get free player inventory slots spigot
public int getFreeSlots(Player player) {
int freeslots = 0;
for (ItemStack it : player.getInventory().getContents()) {
if (it == null || it.getType() == Material.AIR) {
freeslots++;
}
}
freeslots = freeslots - 5; // subtract shield and armor
return freeslots;
}