public class PlayerJoin implements Listener {

    private GradientNick plugin;

    public PlayerJoin(GradientNick plugin) {
        this.plugin = plugin;
    }

    @EventHandler
    public void onJoin(PlayerJoinEvent e) {
        Player p = e.getPlayer();
        String UUID = p.getUniqueId().toString();
        File playersFile = new File(plugin.getDataFolder(), "players.yml");

        if (!playersFile.exists()) {
            try {
                playersFile.createNewFile();
            } catch (IOException exception) {
                exception.printStackTrace();
            }
        }

        YamlConfiguration playersConfig = YamlConfiguration.loadConfiguration(playersFile);

        if (!(p.hasPlayedBefore())) {
            playersConfig.set(UUID + ".firstnickname", p.getDisplayName());
            playersConfig.set(UUID + ".gnickname", p.getDisplayName());
            try {
                playersConfig.save(playersFile);
            } catch (IOException exception) {
                exception.printStackTrace();
            }
            return;
        } else {
            p.setDisplayName(playersConfig.getString(UUID + ".gnickname"));
        }
    }
}