如何改进从文本文件中的收据总额计算总和的程序？

问：

到目前为止，我在下面有这个程序：

import javax.swing.JOptionPane;
import java.io.*;
import java.util.Scanner;
import java.text.DecimalFormat;
class Receipts 
{
    public static void main(String[] args) throws IOException 
    {

        DecimalFormat format = new DecimalFormat("0.00");
        File file = new File("receipt.txt");
        PrintWriter exit = new PrintWriter(
                           new BufferedWriter(
                           new FileWriter(file, true)));

        Scanner scan = new Scanner(file);


        int choice = JOptionPane.showOptionDialog(null, "Hello. What would you like to do today?", "Receipts", JOptionPane.OK_CANCEL_OPTION, JOptionPane.INFORMATION_MESSAGE, null, new String[]{"See my total", "Add a receipt"}, "default");


        switch(choice)
        {
            case 0:  // see my total
            {
                double total = scan.nextDouble();
                String StrTotal = format.format(total); 

                JOptionPane.showMessageDialog(null, "Your total so far is $" + StrTotal + ".");
            }

            case 1:  // add a receipt
            {
                double receipt = Integer.parseInt(JOptionPane.showInputDialog("Enter the amount."));
                double addTotal = scan.nextDouble();
                exit.print(format.format(receipt + addTotal));


                exit.close();

                JOptionPane.showMessageDialog(null, "The amount has been added.");
            }

            default:
            {
                JOptionPane.showMessageDialog(null, "Please input your selection.");
                exit.close();
                break;
            }
        }

    } 
}

在文本文件中，我有一个数字（例如：“0.00”），程序应该获取该数字，然后使用它进行计算。receipt.txt

1. 有一个错误指出：

Exception in thread "main" java.util.InputMismatchException
        at java.util.Scanner.throwFor(Scanner.java:864)
        at java.util.Scanner.next(Scanner.java:1485)
        at java.util.Scanner.nextDouble(Scanner.java:2413)
        at Receipts.main(Receipts.java:26)

我该如何解决这个问题？

2. 有没有办法使它从文本文件中删除总和并将其添加到用户的输入中，然后在文件中再次打印出来以再次使用？或者有更好的方法来重复计算总和？FileWriter