重构13-Extract Method Object（提取方法对象）

public class OrderLineItem {
    public Double Price;
}
public class Order {
    private List<OrderLineItem> OrderLineItems;
    private List<Double> Discounts;
    private Double Tax;
    public Double Calculate() {
        Double subTotal = 0d;
// Total up line items 
for(OrderLineItem lineItem : OrderLineItems) {
            subTotal += lineItem.Price;
}
        // Subtract Discounts 
for(Double discount : Discounts) subTotal -= discount;
// Calculate Tax 
Double tax = subTotal * Tax;
// Calculate GrandTotal 
Double grandTotal = subTotal + tax;
        return grandTotal;
}
}

public class OrderLineItem{
    public Double Price;
}

public class Order {
    public List<OrderLineItem> OrderLineItems;
    public List<Double> Discounts;
    public Double Tax;
    public Double Calculate() {
        return new OrderCalculator(this).Calculate();
}
}

public class OrderCalculator {
    private Double SubTotal;
    private List<OrderLineItem> OrderLineItems;
    private List<Double> Discounts;
    private Double Tax;

    public OrderCalculator(Order order) {
        OrderLineItems = order.OrderLineItems;
Discounts = order.Discounts;
Tax = order.Tax;
}

    public Double Calculate() {
        CalculateSubTotal();
SubtractDiscounts();
CalculateTax();
        return SubTotal;
}

    private void CalculateSubTotal() {
        // Total up line items 
for(OrderLineItem lineItem : OrderLineItems) SubTotal += lineItem.Price;
}
    private void SubtractDiscounts() {
        // Subtract Discounts 
for(Double discount : Discounts) SubTotal -= discount;
}
    private void CalculateTax() {
        // Calculate Tax 
SubTotal += SubTotal * Tax;
}
}