This program calculates a running total.

TotalSales.java

import java.text.DecimalFormat;  
import javax.swing.JOptionPane;

/**
   This program calculates a running total.
*/

public class TotalSales
{
   public static void main(String[] args)
   {
      int days;            // The number of days
      double sales;        // A day's sales figure
      double totalSales;   // Accumulator
      String input;        // To hold the user's input

      // Create a DecimalFormat object to format output.
      DecimalFormat dollar = new DecimalFormat("#,##0.00");

      // Get the number of days.
      input = JOptionPane.showInputDialog("For how many days " +
                                   "do you have sales figures?");
      days = Integer.parseInt(input);

      // Set the accumulator to 0.
      totalSales = 0.0;

      // Get the sales figures and calculate a running total.
      for (int count = 1; count <= days; count++)
      {
         input = JOptionPane.showInputDialog("Enter the sales " +
                                       "for day " + count + ": ");
         sales = Double.parseDouble(input);
         totalSales += sales;   // Add sales to totalSales.
      }

      // Display the total sales.
      JOptionPane.showMessageDialog(null, "The total sales are $" +
                                    dollar.format(totalSales));

      System.exit(0);
   }
}