如何获取“数量”字段中的“项目数”并计算TotalTax？

如何解决如何获取“数量”字段中的“项目数”并计算TotalTax？

toString（）方法将返回一个字符串，其中包括按类别组织的采购数组中的所有项目。 toString（）还将包括税前订单的总成本，税额，总金额（税前+税前）和订购的商品数量。所订购的商品数量由数量字段决定，与存储在采购数组（numpurchases）中的商品数量不同。 这是我的代码：

import java.text.NumberFormat;

public class Receipt {
private Item[] purchases;
private int numpurchases;

public Receipt(int capacity) {
    purchases = new Item[capacity];
    numpurchases = 0;
}

public void add(Item itm) {
        purchases[numpurchases] = itm;
        numpurchases++;
  }
    


public double totalBeforeTax() {
     double total =0;
        for (int i = 0; i < numpurchases; i++) {
            total += purchases[i].getQty()*purchases[i].getPrice();
        }
     return total;
}

//WORK ON THIS!
public double totalTax() {
    return totalBeforeTax();
}

public String toString() {
     StringBuilder info = new StringBuilder();
     //Append the Format the Table of Item
        info.append(String.format("%-15s","Item")).append("\n");
        info.append("---------------------------------------------------").append("\n");
        
        //display Items inside the toString
        for (int i = 0; i < numpurchases; i++) {
            info.append(purchases[i]).append("\n");
        }
        
        //Append the Before Tax,Tax,Total,and Number of Items
        info.append("---------------------------------------------------").append("\n");
        info.append("Before Tax: ").append(NumberFormat.getCurrencyInstance().format(totalBeforeTax())).append("\n");
        info.append("Tax: ").append(NumberFormat.getCurrencyInstance().format(totalTax())).append("\n");
        info.append("Total: ").append(NumberFormat.getCurrencyInstance().format(totalBeforeTax()+totalTax())).append("\n");
       //WORK ON THIS!
       info.append("Number of items: " + purchases[numpurchases]);
     
     return info.toString();
    }
 
}