Get amount of the specified item in one's inventory. Simple, but efficient.

GetItemAmount.java

package code.MarinusLeeuwerik.utils

public class GetItemAmount {
  
  public int getItemAmount(Inventory inventory, Material... search) {
  	
    List<Material> wanted = Arrays.asList(search);
    int amount = 0;
    
    for(ItemStack item : inventory.getContents()) {
    if(item != null && wanted.contains(item.getType()))
    amount += item.getAmount();
    }
  return amount;
  }
}