Thread: Need help understanding this code!

I have a midterm coming up tomorrow night, and it's basically us reading a set of codes and seeing where the errors are and what set of codes are needed to make the program compile succesfully.


Here is the program I am using to review for the midterm, it is a menu which uses frames and panels, and has the user interact with check boxes to pick what items he/she wants, then lists the invoice of all the transactions.

Breakfast.java

import javax.swing.JOptionPane;
import javax.swing.JFrame;
import javax.swing.*;
import java.text.NumberFormat;
import java.awt.Font;
 
public class Breakfast
{
    public static JFrame frame;
    static public void main(String[] args)
    {
      frame = new JFrame ("Ben's Breakfast Bar Menu");  // creates frame with "Ben's Breakfast Bar Menu" as title
      frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE); // Sets the X button to close the program
 
      MenuOptionsPanel panel = new MenuOptionsPanel(); // calls upon MenuOptionsPanel class
      frame.getContentPane().add (panel); 
 
      frame.pack();
      frame.setLocation(200,300);
      frame.setVisible(true);
    }
 
    public static void  displayInvoice(JCheckBox[] items,float[] prices,String[] namesOfItems)
    {		
        NumberFormat money = NumberFormat.getCurrencyInstance();
	float costOfItems=0.0f;
        String message = "";
        message+= "You ordered:\n------------------------------\n";
	message+= String.format("%-24s%6s\n","Food Item","Price")+"\n";	
	for (int index=0; index != items.length; index++)
	{
            if (items[index].isSelected())
            {
		costOfItems += prices[index];
                message += String.format("%-24s%6s\n", namesOfItems[index], 
                                                           money.format(prices[index]));
 
            }
	}
 
	message +="\n------------------------------\n";
	message +=String.format("%-24s%6s","AMOUNT TO PAY",  money.format(costOfItems));
	message +=            "\n------------------------------\n\n";
	message +="Thank you - enjoy your meal!";
 	JTextArea text = new JTextArea(message);
	text.setBorder(null);
	text.setOpaque(false);
	text.setFont(new Font("Monospaced", Font.PLAIN, 14) );
        MenuOptionsPanel.frame2.setVisible(false);
	JOptionPane.showMessageDialog(null, text);        
	}
}

MenuOptionsPanel.java

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
 
public class MenuOptionsPanel extends JPanel
{
 
   private static JLabel prompt;
   private static JRadioButton one, two, three;
   private static MenuPanel mp;
   public static JFrame frame2;
 
   //-----------------------------------------------------------------
   //  Sets up a panel with a label and a set of radio buttons
   //  that present options to the user.
   //-----------------------------------------------------------------
   public MenuOptionsPanel()
   {
      setLayout(new GridLayout(4,1));       
      prompt = new JLabel ("Choose your option?");
      prompt.setFont (new Font ("Helvetica", Font.BOLD, 24));
 
      one = new JRadioButton ("Choose from the Menu");
      one.setBackground (Color.green);
      two = new JRadioButton ("Display Purchase Invoice");
      two.setBackground (Color.green);
      three = new JRadioButton ("Exit");
      three.setBackground (Color.green);
 
      ButtonGroup group = new ButtonGroup();
      group.add (one);
      group.add (two);
      group.add (three);
 
      MenuOptionListener mol = new MenuOptionListener();
      one.addActionListener (mol);
      two.addActionListener (mol);
      three.addActionListener (mol);
 
      add (prompt);
      add (one);
      add (two);
      add (three);
 
      setBackground (Color.green);
      setPreferredSize (new Dimension(400, 100));
   }
 
   //*****************************************************************
   //  Represents the listener for the radio buttons
   //*****************************************************************
   private class MenuOptionListener implements ActionListener
   {
      //--------------------------------------------------------------
      //  Calls the method to process the option for which radio
      //  button was pressed.
      //--------------------------------------------------------------
      public void actionPerformed (ActionEvent event)
      {
         Object source = event.getSource();
 
         if (source == one)
         {
           mp = new MenuPanel(); // instantiates the MenuPanel class 
           frame2 = new JFrame("Breakfast Menu");
           frame2.setDefaultCloseOperation (JFrame.DO_NOTHING_ON_CLOSE);           
           frame2.getContentPane().add(mp);
           frame2.setLocation(500,100);  // Adds parameters and coordinates for the panel 
           frame2.pack();
           frame2.setVisible(true);
         }
         else
            if (source == two)
               Breakfast.displayInvoice(MenuPanel.items, MenuPanel.prices, MenuPanel.namesOfItems);
            else
            {   
                String message; 
                message = "Thanks for Dining at Bobs\nHope you enjoyed your meal";
                JOptionPane.showMessageDialog (null, message);
                System.exit(0);
 
