我的try-catch代码块做错什么了吗？

Question

So we have to make a mortgage calculation project where we have to ask the user to calculate again and now we have to make it so it prints out a error message every time the user enters a string value for any of the inputs. I thought I did it right but something strange happens every time I run it and I can't figure out why and I know it's something wrong with the Try-Catch blocks.

Here are my outputs: http://imgur.com/cbvwM5v

As you can see the third time i run the program I enter a "two" as the second input and it still did the calculations. Then, the third time I tried it, I entered a negative number then a "two" and everything worked the way I wanted it to. Then, the last time I ran it I put a positive number for the first input and it still did the calculations, anything you guys see that might be doing this? Also, I think I may have used the wrong exception, I'm not uite sure what it means, I just guessed. am I supposed to user NumberFormatException and there is also a line under nfe saying that the value is not being used.

Here's my code:

   package MortgageCalculation2c;

import java.text.NumberFormat;
import java.util.Scanner;
/**
 *
 * @author Akira
 */
public class MortgageCalculation2c {

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
     Scanner in = new Scanner(System.in);


    double loanAmount = 0;
    double interestRate = 0;
    double numberYears = 0;
    double months;
    double monthlyPayment;
    double numerator;
    double denominator;
    double formula;
    boolean userInput = true;
    String answer = ("y");


    while (userInput) {
        try {
            loanAmount = 0;
            interestRate = 0;
            numberYears = 0;

   //prompt user for the loan amount       
    System.out.print("Enter the loan amount: ");
    loanAmount = Double.parseDouble(in.nextLine());

    //prompt the user for the interest rate
    System.out.print("Enter the rate: ");
    interestRate = Double.parseDouble(in.nextLine());

    //prompt the user for  thenumber of years
    System.out.print("Enter the number of years: ");
    numberYears = Double.parseDouble(in.nextLine());

        } catch (NumberFormatException nfe) {
            System.out.println("You must enter positive numerical data!");
        }


    //if the user enters a negative number print out a error message, if not, continue calculations
   if ((loanAmount <= 0) || (interestRate <= 0) || (numberYears <= 0)) {
        System.out.println("ALL NUMERICAL VALUES MUST BE POSITIVE!");

    } else {
        //convert the interest rate
       interestRate = interestRate / 100 / 12;

       //the number of years must be converted to months
       months = numberYears * 12;

       //numerator of the monthly payment formula
       numerator = (Math.pow(1 + interestRate, months));

       //denominator of the monthly payment formula
       denominator = ((numerator)-1);

       //the formula equals the numerator divided by the denominator
       formula = ( numerator / denominator );

       //monthly payment calculation
        monthlyPayment = (interestRate * loanAmount * formula);

         //sytem output
        NumberFormat defaultFormat = NumberFormat.getCurrencyInstance();
        System.out.println("The monthly payment is: " + defaultFormat.format(monthlyPayment));

   }

        //prompt the user if they would like to calculate the program again.
        System.out.println("Would you like to calculate again (y/n) : ");

        //if the user enters "y" the program will run again and if the user enters anything else it ends
            answer = in.nextLine();
        answer = answer.toLowerCase();
            if ( answer.equals("y")){
                userInput = true;  //tests the program if it needs to run again
            }else{
                break;  //ends the program
            }

}

}
}

Is there anything that you guys can see that might be the problem?

Answer 1

It seems you need continue in your catch block, so the program flow goes back to the loop.

    ...
    } catch (NumberFormatException nfe) {
        System.out.println("You must enter positive numerical data!");
        continue;   // <---------- here
    }
    ...

If this is not a specific exercise in try-catch construct, it be would be better to use Scanner 's method hasNextDouble() for validatiing and nextDouble for reading and converting the numbers.

It will look something like this:

//prompt user for the loan amount       
loanAmount = enterPositiveDouble("Enter the loan amount: ")

//prompt the user for the interest rate
interestRate = enterPositiveDouble("Enter the rate: ");

//prompt the user for  the number of years
numberYears = enterPositiveDouble("Enter the number of years: ");

and you will have a special static method enterPositiveDouble like following:

static void  enterPositiveDouble(String prompt) {
   Scanner in = new Scanner(System.in);
   boolean ok = true;
   double result = -1; 
   do {
      System.out.print(prompt);
      ok = (in.HasNextDouble() && (result = in.NextDouble()) > 0)
      if ! ok
         System.out.println("You must enter positive numerical data!");
   } while (!ok); 
}

The above is not an optimal code but just the illustration of a possible solution.