@EventHandler
public void arenaSignPlace(SignChangeEvent event)
{
    if(event.getBlock().getType() == Material.WALL_SIGN)
    {
        Sign sign = (Sign) event.getBlock().getState();

        Bukkit.getScheduler().runTaskLater(MyMain.getInstance(), () ->
        {
            sign.setLine(0, "Hello from");

            sign.update(); //wouldnt update if this method wasn't called

            System.out.println("Sign's Lines: " + Arrays.toString(sign.getLines())); //prints [Hello from, , ,] to the console.
        }, 3 * 20);
    }
}