plugin.yml:

version: 1.0
name: Testowy
main: Main.Core
commands:
    convertexp:
        usage: /<command>

listener:

package Listeners;

import org.bukkit.ChatColor;
import org.bukkit.command.Command;
import org.bukkit.command.CommandExecutor;
import org.bukkit.command.CommandSender;
import org.bukkit.entity.Player;

public class ConvertExpCommand implements CommandExecutor {
    @Override
    public boolean onCommand(CommandSender sender, Command cmd, String label, String[] args) {
        Player player = (Player) sender;
        if (sender instanceof Player){
            player.sendMessage(ChatColor.translateAlternateColorCodes('&', "&6Hi"));
        } else {
            sender.sendMessage("Ta komenda jest tylko dla graczy");
        }
        return true;
    }
}

core:

package Main;

import org.bukkit.command.CommandExecutor;
import org.bukkit.plugin.java.JavaPlugin;

import Listeners.ConvertExpCommand;

public class Core extends JavaPlugin{

    @Override

    public void onEnable() {
        this.getCommand("convertexp").setExecutor((CommandExecutor)new ConvertExpCommand());
    }

}