// import all need packages
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.applet.*;
 
//extend JFrame for the user interface and actionlistener for events
public class LCalcApplet extends JApplet implements ActionListener
{
	/*  create instance variables so they can be used in every method
	 *  instance variables allow all method to use them, instead of only
	 *  the method you are working in
	 */
	JButton btnCalc;
	JTextField txtAmount;
	JTextField txtAPR;
	JTextField txtMonths;
	JTextField txtPayment;
	JLabel lblAmount;
	JLabel lblAPR;
	JLabel lblMonths;
	JLabel lblPayment;
 
	/*  create a JMenuBar, used to hold the Menu's such
	 *  as file, edit, and that kind of stuff...On top of that,
	 *  you need the MenuItems...These are the actual items that show
	 *  up on the drop down
	 */
    JMenuBar menuBar;
	JMenu fileMenu;
	JMenuItem exitItem;
	JMenuItem resetItem;
 
	//the classes constructor
	public void init()
	{
		//create the menuBar
		menuBar = new JMenuBar();
		//create the items
		fileMenu = new JMenu("File");
		exitItem = new JMenuItem("Exit");
		resetItem = new JMenuItem("Reset");
		/*  add all the items to the Menu, this is 
		 *  done by the add() method coorisponding to 
		 *  the JMenu class
		 */
		fileMenu.add(exitItem);
		fileMenu.add(resetItem);
		/*  add action listeners to the menu items, this
		 *  will enable us to peform action based on what was
		 *  clicked.
		 */
		exitItem.addActionListener(this);
		resetItem.addActionListener(this);
		/*  once all that is done, you add the 
		 *  menu to the menuBar, and then you set
		 *  the menuBar to be used in the program...Note
		 *  you do not have to add the menubar to the Container
		 *  after setting it.
		 */
		menuBar.add(fileMenu);
		setJMenuBar(menuBar);
 
		/*  JPanels are a type of container...
		 *  It's good to use them as flow control,
		 *  and to group controls
		 */
		JPanel row1 = new JPanel();
		row1.setBackground(Color.white);
		//add the amount textfield and label to the panel
		lblAmount = new JLabel("   Purchase amount: ");
		row1.add(lblAmount);
		/*  The JTextField requires one parameter,
		 *  the number of characters long...So we use 
		 *  ten on these.
		 */
		txtAmount = new JTextField(10);
		txtAmount.setBackground(Color.orange);
		row1.add(txtAmount);
 
		JPanel row2 = new JPanel();
		row2.setBackground(Color.white);
		//add the apr label and textfield to the panel
		lblAmount = new JLabel("  Interest rate(APR): ");
		row2.add(lblAmount);
		txtAPR = new JTextField(10);
		txtAPR.setBackground(Color.orange);
		row2.add(txtAPR);
 
		JPanel row3 = new JPanel();
		row3.setBackground(Color.white);
		//add the month textfield and label to the panel
		lblMonths = new JLabel("Number of months: ");
		row3.add(lblMonths);
		txtMonths = new JTextField(10);
		txtMonths.setBackground(Color.orange);
		row3.add(txtMonths);
 
		JPanel row4 = new JPanel();
		row4.setBackground(Color.white);
		//add the payment label and textfield to the panel
		lblPayment = new JLabel(" Your payment is:    ");
		lblPayment.setForeground(Color.red);
		row4.add(lblPayment);
		txtPayment = new JTextField(10);
		txtPayment.setBackground(Color.orange);
		/*  set this textfield so that it cannot be edited,
		 *  because you don't want the user to enter anything
		 *  in here.
		 */
		txtPayment.setEditable(false);
		row4.add(txtPayment);
 
		JPanel row5 = new JPanel();
		row5.setBackground(Color.white);
		//add the calculate button
		btnCalc = new JButton("Calculate");
		btnCalc.setBackground(Color.cyan);
		/*  add an actionListener to the button, this 
		 *  enables us to perform events if it was clicked
		 */
		btnCalc.addActionListener(this);
		row5.add(btnCalc);
 
