LoanCalcApplet.java

     // A simple loan calculator applet.
     import java.awt.*;
     import java.awt.event.*;
     import javax.swing.*;
     import java.text.*;
     /*
       <applet code="RegPay" width=320 height=200>
</applet> */
     public class RegPay extends JApplet
       implements ActionListener {
       JTextField amount Text, paymentText, periodText, rateText;
       JButton doIt;
       double principal; // original principal
       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() {
         try {
           SwingUtilities.invokeAndWait(new Runnable () {
             public void run() {
               makeGUI(); // initialize the GUI
             }
           });
         } catch(Exception exc) {
System.out.println("Can't create because of "+ exc);
} }
// Set up and initialize the GUI.
private void makeGUI() {
  // Use a grid bag layout.
  GridBagLayout gbag = new GridBagLayout();
  GridBagConstraints gbc = new GridBagConstraints();
  setLayout(gbag);
  JLabel heading = new
         JLabel("Compute Monthly Loan Payments");
  JLabel amountLab = new JLabel("Principal ");
  JLabel periodLab = new JLabel("Years ");
  JLabel rateLab = new JLabel("Interest Rate ");
  JLabel paymentLab = new JLabel("Monthly Payments ");
  amountText = new JTextField(10);
  periodText = new JTextField(10);
  paymentText = new JTextField(10);
  rateText = new JTextField(10);
  // Payment field for display only.
  paymentText.setEditable(false);
  doIt = new JButton("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);
// Create a number format.
nf = NumberFormat.getInstance();
nf.setMinimumFractionDigits(2);
nf.setMaximumFractionDigits(2);
}
/* User pressed Enter on a text field or
   pressed Compute. Display the result if all
   fields are completed. */
public void actionPerformed(ActionEvent ae) {
  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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