public class JTerm {

    private static HashMap<String, Command> commands = new HashMap<>();

    public static void main(String[] args) {
        // init
        init("jterm.command");

        // execute
        executeCommand("echo hello world");
    }

    public static void init(String packageName) {
        URL root = Thread.currentThread().getContextClassLoader().getResource(packageName.replace(".", "/"));
        Arrays.stream(new File(root.getFile()).listFiles()).forEach(file -> {
            try {
                Class clazz = Class.forName(String.format("%s.%s", packageName, file.getName().replaceAll(".class$", "")));
                if (Command.class.isAssignableFrom(clazz)) {
                    commands.put(clazz.getSimpleName().toLowerCase(), (Command) clazz.newInstance());
                }
            } catch (ClassNotFoundException | InstantiationException | IllegalAccessException e) {
                e.printStackTrace();
            }
        });
    }

    public static void executeCommand(String options) {
        String command = options.split(" ")[0];

        if (commands.containsKey(command)) {
            Command currentCommand = commands.get(command);

            if (command.startsWith("-h")) {
                JTerm.out.println(currentCommand.getSyntax());
                return;
            }

            // Parameters of the execute() can be changed to an Array/ArrayList
            currentCommand.execute(options.substring(command.length() + 1));
        } else {
            System.err.printf("Unkown command %s%n", command);
        }
    }
}

abstract class Command {

    protected String syntax;

    public abstract void execute(String parameters);

    public String getSyntax() {
        return syntax;
    }
}

class Echo extends Command {

    public Echo() {
        super.syntax = "echo ...";
    }

    @Override
    public void execute(String parameters) {
        System.out.println(parameters);
    }
}