Java - 覆盖父类方法

Question

我是Java的新手，我正在开发一个项目，可以计算有/无员工折扣的价格。在阅读下面的代码后，有人可以向我解释如何更改子类的父类方法输出，以便为我的程序获取正确的输出吗？

父类（我不允许编辑此内容）：

public class GroceryBill {
    private Employee clerk;
    private List<Item> receipt;
    private double total;
    private double internalDiscount;

    public GroceryBill(Employee clerk) {
        this.clerk = clerk;
        receipt = new ArrayList<Item>();
        total = 0.0;
        internalDiscount = 0.0;
    }

    public void add(Item i) {
        receipt.add(i);
        total += i.getPrice();
        internalDiscount += i.getDiscount();
    }

    public double getTotal() {
        return Math.rint(total * 100) / 100.0;
    }

    public Employee getClerk() {
        return clerk;
    }

    public void printReceipt() {
        System.out.println(this);
    }

    private String valueToString(double value) {
        value = Math.rint(value * 100) / 100.0;
        String result = "" + Math.abs(value);
        if(result.indexOf(".") == result.length() - 2) {
            result += "0";
        }
        result = "$" + result;
        return result;
    }

    public String receiptToString() {
        String build = "items:\n";
        for(int i = 0; i < receipt.size(); i++) {
            build += "   " + receipt.get(i);
            if(i != receipt.size() - 1) {
                build += "\n";
            }
        }
        return build;
    }

    public String toString() {
        return receiptToString() + "\ntotal: " + valueToString(total);
    }

    public String discountToString() {
        return receiptToString() + "\nsub-total: " + valueToString(total) + "\ndiscount: " + valueToString(internalDiscount) + "\ntotal: " + valueToString(total - internalDiscount);
    }

    public static class Employee {
        private String name;

        public Employee(String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }
    }

    public static class Item {
        private String name;
        private double price;
        private double discount;

        public Item(String name, double price, double discount) {
            this.name = name;
            this.price = price;
            this.discount = discount;
        }

        public double getPrice() {
            return price;
        }

        public double getDiscount() {
            return discount;
        }

        private String valueToString(double value) {
            String result = "" + Math.abs(value);
            if(result.indexOf(".") == result.length() - 2) {
                result += "0";
            }
            result = "$" + result;
            return result;
        }

        public String toString() {
            return name + " " + valueToString(price) + " (-" + valueToString(discount) + ")";
        }
    }
}

这是我的代码：

public class DiscountBill extends GroceryBill
{
    private int myDiscountCount;
    private double myDiscountAmount;
    private double myPrice;

    public DiscountBill(Employee clerk, boolean preferred)
    {
        super(clerk);

        String name = "";
        double price = 0;
        double discount = 0;

        Object myItem = new Item(name, price, discount);
        myPrice = ((GroceryBill.Item) myItem).getPrice() - ((GroceryBill.Item) myItem).getDiscount();

        GroceryBill.Item myBill = new GroceryBill.Item(name, price, discount);
        myDiscountAmount = myBill.getDiscount();

        if (myDiscountAmount > 0 && preferred)
        {
            myDiscountCount++;
        }
    }
    /*
    public double getTotal()
    {
        Override goes here?
    }
    */
    public int getDiscountCount()
    {
        return myDiscountCount;
    }
    public double getDiscountAmount()
    {
        return myDiscountAmount;
    }
    public double getDiscountPercent()
    {
        return (myPrice / getDiscountCount()) * 100;
    }
}

最后，here是预期的输出：

P.S。如果我需要提供更多/更少的信息和方法，我可以清理这篇文章或让它更容易理解，请告诉我。如果我的问题太宽泛，请问我对你不了解的内容，我会尽力告诉你！谢谢！