            }
      }
   }
}

MenuPanel.java

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.text.NumberFormat;
 
public class MenuPanel extends JPanel {
    private static NumberFormat fmt = NumberFormat.getCurrencyInstance(); 
    //NumberFormat is abstract base class for all number formats. Provides 
    //interface for formatting and parsing numbers.
    //getCurrencyInstance = returns a currency format for the spcecified locale
 
    //private boolean[] selectedItems;
 
    private JLabel foodColLabel,priceColLabel,priceLabel;
    public static JCheckBox[] items;
    private int numberOfItems;
    private JButton checkOut;
    public static String[] namesOfItems = {"Eggs","Blueberry Pancakes",
                            "Bagels with Cream Cheese",
        		     "English Muffin","Yogurt",
                                 "Corned Beef Hash","Toast",
				     "Fries","Tea","Coffee","Hot Chocolate"};
    public static float[] prices = {2.75f,4.0f,1.5f,0.95f,1.0f,1.75f,
                                    0.75f,1.0f,0.75f,1.20f,1.95f};		
    // sets each price to the corresponding item. iE 2.75 for eggs etc
    public MenuPanel()
    {
 
	numberOfItems = namesOfItems.length;		
	//selectedItems = new boolean[numberOfItems];
		// set layout
	setLayout(new GridLayout(numberOfItems+2,2)); //sets the rows and collums
		// insert labels 
	foodColLabel = new JLabel("MenuItems");
	priceColLabel = new JLabel("Price of Items");
	add(foodColLabel);
	add(priceColLabel);
		// insert a check box for each item
	items = new JCheckBox[numberOfItems];
	for (int index=0; index != numberOfItems; index++)
	// for (initialization; termination; increment
        // Keep in mind: intilization expression initializes the loop, executed 
        // once as the loop beings.
        // When terminaion expression evalutes to false, the loop terminates
 
        {
 
            items[index] = new JCheckBox(namesOfItems[index],false);
            items[index].setBackground (Color.green);
            add(items[index]);
            priceLabel = new JLabel(fmt.format(prices[index]));
            add(priceLabel);                
		//	item[index].addItemListener(this);		}
        }
        checkOut = new JButton("CheckOut");
        ButtonListener bl = new ButtonListener();
        checkOut.addActionListener(bl);
 	add(checkOut);       
        setBackground (Color.green);
        setPreferredSize (new Dimension(400, 500));
   }        
 
 
    private class ButtonListener implements ActionListener
   {
      //--------------------------------------------------------------
      //  Calls the method when Button is pressed
      //--------------------------------------------------------------
      public void actionPerformed (ActionEvent event)
      {
         Object source = event.getSource();
 
         if (source == checkOut)
         {
         //  Breakfast.frame.setVisible(false);
           Breakfast.displayInvoice(items,prices,namesOfItems);
         }
      }
   }
}

As you can see I have added some notes, however I am having trouble understanding the boolean arguements in the code.

For example, in the MenuPanel.java class, there is a segment of the code which states:

    {
 
            items[index] = new JCheckBox(namesOfItems[index],false);
            items[index].setBackground (Color.green);
            add(items[index]);
            priceLabel = new JLabel(fmt.format(prices[index]));
            add(priceLabel);                
		//	item[index].addItemListener(this);		}
        }

What does the items[index] = new JCheckBox(namesOfItems[index],false); what does the "false" mean?

Same thing here:

   if (source == checkOut)
         {
         //  Breakfast.frame.setVisible(false);
           Breakfast.displayInvoice(items,prices,namesOfItems);
         }

I am having trouble understanding the boolean value of (false)



Another question:
message +=String.format("%-24s%6s","AMOUNT TO PAY", money.format(costOfItems));

I am having trouble understanding the %-24s and %6s, I know this statement is for the placement of the expression, so -24s would mean 24 characters to the left correct? and 6s would mean 6 characters to the right? Does the s mean string? So it would mean 24 strings to the left? What does the s represent? Also, what is the difference between %-24s and %-24f? Is the usage of s and f depending on what value the expression is printing?

What does the items[index] = new JCheckBox(namesOfItems[index],false); what does the "false" mean?

What does the documentation say?

Answers to both of your questions could have been found through any decent search engine and a little effort. The documentation to the language answers both questions.

Correction. ...answers all of your questions. Do some reading and ask any questions you have left.