Bruce - 5 months ago 12

Java Question

Foreword: sorry for all the code, I know most of it is redundant for this question.

This class takes a description (size), a cost, and a quantity as arguments and returns the number of times the object was created (transaction_number), the total quantity specified for all the objects created (total_quantity), and (is supposed to) return the total cost of all the object created.

`public class SalesTransaction`

{

private static int counter = 1;

public final int transaction_number;

private static int order_quantity = 0;

public final int total_quantity;

private static double temp_grand_total = 0;

public final double grand_total;

private String size;

private double cost;

private int quantity;

public SalesTransaction(String size, double cost, int quantity)

{

this.size = size;

this.cost = cost;

this.quantity = quantity;

this.transaction_number = counter++;

order_quantity += quantity;

this.total_quantity = order_quantity;

temp_grand_total += totalCostAfterTax(cost*quantity); // this is wrong!

this.grand_total = temp_grand_total;

}

public static double discountAmount(int quantity, double cost)

{

double discount_amount = 0;

if (quantity > 20)

{

discount_amount = cost * 0.10;

}

return discount_amount;

}

public static double totalCostBeforeTax(SalesTransaction temp)

{

double total_cost;

int quantity = temp.quantity;

double cost = temp.cost;

total_cost = quantity * cost;

double discount_amount = discountAmount(quantity, total_cost);

total_cost = total_cost - discount_amount;

return total_cost;

}

public static double totalCostAfterTax(double total_cost)

{

total_cost = total_cost * 1.15;

return total_cost;

}

public static void printStats(SalesTransaction temp)

{

System.out.println("Transaction Number: " + temp.transaction_number);

System.out.println("Size: " + temp.size);

System.out.println("Cost Per Table: "+ temp.cost);

System.out.println("Number of Tables: " + temp.quantity);

System.out.println("Total Tables So Far: " + temp.total_quantity);

double total_cost_before_tax = totalCostBeforeTax(temp);

double discount_amount = discountAmount(temp.quantity, total_cost_before_tax);

System.out.println("Discount: " + discount_amount);

double total_cost_after_tax = totalCostAfterTax(total_cost_before_tax);

temp.temp_grand_total = total_cost_after_tax;

System.out.println("Cost for this transaction: " + total_cost_after_tax);

System.out.println("Total cost: "+ temp.grand_total);

System.out.println();

}

}

And this is just a tester class.

`public class SalesTester`

{

public static void main(String[] args)

{

SalesTransaction one = new SalesTransaction("Small", 10.0, 10);

one.printStats(one);

SalesTransaction two = new SalesTransaction("Medium", 20.0, 30);

two.printStats(two);

SalesTransaction three = new SalesTransaction("Large", 30.0, 40);

three.printStats(three);

}

}

The problem is that I can't figure out how to store the grand_total. I tried doing it the same way I stored the total_quantity but I can see why that isn't working.

How can I keep track of the grand total of all the transactions (objects) so I can then print it out on the console?

I assume there's another way of expressing this in the constructor but I'm not sure how and I couldn't find any examples of this.

Answer

The simplest solution is to use a `static`

field on your `SalesTransaction`

class which holds the `grand_total`

. A `static`

variable is shared by all instances of a class.

```
private static double grandTotal = 0;
public SalesTransaction(double cost) {
grandTotal += cost;
}
```

**However**, this has some disadvantages in the long run. It would mean you can't have transactions as members of different grand totals. This is why it's called the singleton anti-pattern.

A much better way to solve the problem is to make an additional class such as `TransactionGroup`

, which contains `SalesTransaction`

objects in a `List`

, and sums together the costs when needed.

```
public class TransactionGroup {
private List<SalesTransaction> transactions = new ArrayList<>();
public void addTransaction(SalesTransaction st) {
transactions.add(st);
}
public double getGrandTotal() {
double sum = 0;
for (SalesTransaction st : transactions) {
sum += st.getCost();
}
return sum;
}
}
```