GameMode Fly Checking

Main.java

package elite.gm;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.util.List;
import org.bukkit.Bukkit;
import org.bukkit.ChatColor;
import org.bukkit.GameMode;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerCommandPreprocessEvent;
import org.bukkit.plugin.java.JavaPlugin;

public class Main extends JavaPlugin implements Listener{

    @Override
    public void onEnable(){
        Bukkit.getServer().getPluginManager().registerEvents(this, this);
    }
   
    @EventHandler
    public void onPlayerCommandPreprocess(PlayerCommandPreprocessEvent e) {
        Player p = e.getPlayer();
        
        String[] args = e.getMessage().toLowerCase().split("\\s+");
        if (args.length > 1){
            if (args[0].equals("/gamemode")) {
                if (args.length == 2){
                    //Check if player typing command has permission
                    e.setCancelled(true);
                    
                    Boolean pFlying = p.isFlying();
                    
                    switch (args[1]) {
                        case "survival":
                        case "0":
                            p.setAllowFlight(true); //Change value depending on server
                            
                            gameModeChange(p, GameMode.SURVIVAL);
                            
                            p.setFlying(pFlying);
                            break;
                        case "creative":
                        case "1":
                            p.setAllowFlight(true);
                            
                            gameModeChange(p, GameMode.CREATIVE);
                            
                            p.setFlying(pFlying);
                            break;
                        case "adventure":
                        case "2":
                            p.setAllowFlight(false); //Change value depending on server
                            
                            gameModeChange(p, GameMode.ADVENTURE);
                            
                            p.setFlying(false); //Keep set flying as variable, unless you don't want them to fly
                            break;
                        case "spectator":
                        case "3":
                            p.setAllowFlight(true);
                            
                            gameModeChange(p, GameMode.SPECTATOR);
                            
                            p.setFlying(pFlying);
                            break;
                        default:
                            p.sendMessage(ChatColor.RED + "Usage: /gamemode <mode> [player]");
                            p.sendMessage(ChatColor.YELLOW + "Survival | Creative | Adventure | Spectator");
                            break;
                    }
                }
                if (args.length == 3){
                    e.setCancelled(true);
                    
                    //Also check if player typing command has permission
                    
                    List<Player> players = p.getWorld().getPlayers();
                    
                    if (players.contains(Bukkit.getPlayer(args[2]))){
                        
                        Player target = Bukkit.getPlayer(args[2]);
                        
                        Boolean pFlying = target.isFlying();
                        
                        switch (args[1]) {
                            case "survival":
                            case "0":
                                target.setAllowFlight(true); //Change value depending on server
                                
                                gameModeChange(target, GameMode.SURVIVAL);
                                
                                target.setFlying(pFlying);
                                break;
                            case "creative":
                            case "1":
                                target.setAllowFlight(true);
                                
                                gameModeChange(target, GameMode.CREATIVE);
                                
                                target.setFlying(pFlying);
                                break;
                            case "adventure":
                            case "2":
                                target.setAllowFlight(false); //Change value depending on server
                                
                                gameModeChange(target, GameMode.ADVENTURE);
                                
                                target.setFlying(false); //Keep set flying as variable, unless you don't want them to fly
                                break;
                            case "spectator":
                            case "3":
                                target.setAllowFlight(true);
                                
                                gameModeChange(target, GameMode.SPECTATOR);
                                
                                target.setFlying(pFlying);
                                break;
                            default:
                                p.sendMessage(ChatColor.RED + "Usage: /gamemode <mode> [player]");
                                p.sendMessage(ChatColor.YELLOW + "Survival | Creative | Adventure | Spectator");
                                break;
                        }
                    }else{
                        p.sendMessage(ChatColor.RED + "Cannot set GameMode of: " + ChatColor.WHITE + args[2]);
                        p.sendMessage(ChatColor.RED + "Usage: /gamemode <mode> [player]");
                        p.sendMessage(ChatColor.YELLOW + "Survival | Creative | Adventure | Spectator");
                    }
                }
            }
        }
    }
    
    public void gameModeChange(Player p, GameMode g){
        
        if (g.equals(GameMode.SURVIVAL)){
            sendGameUpdate(p, 0);
        }
        
        if (g.equals(GameMode.CREATIVE)){
            sendGameUpdate(p, 1);
        }
        
        if (g.equals(GameMode.ADVENTURE)){
            sendGameUpdate(p, 2);
        }
        
        if (g.equals(GameMode.SPECTATOR)){
            sendGameUpdate(p, 3);
        }
    }
    
    public void sendGameUpdate(Player p, int gameMode){
        try {
            Constructor<?> gmConstruct= getNMSClass("PacketPlayOutGameStateChange").getConstructor(int.class, float.class);

            Object packet = gmConstruct.newInstance(3, gameMode);

            sendPacket(p, packet);
        } catch (SecurityException | IllegalArgumentException | IllegalAccessException | InvocationTargetException | NoSuchMethodException | InstantiationException ex) {
        }
    }


    public void sendPacket(Player p, Object packet) {
        try {
            Object handle = p.getClass().getMethod("getHandle").invoke(p);
            Object playerConnection = handle.getClass().getField("playerConnection").get(handle);
            playerConnection.getClass().getMethod("sendPacket", getNMSClass("Packet")).invoke(playerConnection, packet);
        }

        catch (NoSuchMethodException | SecurityException | IllegalAccessException | IllegalArgumentException | InvocationTargetException | NoSuchFieldException ex) {
        }
    }

    public Class<?> getNMSClass(String name) {
        String version = Bukkit.getServer().getClass().getPackage().getName().split("\\.")[3];
        
        try {
            return Class.forName("net.minecraft.server." + version + "." + name);
        }

        catch (ClassNotFoundException ex) {
            return null;
        }
    }
}

Suggestions.md

• Move version to constructor | Initialize it onEnable()
• Store Flight Boolean and GameMode variables onPlayerQuit | onPlayerJoin set Flight based on GameMode
• Check SystemCommandEvent for GameMode command | Cancel the event | Pass arguments to a custom method to set Flight and GameMode
• Add Permission check -> See if player is allowed to change their GameMode manually