While loop in monthly amortization calculator runs after 0 [Java]

Question

I am almost finished with programming a monthly amortization calculator that is based on the user's principal amount, monthly interest and monthly payment. But there is on last problem that I can't seem to figure out. I seemed to have set the limit at 0 but it still shows the first amount if negative money was a thing. Here is the code for a better understanding of what I mean:

import java.util.Scanner;

public class Amortization {

   public static void main(String []args){

       Scanner input = new Scanner(System.in);
       int month = 1;
       int year = 0;

       double balance;
       double rate;
       double payment;
       double principal;
       double calculated_interest;
       double actual_payment;
       double principal_amt;

       System.out.println("What is your principal amount?"); principal = input.nextDouble(); balance = principal;
       System.out.println("What is your monthly interest rate in decimal?"); rate = input.nextDouble();
       System.out.println("What is your monthly payment?"); payment = input.nextDouble();


       while(balance>0){

          if(month == 13){
             year++;
             month = 1;
          }

          calculated_interest = ((int)(Math.round(balance*rate*100)))/100.0;
          principal_amt = ((int)(Math.round((payment-calculated_interest))*100))/100.0;
          actual_payment = ((int)(Math.round((payment-calculated_interest)*100)))/100.0;

          System.out.println("Year " + year + ", " + "Month " + month + ":");
          System.out.println("Your interest amount is " + "$" + calculated_interest);
          System.out.println("Your principal amount " + "$" + principal_amt);
          balance = ((int)(Math.round((balance-actual_payment)*100)))/100.0;

          System.out.println("Your new balance is " + "$" + balance);
          System.out.println();

          month++;
      }
      input.close();
     }
  }

Answer 1

There are few problems with your code.

• Sum of interest and principal payment of every month is not equal to monthly payment.
• "actual_payment" calculated by you is not required.
• You don't need to round your payments to integers.

• Because you are taking monthly payments from user, user will have to pay less than monthly payment in last month and that's what causing the -ve If you want your payments to be equal every month you'll have to calculate monthly payments yourself.

  public static void main(String[] args) {
      Scanner input = new Scanner(System.in);
      int month = 1;
      int year = 0;
  
      double balance;
      double rate;
      double payment;
      double principal;
      double calculated_interest;
      double actual_payment;
      double principal_amt;
  
      System.out.println("What is your principal amount?");
      principal = input.nextDouble();
      balance = principal;
      System.out.println("What is your monthly interest rate in decimal?");
      rate = input.nextDouble();
      System.out.println("What is your monthly payment?");
      payment = input.nextDouble();
  
      while (balance > 0) {
  
          if (month == 13) {
              year++;
              month = 1;
          }
          calculated_interest = (balance * rate * 100) / 100.0;
          principal_amt = payment - calculated_interest;
  
          System.out.println("Year " + year + ", " + "Month " + month + ":");
          System.out.println("Your interest amount is " + "$" + calculated_interest);
  
          if (balance > payment) {
  
              balance = ((int) (Math.round((balance - principal_amt) * 100))) / 100.0;
              System.out.println("Your principal amount " + "$" + principal_amt);
              System.out.println("Your new balance is " + "$" + balance);
          } else {
              System.out.println("Your principal amount " + "$" + (balance - calculated_interest));
              System.out.println("Your new balance is " + "$" + 0);
              System.out.println("You'll only pay $" + (calculated_interest + balance) + " this month.");
              break;
          }
  
          System.out.println();
  
          month++;
      }
      input.close();
  }
  

Answer 2

The problem is when the loop is at Year 29, month 12 the values are

Year 29, Month 12:
Your interest amount is $20.9
Your principal amount $4176.0
Your new balance is $3.45

Now, at this the value of balance is not less than 0, so while will be true and when the new values are calculated the balance (balance = ((int)(Math.round((balance-actual_payment)*100)))/100.0;) becomes negative as balance - atual_payment will return negative because the previous balance is $3.45.

One of the solution, add another if condition in the while loop after computing the balance.

while(balance>0){

      calculated_interest = ((int)(Math.round(balance*rate*100)))/100.0;
      principal_amt = ((int)(Math.round((payment-calculated_interest))*100))/100.0;
      actual_payment = ((int)(Math.round((payment-calculated_interest)*100)))/100.0;
      balance = ((int)(Math.round((balance-actual_payment)*100)))/100.0;

      if (balance < 0){
        break; //this will break out of the loop
      }

      if(month == 13){
         year++;
         month = 1;
      }
      System.out.println("Year " + year + ", " + "Month " + month + ":");
      System.out.println("Your interest amount is " + "$" + calculated_interest);
      System.out.println("Your principal amount " + "$" + principal_amt);

      System.out.println("Your new balance is " + "$" + balance);
      System.out.println();

      month++;
  }

This code will not print out negative balance.