;
	}
	public static class Parameters {
	private final Map<String, Object> params = new HashMap<>();
	public void add(String name, Object value) {
	params.put(name, value);
	}
	public String getString(String name, String defaultValue) {
	Object value = params.get(name);
	if (value == null) {
	return defaultValue;
	}
	if (value instanceof String) {
	return (String) value;
	}
	throw new RuntimeException(
	String.format("'%s' doesn't look like a string: %s", name, value));
	}
	public int getInteger(String name, int defaultValue) {
	Object value = params.get(name);
	if (value == null) {
	return defaultValue;
	}
	if (value instanceof Integer) {
	return (Integer) value;
	}
	throw new RuntimeException(
	String.format("'%s' doesn't look like an integer: %s", name, value));
	}
	public boolean getBoolean(String name, boolean defaultValue) {
	Object value = params.get(name);
	if (value == null) {
	return defaultValue;
	}
	if (value instanceof Boolean) {
	return (Boolean) value;
	}
	throw new RuntimeException(
	String.format("'%s' doesn't look like an integer: %s", name, value));
	}
	}
	public static void main(String[] args) {
	Parameters params = parse(args);
	System.out.println("host = " + params.getString("host", "localhost"));
	System.out.println("port = " + params.getInteger("port", 80));
	System.out.println("verbose = " + params.getBoolean("verbose", false));
	}
	}

view raw CommandLine.java hosted with ❤ by GitHub

You can add more parameters to the "while" loop. Here is how it can be used:

$ javac -d . CommandLine.java 
$ java CommandLine
host    = localhost
port    = 80
verbose = false
$ java CommandLine --host test
host    = test
port    = 80
verbose = false
$ java CommandLine --host test --port 81
host    = test
port    = 81
verbose = false
$ java CommandLine --host test --port 81 --verbose
host    = test
port    = 81
verbose = true
$ java CommandLine --host test --port 81 --verbose --a
Exception in thread "main" java.lang.RuntimeException: Unexpected parameter: --a
 at CommandLine.parse(CommandLine.java:30)
 at CommandLine.main(CommandLine.java:91)
$ java CommandLine --host test --port
Exception in thread "main" java.lang.RuntimeException: No path specified for --output option
 at CommandLine.parse(CommandLine.java:21)
 at CommandLine.main(CommandLine.java:91)
$ java CommandLine --host test --port
Exception in thread "main" java.lang.RuntimeException: No path specified for --port option
 at CommandLine.parse(CommandLine.java:21)
 at CommandLine.main(CommandLine.java:91)

Enjoy!