//imports
import java.util.ArrayList;
import org.bukkit.entity.Player;

//1.
public static ArrayList<Player> Baumodus = new ArrayList<>();

//2.
@Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
        Player p = null;
        if (sender instanceof Player){
            p = (Player) sender;
        }

        if (command.getName().equalsIgnoreCase("bauen")){
            Baumodus.add(p);
            player.getInventory().clear();
            p.setGameMode(GameMode.CREATIVE);
            return true;
        } else if (command.getName().equalsIgnoreCase("spielen")){
            Baumodus.remove(p);
            p.setGameMode(GameMode.SURVIVAL);
            return true;
        }

        return false;

    }