Java application calculate monthly loan payments

Question

This is due Feb. 19 and I can't get the for statement to calculate right. Can someone tell me what I'm doing wrong. I'll copy and paste the program:
import java.text.DecimalFormat;
import javax.swing.*; // import package javax.swing
public class Lab4 {
     public static void main( String args[] )
     double L, // loan amount
          R, // monthly interest rate
          P, // monthly loan payment
          S, // used for counting
          APR, // annual % for interest
          balance,
          newBalance,
          principle,
          interestPart;
     int Y,
          M = 0,
          count = 1;
     // read in Loan amount from user
     String Loan =
          JOptionPane.showInputDialog(
          "Enter Amount for Loan/Mortgage" );
     // read in Interest Rate from user
     String InterestRate =
          JOptionPane.showInputDialog(
          "Enter Interest Rate (APR) .XXX" );
     // read Period of Years for Loan
     String Year =
          JOptionPane.showInputDialog(
          "Enter Period (Years) for Loan/Mortgage");
     // convert from type String to type Double
     L = Double.parseDouble ( Loan );
     APR = Double.parseDouble ( InterestRate );
     Y = Integer.parseInt ( Year );
          R = APR / 12;
          S = 0;
     // create JTextArea to display output
     JTextArea outputTextArea = new JTextArea(20,50);
     JScrollPane scroller = new JScrollPane(outputTextArea);
     // used to set the decimal places
DecimalFormat twoDigits = new DecimalFormat( "0.00" );
     while ( M < (Y*12))
          S += Math.pow(1.0 + R, M);
          P = L * (1.0 + R * S) / S;
     // set first line of text in outputTextArea
     outputTextArea.setText( "Loan/Mortgage Amount: $" +           twoDigits.format( L ) + "\nInterest Rate: " +
          APR * 100 + "%" + "\nLoan Period (Years): " +
          Y + "\nMonthly Payment: $" + twoDigits.format( P ) +
          "\n\n\tAMORTIZATION SCHEDULE\n\n" + "Payment " +
          " Principle Part " + " Interest Part " +
          " Loan\n" + "Number " + " of Payment"
          + " of Payment " + " Balance\n\n");
          M = M +1;
     for ( count = 0; count <= (Y*12); count++ )
          S += Math.pow(1.0 + R, M);
          P = L * (1.0 + R * S) / S;
          interestPart = balance * (APR / 12)*10;
          principle = P - interestPart;
          newBalance = balance - principle;
          outputTextArea.append(count + "\t" +                     twoDigits.format(principle) +
          "\t" + twoDigits.format(interestPart) + "\t" +                twoDigits.format(newBalance) + "\n");
          M = M + 1;
          balance = newBalance;
     JOptionPane.showMessageDialog( null,                scroller, "Loan/Mortgage Data",
          JOptionPane.INFORMATION_MESSAGE );
          System.exit( 0 );

Answer 1

Yep,
1. remove the text areas from the code
2. // comment out the OptionPanes for now and put amounts in manually eg
years = 25;
interestRate = 12.7;
amountToBorrow = 50000;
3. In psudocode (I'm not sure what your formulas are doing here???)
int months = yearsFortheRepayment * 12;
for(int i=0; i < months; i++){
total = total * monthlyInterestRate; //or whatever the formula is here
4. Comment out the rest of the code and test it up to here with;-
System.out.println("Test, is this part right? Total payment="+total);
Is it the right figure?
5. Do the same for the second formula
6. If this is right, put back (uncomment) the JOptionPane's and take out these;-
years = 25;
interestRate = 12.7;
amountToBorrow = 50000;
Then see if it still works the same with the same figures.
7. Display the results for your 2 stages with a JOP.showMessageDialog.
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
If you post your code again use the your code in here tags to format it nicely and follow coding conventions they're there for a very good reason ie: indenting and naming - some of the variable identifier names you have used are not good, making the code hard to track. Good luck.