		/*  Construct a container object, and
		 *  then add all of the JPanels to it. 
		 *  You set the layout to the container, so 
		 *  that you have some kind of flow control in the
		 *  program.
		 */
		Container content = getContentPane();
		//we will use a flowlayout
		content.setLayout(new FlowLayout());
		content.add(row1);
		content.add(row2);
		content.add(row3);
		content.add(row4);
		content.add(row5);
		content.setBackground(Color.white);
		//set the content pane of the frame
		setContentPane(content);
		//last, set it visible
		setVisible(true);
 
	}
 
	public void actionPerformed(ActionEvent ae)
	{
		/*  check to see if the event was fired by the
		 *  calculate button, if so peform the correct operation
		 */
		if (ae.getSource() == btnCalc)
		{
			/*  use some if/else statements to check and make sure
			 *  that all the textfields have some value in them
			 */
			if (txtAmount.getText().equals(""))
			{
				JOptionPane.showMessageDialog(this,"Please enter an amount");
					// invoke a class method that will show a simple message to the user
			}
			else if(txtAPR.getText().equals(""))
			{
				JOptionPane.showMessageDialog(this,"Please enter the interest rate");
					// invoke a class method that will show a simple message to the user
			}
			else if (txtMonths.getText().equals(""))
			{
				JOptionPane.showMessageDialog(this,"Please enter length of months");
					// invoke a class method that will show a simple message to the user
			}
			/*  if we make it to here, all values are filled, and ready to be proccessed
			 *  so we use a try cacth statement to make sure the values wasn't anything other
			 *  than numeric.
			 */
			else 
			{	
				try {
					//create new method used to calculate and show the monthly payment
					getAndShowPayment();
				}
				catch (ArithmeticException mathE)
				{
					//catch an arithmetic exception, and if it was caught give the user
					//some feedback
					JOptionPane.showMessageDialog(this,"Error in arithmetic");
				}
			}
		}
		//just in case the selection was the exit item
		else if (ae.getSource() == exitItem)
		{
			/* call the System classes exit() method, and 
			 * place 0 inside of the paranthesis.  This will make
			 * the program exit.
			 */
			System.exit(0);
		}
		//if the reset menu item was selected
		else if (ae.getSource() == resetItem)
		{
			/*  reset all the values in the  textfields to null,
			 *  we do this by callling the textfields setText() method
			 *  and putting null values in there
			 */
				txtAmount.setText("");
				txtAPR.setText("");
				txtMonths.setText("");
			    txtPayment.setText("");
		}
	}
 
	/*  This is a custom method that returns void, or nothing...
	 *  it will calculate the values, and print it out to the user
	 */
	public void getAndShowPayment()
	{
				/*  Use the double's wrapper class(Double) to parse
				 *  the text into a double...Do this for all three textfields
				 */
				double dAmount = Double.valueOf(txtAmount.getText());
				double dAPR = Double.valueOf(txtAPR.getText());
				double dMonths = Double.valueOf(txtMonths.getText());
 
				/* This is the forumula to calcuate the mortgage:
				 * a -- loan amount
				 * m -- # of months to repay
				 * i -- monthly interest
				 *
				 *Monthly payment:
				 * 
				 *  a(i)
				 * -------- (divide)
				 *  1 - (1/1+i)^m
				 */
 
				/* this is the interest rate, we divide by 1200 because we
				 * want the user to be able to enter the intrest rate in this format:
				 * 6.5 instead of .065
				 */
				double rate = (dAPR / 1200);
				//simply calculate the payment using the above formula.
 
				double dgetMonthlyPayment = (dAmount * rate / (1 - (Math.pow(1 / (1 + rate), dMonths))));
				/*  use the decimal format class to create a new decimal format, this way the calculation
				 *  will be to two decimal places
				 *  we precede it with java.text because this is the class that contains the 
				 *  decimal format, and we don't to import the statement
				 */
 				java.text.DecimalFormat dec = new java.text.DecimalFormat(",###.00");
 				//show the amount by calling the decimalFormat.format() method
				txtPayment.setText("$ " + dec.format(dgetMonthlyPayment));
	}
}

<HTML>
<HEAD>
</HEAD>
<BODY BGCOLOR="000000">
<CENTER>
<APPLET
	code	= "LCalcApplet.class"
	width	= "370"
	height	= "600"
	>
</APPLET>
</CENTER>
</BODY>
</HTML>