public class TestPlugin extends JavaPlugin implements Listener {
  
    @Override
    public void onEnable() {
        // Listen to events
        Bukkit.getPluginManager().registerEvents(this, this);
    }
  
    @EventHandler
    public void onJoin(final PlayerJoinEvent event) {
        // Hide all players from each other on the next tick
        new BukkitRunnable() {  
            public void run() {
                Player player = event.getPlayer();
              
                for (Player other : Bukkit.getOnlinePlayers()) {
                    if (other.equals(player)) continue; // Skip self
                  
                    // Hide from each other
                    other.hidePlayer(player);
                    player.hidePlayer(other);
                  
                    // Confirm in console that this code is getting executed
                    getLogger().log(Level.INFO, "Hiding " + other.getName() + " and " + player.getName() + " from each other by task");
                }
            }
        }.runTaskLater(this, 1);
    }
  
    @EventHandler
    public void onCommand(PlayerCommandPreprocessEvent event) {
        if (event.getMessage().equalsIgnoreCase("/hideall")) {
            Player player = event.getPlayer();
          
            for (Player other : Bukkit.getOnlinePlayers()) {
                if (other.equals(player)) continue; // Skip self
              
                // Hide from each other
                other.hidePlayer(player);
                player.hidePlayer(other);
              
                // Confirm in console that this code is getting executed
                getLogger().log(Level.INFO, "Hiding " + other.getName() + " and " + player.getName() + " from each other by /hideall");
            }
        }
    }
}