A simple loan calculator applet
/*
* Chapter 9 - Financial Applets and Servlets The Art of Java by Herbert Schildt
* and James Holmes McGraw-Hill/Osborne 2003
*
*/
import java.applet.Applet;
import java.awt.Button;
import java.awt.Graphics;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Label;
import java.awt.TextField;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.NumberFormat;
/*
* <applet code="RegPay" width=280 height=200> </applet>
*/
public class RegPay extends Applet implements ActionListener {
TextField amountText, paymentText, periodText, rateText;
Button doIt;
double principal; // original princial
double intRate; // interest rate
double numYears; // length of loan in years
/*
* Number of payments per year. You could allow this value to be set by the
* user.
*/
final int payPerYear = 12;
NumberFormat nf;
public void init() {
// Use a grid bag layout.
GridBagLayout gbag = new GridBagLayout();
GridBagConstraints gbc = new GridBagConstraints();
setLayout(gbag);
Label heading = new Label("Compute Monthly Loan Payments");
Label amountLab = new Label("Principal");
Label periodLab = new Label("Years");
Label rateLab = new Label("Interest Rate");
Label paymentLab = new Label("Monthly Payments");
amountText = new TextField(16);
periodText = new TextField(16);
paymentText = new TextField(16);
rateText = new TextField(16);
// Payment field for display only.
paymentText.setEditable(false);
doIt = new Button("Compute");
// Define the grid bag.
gbc.weighty = 1.0; // use a row weight of 1
gbc.gridwidth = GridBagConstraints.REMAINDER;
gbc.anchor = GridBagConstraints.NORTH;
gbag.setConstraints(heading, gbc);
// Anchor most components to the right.
gbc.anchor = GridBagConstraints.EAST;
gbc.gridwidth = GridBagConstraints.RELATIVE;
gbag.setConstraints(amountLab, gbc);
gbc.gridwidth = GridBagConstraints.REMAINDER;
gbag.setConstraints(amountText, gbc);
gbc.gridwidth = GridBagConstraints.RELATIVE;
gbag.setConstraints(periodLab, gbc);
gbc.gridwidth = GridBagConstraints.REMAINDER;
gbag.setConstraints(periodText, gbc);
gbc.gridwidth = GridBagConstraints.RELATIVE;
gbag.setConstraints(rateLab, gbc);
gbc.gridwidth = GridBagConstraints.REMAINDER;
gbag.setConstraints(rateText, gbc);
gbc.gridwidth = GridBagConstraints.RELATIVE;
gbag.setConstraints(paymentLab, gbc);
gbc.gridwidth = GridBagConstraints.REMAINDER;
gbag.setConstraints(paymentText, gbc);
gbc.anchor = GridBagConstraints.CENTER;
gbag.setConstraints(doIt, gbc);
// Add all the components.
add(heading);
add(amountLab);
add(amountText);
add(periodLab);
add(periodText);
add(rateLab);
add(rateText);
add(paymentLab);
add(paymentText);
add(doIt);
// Register to receive action events.
amountText.addActionListener(this);
periodText.addActionListener(this);
rateText.addActionListener(this);
doIt.addActionListener(this);
nf = NumberFormat.getInstance();
nf.setMinimumFractionDigits(2);
nf.setMaximumFractionDigits(2);
}
/*
* User pressed Enter on a text field or pressed Compute.
*/
public void actionPerformed(ActionEvent ae) {
repaint();
}
// Display the result if all fields are completed.
public void paint(Graphics g) {
double result = 0.0;
String amountStr = amountText.getText();
String periodStr = periodText.getText();
String rateStr = rateText.getText();
try {
if (amountStr.length() != 0 && periodStr.length() != 0
&& rateStr.length() != 0) {
principal = Double.parseDouble(amountStr);
numYears = Double.parseDouble(periodStr);
intRate = Double.parseDouble(rateStr) / 100;
result = compute();
paymentText.setText(nf.format(result));
}
showStatus(""); // erase any previous error message
} catch (NumberFormatException exc) {
showStatus("Invalid Data");
paymentText.setText("");
}
}
// Compute the loan payment.
double compute() {
double numer;
double denom;
double b, e;
numer = intRate * principal / payPerYear;
e = -(payPerYear * numYears);
b = (intRate / payPerYear) + 1.0;
denom = 1.0 - Math.pow(b, e);
return numer / denom;
}
}
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