totalAmountCalculator.java

public double totalAmount(List<Items> items) {
  double total = 0.0;
  for (item : items) {
    if (item.getCategory() == "DISCOUNT") {
      total += 0.95 * item.getPrice();
    }
    else if (item.getCategory() == "WEIGHT") { 
      total += item.getQuantity()*5/1000);
    }
    else if (item.getCategory() =="SPECIAL") {
      total += 0.8*item.getPrice();
    }
    // more rules are coming!
  }
  return total;
}