Question

I’m trying to code a loan calculator. I seem to be having issues. I am trying to get an input from the user and validate the input. I know I am doing it wrong the problem is I’m scratching my head wondering how to do it right.
I get a red line on the d = getDouble(sc, prompt); and the i = getInt(sc, prompt); which I understand I don’t have that coded correctly. I’m just unsure how to go about fixing it.

I also have to validate the continue statement which I wasn’t to sure the best way to go about that and finally the instructor expects the code to be 80 lines or less which I am right about 80 lines. I guess I’m looking for a better way to do this but being new I’m scratching my head and I’m hoping someone can lend a hand.

As always I really appreciate the help.

   import java.util.Scanner;
   import java.text.NumberFormat;

    public class LoanCalculator
    {   
        public static double getDoubleWithinRange(Scanner sc, String prompt, double min, double max)
        {
          double d = 0.0;
          boolean isValid = false;
          while(isValid == false);
           {
                d = getDouble(sc, prompt);
                if (d <= min)
                   {
                   System.out.println("Error! Number must be greater tha 0.0");
                   }
                   else if (d >= max)
                  {
                     System.out.println("Error number must be less than 1000000.0");
                }
                else 
                    isValid = true;
            }
            return d;       
        }
            public static int getIntWithinRange(Scanner sc, String prompt, int min, int max)
        {
            int i = 0;
            boolean isvalid = false;
            while(isvalid == false)
             {
                i = getInt(sc, prompt);
                if (i <= min)
                       System.out.println("Error! Number must be more than 0");
                else if (i >= max)
                    System.out.println("Error! Number must be less than 100");
                else 
                    isvalid = true;
            }   
        }
          public static void main(String[] args)
         {
            System.out.println("Welcome to the loan calculator");
            Scanner sc = new Scanner(System.in);
            String choice = "y";
            while (choice.equalsIgnoreCase("y"))
            {
                System.out.println("DATA ENTRY");
                double loanAmount = getDoubleWithinRange(sc, "Enter loan amount: ", 0.0, 1000000.0);
                double interestRate = getDoubleWithinRange(sc, "Enter yearly interest rate: ", 0, 20);
                int years = getIntWithinRange(sc, "Enter number of years: ", 0, 100);
                int months = years * 12;

                double monthlyPayment = loanAmount * interestRate/
                        (1 - 1/Math.pow(1 + interestRate, months));

                NumberFormat currency = NumberFormat.getCurrencyInstance();
                NumberFormat percent = NumberFormat.getPercentInstance();
                percent.setMaximumFractionDigits(3);
                System.out.println("RESULST");
                System.out.println("Loan Amount" + currency.format(loanAmount));
                System.out.println("Yearly interest rate: " + percent.format(interestRate));
                System.out.println("Number of years: " + years);
                System.out.println("Monthly payment: " + currency.format(monthlyPayment));

                System.out.println();
                System.out.println("Continue? (y/n): ");
                choice =sc.next();
                System.out.println();

            }           
        }
    }

Answer 1

I think getDouble and getInt are string functions so you would have to get a string first then call those methods. However, since you have a scanner, I assume you want to use that with the nextXXX methods:

Scanner sc = new Scanner (System.in);
double d = sc.nextDouble();

You can use this complete snippet for educational purposes:

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

        System.out.print("Enter your double: ");
        double d = sc.nextDouble();

        System.out.print("Enter your integer: ");
        int i = sc.nextInt();

        System.out.println("You entered:  " + d + " and " + i);
    }
}

Transcript:

Enter your double: 3.14159
Enter your integer: 42
You entered:  3.14159 and 42

Basically, the process is:

• Instantiate a scanner, using the standard input stream.
• Use print for your prompts.
• Use the scanner nextXXX methods for getting the input values.

---

A little more assistance here, based on your comments.

In your main function, you have:

double loanAmount = getDoubleWithinRange(sc, "Enter loan amount: ", 0.0, 1000000.0)

and that function has the prototype:

public static double getDoubleWithinRange(
    Scanner sc, String prompt, double min, double max)

That means those variables in the prototype will be set to the values from the call. So, to prompt for the information, you could use something like (and this is to replace the d = getDouble(sc, prompt); line):

System.out.print(prompt);
double d = sc.nextDouble();

And there you have it, you’ve prompted the user and input the double from them. The first line prints out the prompt, the second uses the scanner to get the input from the user.

As an aside, your checks for the minimum and maximum are good but your error messages have hard-coded values of 0 and 100K. I would suggest that you use the parameters to tailor these messages, such as changing:

System.out.println("Error! Number must be greater tha 0.0");

into:

System.out.println("Error! Number must be greater than " + min);

That way, if min or max change in future , your users won’t get confused 🙂

---

I’ll leave it up to you to do a similar thing for the integer input. It is your homework, after all 🙂