按揭买房月还换程序

//A simple loan calculator applet.

import java.awt.*;

import java.awt.event.*;

import java.applet.*;

import java.text.*;

/*

 <applet code="RegPay" width=280 height=200>

 </applet>

 */

 public class RegPay extends Applet

 implements ActionListener

 {

      TextField amountText,paymentText,periodText,rateText;

      Button doIt;

      double principal;

      double intRate;

      double numYears;

      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("Computer 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);

             paymentText=new TextField(16);

             periodText=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("");

            }

            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;

       }

 }