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package me.illuminatiproductions.youtubeplugin;

import me.illuminatiproductions.youtubeplugin.commands.VanishCommand;
import me.illuminatiproductions.youtubeplugin.events.JoinEvent;
import org.bukkit.entity.Player;
import org.bukkit.plugin.java.JavaPlugin;

import java.util.ArrayList;

public class YoutubePlugin extends JavaPlugin {

    public ArrayList<Player> invisible_list = new ArrayList<>();

    @Override
    public void onEnable() {
        // Plugin startup logic

        getCommand("vanish").setExecutor(new VanishCommand(this));

        getServer().getPluginManager().registerEvents(new JoinEvent(this), this);
    }

}

package me.illuminatiproductions.youtubeplugin.commands;

import me.illuminatiproductions.youtubeplugin.YoutubePlugin;
import org.bukkit.Bukkit;
import org.bukkit.command.Command;
import org.bukkit.command.CommandExecutor;
import org.bukkit.command.CommandSender;
import org.bukkit.entity.Player;

public class VanishCommand implements CommandExecutor {

    YoutubePlugin plugin;

    public VanishCommand(YoutubePlugin plugin) {
        this.plugin = plugin;
    }

    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {

        if (sender instanceof Player){
            Player player = (Player) sender;
            if (plugin.invisible_list.contains(player)){
                //Since they are on the list, remove them and make them visible
                for (Player people : Bukkit.getOnlinePlayers()){
                    people.showPlayer(plugin, player);
                }
                plugin.invisible_list.remove(player);
                player.sendMessage("You are now visible to other players");
            }else if (!plugin.invisible_list.contains(player)){
                //Hide yourself from every other player on the server
                for (Player people : Bukkit.getOnlinePlayers()){
                    people.hidePlayer(plugin, player); //Hides you from that player
                }
                plugin.invisible_list.add(player);
                player.sendMessage("You are now invisible to other players");
            }
        }
        return true;
    }
}

package me.illuminatiproductions.youtubeplugin.events;

import me.illuminatiproductions.youtubeplugin.YoutubePlugin;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerJoinEvent;

public class JoinEvent implements Listener {

    YoutubePlugin plugin;

    public JoinEvent(YoutubePlugin plugin) {
        this.plugin = plugin;
    }

    @EventHandler
    public void PlayerJoin(PlayerJoinEvent e){
        //When a player joins, make the players on the invisible list invisible for them
        Player player = e.getPlayer();
        for (int i = 0;i < plugin.invisible_list.size(); i++){
            player.hidePlayer(plugin, plugin.invisible_list.get(i));
        }
    }

}