extras/src/main/java/pw/kaboom/extras/commands/CommandUsername.java

package pw.kaboom.extras.commands;

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

import com.destroystokyo.paper.profile.PlayerProfile;

public final class CommandUsername implements CommandExecutor {
    private long millis;

    @Override
    public boolean onCommand(final CommandSender sender, final Command command, final String label,
                             final String[] args) {
        if (sender instanceof ConsoleCommandSender) {
            sender.sendMessage("Command has to be run by a player");
        } else {
            final Player player = (Player) sender;

            final String nameColor = ChatColor.translateAlternateColorCodes(
                '&', String.join(" ", args));
            final String name = nameColor.substring(0, Math.min(16, nameColor.length()));

            final long millisDifference = System.currentTimeMillis() - millis;

            if (args.length == 0) {
                player.sendMessage(ChatColor.RED + "Usage: /" + label + " <username>");
            } else if (name.equals(player.getName())) {
                player.sendMessage("You already have the username \"" + name + "\"");
            } else if (millisDifference <= 2000) {
                player.sendMessage("Please wait a few seconds before changing your username");
            } else {
                if (Bukkit.getPlayer(name) != null
                        && Bukkit.getPlayer(name).isOnline()) {
                    player.sendMessage("A player with that username is already logged in");
                    return true;
                }

                final PlayerProfile profile = player.getPlayerProfile();

                profile.setName(name);

                player.setPlayerProfile(profile);

                millis = System.currentTimeMillis();

                player.sendMessage("Successfully set your username to \"" + name + "\"");
            }
        }
        return true;
    }
}