DiscountedItem.java

Invoice.java

import java.util.*;
import javax.swing.event.*;

/**
   An invoice for a sale, consisting of line items.
*/
public class Invoice
{
   /**
      Constructs a blank invoice.
   */
   public Invoice()
   {
      items = new ArrayList();
      listeners = new ArrayList();
   }

  /**
      Adds an item to the invoice.
      @param item the item to add
   */
   public void addItem(LineItem item) 
   { 
      items.add(item); 
      // notify all observers of the change to the invoice
      ChangeEvent event = new ChangeEvent(this);
      for (int i = 0; i < listeners.size(); i++)
      {
         ChangeListener listener 
            = (ChangeListener) listeners.get(i);
         listener.stateChanged(event);
      }
   }

   /**
      Adds a change listener to the invoice.
      @param listener the change listener to add
   */
   public void addChangeListener(ChangeListener listener) 
   { 
      listeners.add(listener); 
   }

   /**
      Gets an iterator that iterates through the items.
      @return an iterator for the items
   */
   public Iterator getItems()
   {
      return new
         Iterator()
         {
            public boolean hasNext() 
            { 
               return current < items.size(); 
            }
            public Object next() 
            { 
               Object r = items.get(current);
               current++; 
               return r;
            }
            public void remove()
            {
               throw new UnsupportedOperationException();
            }
            private int current = 0;
         };
   }

   public String format(InvoiceFormatter formatter)
   {
      String r = formatter.formatHeader();
      Iterator iter = getItems(); 
      while (iter.hasNext())
      {
         LineItem item = (LineItem) iter.next();
         r += formatter.formatLineItem(item);
      }
      return r + formatter.formatFooter();
   }

   private ArrayList items;
   private ArrayList listeners;
}

/**
   This interface describes the tasks that an invoice
   formatter needs to carry out.
*/
public interface InvoiceFormatter
{
   /**
      Formats the header of the invoice.
      @return the invoice header
   */
   String formatHeader();
   /**
      Formats a line item of the invoice.
      @return the formatted line item
   */
   String formatLineItem(LineItem item);
   /**
      Formats the footer of the invoice.
      @return the invoice footer
   */
   String formatFooter();
}

LineItem.java

/**
   A line item in an invoice.
*/
public interface LineItem
{
   /**
      Gets the price of this line item.
      @return the price
   */
   double getPrice();
   /**
      Gets the description of this line item.
      @return the description
   */   
   String toString();
}

Product.java

/**
   A product with a price and description.
*/
public class Product implements LineItem
{
   /**
      Constructs a product.
      @param description the description
      @param price the price
   */
   public Product(String description, double price)
   {
      this.description = description;
      this.price = price;
   }
   public double getPrice() { return price; }
   public String toString() { return description; }
   private String description;
   private double price;
}

SimpleFormatter.java

/**
   A simple invoice formatter.
*/
public class SimpleFormatter implements InvoiceFormatter
{
   public String formatHeader()
   {
      total = 0;
      return "     I N V O I C E\n\n\n";
   }

   public String formatLineItem(LineItem item)
   {
      total += item.getPrice();
      return item.toString() + ": $" 
         + item.getPrice() + "\n";
   }

   public String formatFooter()
   {
      return "\n\nTOTAL DUE: $" + total + "\n";
   }
   
   private double total;
}

InvoiceTest.java

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;

/**
   A program that tests the invoice classes.
*/
public class InvoiceTest
{
   public static void main(String[] args)
   {
      final Invoice invoice = new Invoice();
      final InvoiceFormatter formatter = new SimpleFormatter();

      // this text area will contain the formatted invoice
      final JTextArea textArea = new JTextArea(20, 40);

      // when the invoice changes, update the text area
      ChangeListener listener = new
         ChangeListener()
         {
            public void stateChanged(ChangeEvent event)
            {
               textArea.setText(invoice.format(formatter));
            }
         };
      invoice.addChangeListener(listener);

      // add line items to a combo box
      final JComboBox combo = new JComboBox();   
      Product hammer = new Product("Hammer", 19.95);
      Product nails = new Product("Assorted nails", 9.95);
      combo.addItem(hammer);
      Bundle bundle = new Bundle();
      bundle.add(hammer);
      bundle.add(nails);
      combo.addItem(new DiscountedItem(bundle, 10));

      // make a button for adding the currently selected
      // item to the invoice
      JButton addButton = new JButton("Add");
      addButton.addActionListener(new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               LineItem item = (LineItem) combo.getSelectedItem();
               invoice.addItem(item);
            }
         });

      // put the combo box and the add button into a panel
      JPanel panel = new JPanel();
      panel.add(combo);
      panel.add(addButton);

      // add the text area and panel to the content pane
      JFrame frame = new JFrame();
      Container contentPane = frame.getContentPane();
      contentPane.add(new JScrollPane(textArea), 
         BorderLayout.CENTER);
      contentPane.add(panel, BorderLayout.SOUTH);
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.pack();
      frame.setVisible(true);   
   }
}

Results