Repeating values

Question

I have a project that has to ask the user for a credit balance and a rate, and then output how much you would have to pay for the next 12 months if you payed the mininum payment(calculated by: balance * 0.03, and if thats lower than 10 then the mininum payment is $10). I am cannot figure out why "balance * 0.03" will not increase in value of "minPay" as it loops.

import java.util.Scanner;
public class Project1Term2 
{
public static void main(String[] args) 
{
    Scanner in = new Scanner(System.in);

    System.out.println();
    System.out.print("Enter credit card balance: ");
    double balance = in.nextDouble();
    System.out.print("Enter interest rate: ");
    double interest = in.nextDouble();

    interest = interest / 100;

    for(int i = 0; i <= 12; i++)
    {
        Calculate out = new Calculate(balance, interest);
        System.out.println(out.getNewValue());
        balance = out.getNewValue();
    }   
}  
}

class Calculate
{
private final double rate;
private double balance;
private double minPay;
private double newBalance;

Calculate(double balance, double rate)
{
    this.balance = balance;
    this.rate = rate;
}

double getNewValue()
{
    if((balance * 0.03) < 10)
    {
        minPay = 10;
    }
    else
    {
        minPay = balance * 0.03;
    }
    newBalance = (balance - minPay) + (balance * rate);

    return newBalance;
}
}

Answer 1

You are creating a new instance of Calculate on every loop. That´s wrong on many ways. One consequence is that you´re never incrementing minPay , as each instance is used only once.

Answer 2

If you are simply looking for an answer as to why your balance wasn't decreasing, the answer is that your logic was incorrect.

newBalance = (balance - minPay) + (balance * rate);

Should be:

newBalance = balance + (balance * rate) - minPay ;

You want to add the interest to the current balance, and then subtract whatever the minimum payment is. Running your program with that logic gave me:

Enter credit card balance: 100
Enter interest rate: .1
90.1
80.1901
70.2702901
60.340560390099995
50.4009009504901
40.45130185144059
30.49175315329203
20.522244906445323
10.542767151351768
0.5533099185031194
-9.446136771578377
-19.455582908349953
-29.475038491258303

You could also work on some better coding practices. For example, you shouldn't instantiate a new instance of the class every loop. Instead, continue to use the same class instance. You should also check to see if the balance is 0, for example the output I got from running your program allowed me to pay into the negatives.

Hope this helps, feel free to ask questions if it doesn't make sense.