Number Input Validation

Question

I'm having a lot of trouble validating that only a number within the specified range can be taken as input in this program that calculates payments on a loan. The loops repeat and prompt for correct input IF the values inputted are numbers. I need the program to account for Strings and characters, so that if a string or char is entered in the loan amount, year amount, or interest rate amount, the user is prompted again until a number with no chars or strings present is input. I tried everything; I've changed all of my methods to strings and tried to parse the strings into integers, a waste of about 2 hours; I've tried using exception handling, I cannot get it to work right and there's too much overhead for that anyway. If there was just a static method such as NotANumber (NaN) for primitive types so that I could validate at the same time as I take the <variable>.nextDouble, I wouldn't have a problem. Any help please?
public class Payment
    private double la;
    private double mir;
    private int yrs;
    private double monthlyPayment;
    Scanner inp = new Scanner(System.in);
    public Payment()     //no argument constructor
        la =  0;
        mir = 0;
        yrs = 0;
    public static double monthlyPayment(double loanAmount, double monthlyInterestRate, int years)
        int months = 12;// *= years;
        double temp = monthlyInterestRate;
        months *= years;
        monthlyInterestRate = monthlyInterestRate * .01;
        monthlyInterestRate /= 12;
        double monthlyPayment = loanAmount * monthlyInterestRate / (1 - 1/Math.pow(1 + monthlyInterestRate, months));
        System.out.println("Loan amount: " + loanAmount);
        System.out.println("Yearly interest rate: " + temp);
        System.out.println("Years: " + years);
        System.out.println("Monthly payment: " + monthlyPayment);
        return 1;
    public void input()
        char c = ' ';
        String s = "";
        double loan;
        double monthlyInterestRate;
        int years;
        Scanner in = new Scanner(System.in);
        do
            do
                System.out.println("Enter loan amount: ");
                loan = in.nextDouble();
            while(loanAmountFalse(loan));
            setLoanAmount(loan);
            do
                System.out.print("Enter monthly interest rate: ");
                monthlyInterestRate = in.nextDouble();
            while(monthlyInterestRateFalse(monthlyInterestRate));
            setMonthlyInterestRate(monthlyInterestRate);
            do
                System.out.print("Enter years: ");
                years = in.nextInt();
            while(yearsFalse(years));
            setYears(years);
            monthlyPayment(loan, monthlyInterestRate, years);
        while(continueOK());       
    private boolean continueOK()
        boolean cont = true;
        String s = "";
        char c = ' ';
        Scanner in = new Scanner(System.in);
        do                                   //allows prompt to repeat if 'y' or 'n' is not entered
            System.out.print("Enter 'y' to continue or 'n' to end program:");
            s = in.next();
            s = s.toLowerCase();            //input is not case sensitive
            c = s.charAt(0);
            if(c == 'n')
                System.out.println("System exiting...");
                System.exit(0);
        while(c != 'y');
        return true;
    private boolean loanAmountFalse(double loanAmount)
        if(loanAmount < 0)
            System.out.println("Loan amount UNDER 0... error...");
            return true;
        else if(loanAmount > 1000000)
            System.out.println("loan amount OVER 1000000...");
            return true;
        else
            System.out.println("loanAmOK method reached...");
            return false;
    private boolean monthlyInterestRateFalse(double monthlyInterestRate)
        if(monthlyInterestRate < 0)
            System.out.println("ERROR: Interest rate must be over 0...");
            return true;
        else if(monthlyInterestRate > 20)
            System.out.println("Error: Interest rate must be under 20...");
            return true;
        else
            System.out.println("good to go...");
            return false;
    private boolean yearsFalse(int years)
        if(years < 0)
            System.out.println("ERROR: years must be over 0...");
            return true;
        else if(years > 100)
            System.out.println("ERROR: years must be under 100...");
            return true;
        else
            System.out.println("Years ok");
            return false;
    public void setLoanAmount(double loanAmount)
        if(loanAmountFalse(loanAmount))
            System.out.print("Error: loan amount must be greater than 0 and less than $1,000,000.");
        else
            this.la = loanAmount;
    public void setMonthlyInterestRate(double monthlyInterestRate)
        if(monthlyInterestRateFalse(monthlyInterestRate))
            System.out.println("Error: interest rate must be greater than 0 and less than 20.");
        else
            this.mir = monthlyInterestRate;
    public void setYears(int years)
        if(yearsFalse(years))
            System.out.println("Error: years must be greater than 0 and less than 100.");
        else
            this.yrs = years;
}

Answer 1

flounder wrote:
Or wrap a try statement around the call to nextDouble.Or use Scanner.hasNextDouble() to determine if the next token can successfully be parsed as a double.