Trying to make a pop up message for a loan calculator

Question

Yes this is very basic, but I am very basic. I am using the init() method, and paint() method "classes" as applet to make a loan calculator. I have the calculating all done, and correct, but if someone puts in a nonreal number, or an amount they want to take in less than 0 I want the pop up message tell them they are crazy. How would I create a message popup? The code below is the full code for my loan calculator.
* Programmer: Carl Finch
* Program: The Bank of America Loan Calculator(not affiliated with Bank of * America just a goof)
* Date: 01/12/05
* Purpose: Learn the paint() function, action performed function, and
* all their "special methods"
import java.awt.*;
import java.awt.event.*;
import java.text.*;
import java.applet.*;
public class Loan extends Applet implements ActionListener
// Declare the vars
TextField txtAmo;
TextField txtRat;
TextField txtMon;
TextField txtPay;
Button btnCal;
public void init()
//set the layout of the textfields, layouts, and the big o butotn.
setLayout(null);
txtAmo = new TextField (25);
txtRat = new TextField(5);
txtMon = new TextField(6);
txtPay = new TextField(8);
btnCal = new Button("Calculate");
txtAmo.setBounds(145,25,80,30);
txtRat.setBounds(145,65,80,30);
txtMon.setBounds(145,105,80,30);
txtPay.setBounds(145,145,80,30);
btnCal.setBounds(145,200,80,30);
//add all to applet
add(txtAmo);
add(txtRat);
add(txtMon);
add(txtPay);
add(btnCal);
btnCal.addActionListener(this);
//Create graphics for the applet
public void paint(Graphics g)
// simple text displayed on applet
g.setColor(Color.white);
g.fillRect(0, 0, 500, 500);
g.setColor(Color.blue);
g.drawString("Bank of America Loan Calculator", 145, 20);
g.setColor(Color.blue);
g.drawString("Loan Amount",30,35);
g.drawString("APR (%)", 30,75);
g.drawString("Months to Repay",30,115);
g.drawString("Monthly Payment:",30,155);
//Adding image method
Image money;
money = getImage(getDocumentBase(), "money.jpg");
g.drawImage(money, 300,50, this);
//Action Performed
public void actionPerformed(ActionEvent ae)
//Declaring the vars needed.
//try
DecimalFormat money = new DecimalFormat("$#.00");
double a = Double.valueOf(txtAmo.getText()).doubleValue();
double r = Double.valueOf(txtRat.getText()).doubleValue();
int m = Integer.parseInt(txtMon.getText().trim());
r=r/1200;
//The math of the monthly payment
double p=a*r/(1-Math.pow(1/(1+r),m));
txtPay.setText(""+money.format(p));
/*catch(ArithmeticException error)
//Find option for java.awt
//Adding some error messages for the technology inleterates.
Sorry I had to display the code, but I don't know how to do attachements

Answer 1

javax.swing.JOptionPane.showMessageDialog(null, "Message here");