Thread: Beginner at Java, Need help with my project its due tuesday.

Guidelines

This assignment is comprised of modifying your program which will calculate a monthly a moritzed amount (e.g. a house mortgage). For this program, you are to add the logic to prompt the user if they wish to enter the data again, then loop if they enter a "y" value; otherwise perform the calculations and the error checking as in the previous program.

Enter loan amount: 100000
Enter rate: 6
Enter number years: 30

The monthly payment is: $599.55

Would you like to calculate again (y/n): y

Enter loan amount: -100000
Enter rate: 6
Enter number years: 30

You must enter positive numeric data!

Would you like to calculate again (y/n): n

__________________________________________________ __________________

package monthypayment2;
import java.text.NumberFormat;
import java.util.Scanner;
 
 
public class Monthypayment2 {
    private static double monthlypay1;
    private static String choice;
    private static boolean n;
    private static String False;
    private static boolean y;
 
 
 
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
 
        //format number and currency
       NumberFormat NF = NumberFormat.getCurrencyInstance();
       NumberFormat NI = NumberFormat.getNumberInstance();
 
 
        double months = 1;
        double numyears;
        double loanamount;
        double rate;
 
       int repeat = 0;   //condition for loop
 
       do {
 
 
 
        //prompt loan amount
        System.out.print("Enter Loan Amount:");                           
        loanamount = in.nextDouble();
 
        //prompt rate
        System.out.print("Enter Rate:");
         rate = in.nextDouble();
 
         //prompt years
        System.out.print("Enter Number of Years:");
        numyears = in.nextDouble();
 
 
      //calculation monthly
       monthlypay1 = (loanamount * rate / 12) / (Math.pow(1 + rate, months)) / (Math.pow(1 + rate, months)-1);
 
 
        System.out.println("The Monthly Payment is: $" + monthlypay1);
 
         y = true;
         n = false;
 
        if (monthlypay1 < 0) {
            System.out.print("You need to enter positive numerical data!");}
        else {
            System.out.print("Would you like to continue calculations(y/n)?");
            choice = in.nextLine();
        }
       } while (choice.equals("y"));
 
                while(choice.equals("n")) {
            break; }
               }
 
 
            }

Basically when I run the program the Math is still off despite me getting help and doing all suggestions that users gave me on here. I compared it with the formula and it's correct.
When i put in a negative loan amount i get this error
"Exception in thread "main" java.lang.NullPointerException"
at line 65(which would be

 while (choice.equals("y"));

after asking if I would like to calculate again.
I've restarted a couple times on the project but I keep running into the same problems.
Can anyone help me?

https://scontent-b-iad.xx.fbcdn.net/...95&oe=526636AF <--- formula that I am using.

for the math I tried breaking it down but still doesn't calculate correctly.

double calc1 = loan * rate;
        double calc2 = Math.pow(1 + rate, months);
        double calc3 = Math.pow(1 + rate, months)-1;
        double calc4 = calc1 / calc2;
        monthlypay1 = calc4 / calc3;