A simple loan calculator applet : Applet « Swing JFC « Java

A simple loan calculator applet

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();

    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.

    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.

    // Register to receive action events.

    nf = NumberFormat.getInstance();

   * User pressed Enter on a text field or pressed Compute.
  public void actionPerformed(ActionEvent ae) {

  // 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();


      showStatus(""); // erase any previous error message
    } catch (NumberFormatException exc) {
      showStatus("Invalid Data");

  // 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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