Jeremy B Jeremy B - 1 month ago 8
Java Question

How to validate a string that the user entered? (Java)

I have edited this question but at the bottom of the program I have tried to code a switch statement to validate some code. I'm trying to ask the user to enter Y/N to continue. If they enter Y the program should restart. If they enter N the program should end. If they enter Z they should get an error message and be prompted again with the continue y/n?
Right now if I typed a random letter I get sent back to the data entry section and I'm not sure why.

Also I noticed my calculations are off. I'm assuming it has something to do with the fact I'm not using the bigdecimal class. Currently if I typed 5.6 for the percent it automatically assumes it's 560%

I'm new to Java and unfortunately struggling. Someone was kind enough to help me last night without blatantly giving me the answers which I appreciated because it made me hash things out on my own.

Any help is definately appreciated since I'm having a hard time with this class.
THank you!

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

public class LoanCalculator
{


public static double getDoubleWithinRange(Scanner sc, String prompt,
double min, double max) //validate double
{
double d = 0;
boolean isValid = false;
while(isValid == false)
{
System.out.println(prompt); //print prompt for double
if (sc.hasNextDouble())
{
d = sc.nextDouble();

if (d <= min) //if input <= minimum double
{
System.out.println("Error! Number must be greater than " + min);
System.out.println();//print error grater than min double
}
else if (d >= max) // if input >= maximum double value
{
System.out.println("Error number must be less than " + max);
System.out.println();//print error less than max double value
}

else
isValid = true; //else double is valid
}
else
{
System.out.println("Error! Invalid decimal value.");
System.out.println();
sc.nextLine();
}
}

return d; //return double
}
//validate integer
public static int getIntWithinRange(Scanner sc, String prompt, int min, int max)
{
int i = 0;
boolean isValid = false;
while(isValid == false)
{
System.out.println(prompt); //print prompt for integer
if (sc.hasNextInt())
{
i = sc.nextInt();

if (i <= min) //if input <= minimum integer
{
System.out.println("Error! Number must be greater than " + min);
System.out.println();//print error grater than min integer
}
else if (i >= max) // if input >= maximum integer value
{
System.out.println("Error number must be less than " + max);
System.out.println();//print error less than max integer value
}

else
isValid = true; //else integer is valid
}
else
{
System.out.println("Error! Invalid integer value.");
System.out.println();
sc.nextLine();
}
}

return i; //return integer
}

public static void main(String[] args)
{
System.out.println("Welcome to the loan calculator"); //welcome user to loan calculator
Scanner sc = new Scanner(System.in); // create new scanner
String choice = "y"; //start choice loop
while (choice.equalsIgnoreCase("y"))
{
System.out.println();
System.out.println("DATA ENTRY");
double loanAmount = getDoubleWithinRange(sc, "Enter loan amount: ",
0.0, 1000000.0); //get user input for loanAmount
//get user input for interestRate
double interestRate = getDoubleWithinRange(sc,
"Enter yearly interest rate: ", 0, 20);
//get user input for years
int years = getIntWithinRange(sc, "Enter number of years: ", 0, 100);
int months = years * 12; // calculate years to months
//calculate monthly payment
double monthlyPayment = loanAmount * interestRate/
(1 - 1/Math.pow(1 + interestRate, months));
//import currency instance
NumberFormat currency = NumberFormat.getCurrencyInstance();
// import percent instance
NumberFormat percent = NumberFormat.getPercentInstance();
percent.setMinimumFractionDigits(1); //set fraction digits for percent
System.out.println("RESULST"); //print results
//print loanAmount
System.out.println("Loan Amount: " + currency.format(loanAmount));
//print interestRate
System.out.println("Yearly interest rate: " + percent.format(interestRate));
System.out.println("Number of years: " + years); //print years
//print monthlyPayment
System.out.println("Monthly payment: " + currency.format(monthlyPayment));

boolean quit = false;
do{

System.out.println();
System.out.println("Continue? (y/n): "); //prompt user to continue
String userinput1 = sc.next();


char choice1 = userinput1.toLowerCase().charAt(0);
switch(choice1){
case 'y':

break;
case 'n':
// case n, do something here
quit = true;
break;
case ' ':
System.out.println("Error! This entry is required. Try again.");
break;
default:
System.out.println("Error! Entry must be 'y or 'n''");
break;
}
}while (!quit);

}

}
}

Answer

Use a switch like this:

boolean quit = false;
do{ // change your while by this

    // your own stuff here


    // then after all your program stuff
    boolean choiceIsOK = false;
    do{
    String userinput = sc.next();
    char choice = userinput.toLowerCase().charAt(0);
    switch(choice){
    case 'y':
        // case y, do nothing, you could even remove that case.
        choiceIsOK = true;
        break;
    case 'n':
        // case n, do something here
        choiceIsOK = false;
        quit = true;
        break;
    default:
        // error or warning
        System.out.println("Type Y or N to respectively continue or quit");
        break;
    }
    }while(!choiceIsOK);
}while (!quit);

In your code, it will look like this:

public static void main(String[] args)
{
System.out.println("Welcome to the loan calculator"); //welcome user to loan calculator
Scanner sc = new Scanner(System.in); // create new scanner
//String choice = "y"; //start choice loop [no need for that line anymore]
boolean quit = false; 
do{
        System.out.println();
        System.out.println("DATA ENTRY");
        double loanAmount = getDoubleWithinRange(sc, "Enter loan amount: ", 
                0.0, 1000000.0); //get user input for loanAmount
        //get user input for interestRate
        double interestRate = getDoubleWithinRange(sc, 
                "Enter yearly interest rate: ", 0, 20); 
        //get user input for years
        int years = getIntWithinRange(sc, "Enter number of years: ", 0, 100); 
        int months = years * 12; // calculate years to months
        //calculate monthly payment
        double monthlyPayment = loanAmount * interestRate/
                (1 - 1/Math.pow(1 + interestRate, months)); 
        //import currency instance
        NumberFormat currency = NumberFormat.getCurrencyInstance(); 
        // import percent instance
        NumberFormat percent = NumberFormat.getPercentInstance(); 
        percent.setMinimumFractionDigits(1); //set fraction digits for percent 
        System.out.println("RESULST"); //print results 
        //print loanAmount
        System.out.println("Loan Amount: " + currency.format(loanAmount)); 
        //print interestRate
        System.out.println("Yearly interest rate: " + percent.format(interestRate)); 
        System.out.println("Number of years: " + years); //print years
         //print monthlyPayment
        System.out.println("Monthly payment: " + currency.format(monthlyPayment)); 


        // then after all your program stuff
        boolean choiceIsOK = false;
            do{

                System.out.println();
                System.out.println("Continue? (y/n): "); //prompt user to continue 
                String userinput1 = sc.next();
                    char choice1 = userinput1.toLowerCase().charAt(0);
                    switch(choice1){
                    case 'y':
                        // case y, do nothing, you could even remove that case.
                        choiceIsOK = true;
                        break;
                    case 'n':
                        // case n, do something here
                        choiceIsOK = true; // I fixed this
                        quit = true;
                        break;
                    default:
                        // error or warning
                        System.out.println("Type Y or N to respectively continue or quit");
                        break;
                    }
                }while(!choiceIsOK);

    }while(!quit);

    }           
}
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