问如何在java中重写抽象方法？

EN

package homeFolder;

import java.util.Scanner;

public class BuyNItemsGetOneFree extends DiscountPolicy {

    // Below are your fields, or instance variables. Notice the lack of static.
    final double itemCost;
    final int count;
    final int n;

    public static void main(String[] args) {

        // Now you have an instance to work with.
        BuyNItemsGetOneFree sale = newInstance();

        for(int i = 1; i <= sale.count; i ++) {

            double discount = sale.computeDiscount(i, itemCost);

            System.out.println(i + " items at " + sale.itemCost +
                                "; discount is " + discount);

        }

    }

    // Only the factory method can access this.
    private BuyNItemsGetOneFree (double itemCost, int count, int n) {
        this.itemCost = itemCost;
        this.count = count;
        this.n = n;
    }

    public static BuyNItemsGetOneFree newInstance() {

        // The scanner really only needs to be used here.
        Scanner input = new Scanner(System.in);

        // Initilizes itemCost
        System.out.print("Enter the cost of the item: ");
        double itemCost = input.nextDouble();

        // Initilizes count
        System.out.print("How many items are you going to buy: ");
        int count = input.nextInt();

        // Initilizes n
        System.out.print("How many items must you buy till you got one free? ");
        int n = input.nextInt();

        // Constructs and returns
        return new BuyNItemsGetOneFree(itemCost, count, n);
    }

    @Override // Always add this when overriding a method.
    public double computeDiscount(int count, double itemCost) {

        double discount = 0;

        if((count % n) == 0)
            discount += itemCost;

        return discount;
    }
}