[X] My Assistant

[confused.help]

I'm trying to write this program to display an amortization schedule.... I am nearly finished, but my final principal is coming up negative, and I've tried everything I can think of to fix this but nothing is working... any ideas?? Thanks..

CODE

import java.util.Scanner;
import java.text.DecimalFormat;

public class Mortgage
{
      
    public static void main (String args[])
    {  
        Scanner input = new Scanner(System.in);
  
        int choice;
        double principal;
        double yearlyInterest;
        int paybackYears;
        double monthlyPmt;

        System.out.println("Select one of the following options");
        System.out.println();
        System.out.println("[1]   Monthly Loan Payment Computation");
        System.out.println("[2]   Amortization Schedule");
        System.out.println("[3]   Credit Card Pay-Off Schedule");
        System.out.println();
        System.out.print("Enter your choice  ===>>  ");
        choice = input.nextInt();
        System.out.println();

        switch (choice)
        {
            case 1:
                System.out.print("Enter loan amount       ===>>  ");
                principal = input.nextDouble();
                System.out.print("Enter annual percent    ===>>  ");
                yearlyInterest = input.nextDouble();
                System.out.print("Enter years to pay back ===>>  ");
                paybackYears = input.nextInt();
                Interest loan1 = new Interest(principal, yearlyInterest, paybackYears,0);
                loan1.computePayment();
                break;
            case 2:
                System.out.print("Enter principle balance  ===>>  ");
                principal = input.nextDouble();
                System.out.print("Enter annual percent     ===>>  ");
                yearlyInterest = input.nextDouble();
                System.out.print("Enter monthly payment    ===>>  ");
                monthlyPmt = input.nextDouble();
                Interest loan2 = new Interest(principal, yearlyInterest, 0, monthlyPmt);
                loan2.amortization();
                break;
                    }
        System.out.println();
    }
    
}



class Interest
{

    private double principal;
    private double percent;
    private int years;
    private double monthlyPmt;
    private int months;
    private int pmtNr;
    private double monthlyRate;
    private double interestPmt;
    private double principalPmt;
    private double totalPmt;
    private double totalInt;

    public Interest(double la, double yi, int py, double mp)
    // Do not alter anything in the constructor!
    {
        principal = la;
        percent = yi;
        years = py;
        monthlyPmt = mp;
        monthlyRate = percent/1200;
        months = years * 12;
        pmtNr = 0;
    }


    private double round(double x)
    {
        return (double) Math.round(100 * x) / 100;
    }
    
    
    public void computePayment()
    {
        System.out.println();
        System.out.println("Monthly Loan Payment Computation");
        System.out.println();
        
        DecimalFormat output = new DecimalFormat("##.0");
        monthlyPmt = (((monthlyRate)* Math.pow(1+monthlyRate,months))/(Math.pow(1+monthlyRate,months)-1))*principal;
        monthlyPmt = round(monthlyPmt);
        totalPmt =(months*monthlyPmt);
        totalInt = totalPmt - principal;
        
        System.out.println("Loan Amount:     " + principal);
        System.out.println("Yearly Interest: " + percent);
        System.out.println("Payback Years:   " + years);
        System.out.println("Monthly Payment: " + monthlyPmt);
        System.out.println("Total Payments:  " + output.format(totalPmt));
        System.out.println("Total Interest:  " + output.format(totalInt));
    }
//This is where I'm having trouble....
    public void amortization()
    {
        System.out.println();
        System.out.println("Amortization Schedule");
        System.out.println();
        System.out.println("Loan Amount:     " + principal);
        System.out.println("Yearly Interest: " + percent);
        System.out.println("Montly Payment:  " + monthlyPmt);
        double Loan = principal;
        

        do
        {
            interestPmt = round(principal*monthlyRate);
            principalPmt = round(monthlyPmt - interestPmt);
            principal = round(principal - principalPmt);
            
            
            pmtNr ++;
            

            System.out.println(pmtNr+"    \t"+monthlyPmt+"    \t"+interestPmt+"    \t"+principalPmt+"    \t"+principal);
        }
        while (principal > 0);
        
        totalPmt = pmtNr * monthlyPmt;
        totalInt = round(totalPmt - Loan);
    
    
        System.out.println("Payback Months:  " + pmtNr);  
        System.out.println("Total Payments:  " + totalPmt);
        System.out.println("Total Interest:  " + totalInt);
    }