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How to find out which variable is throwing an exception?

I'm writing a program that culculates tip and total from bill and tip rate.
public void takeUserInput() {
Scanner sc = new Scanner(System.in);
double billAmount;
int tipRate;
try {
System.out.print("What is the bill? ");
billAmount = sc.nextDouble();
System.out.print("What is the tip percentage? ");
tipRate = sc.nextInt();
tc.calculate(billAmount, tipRate);
} catch (InputMismatchException e1) {
String errorMessage = "Please enter a valid number for the ";
// errorMessage += billAmount or
// errorMessage += tipRate ?
}
I'm looking for a way to find out which variable throws InputMismatchException, so I can add which variable name into variable errorMessage and print to the screen.

There are various simple ways to get there:
Call hasNextXxx() prior calling nextXxx().
If you go for one try/catch block per input, it is very clear within your catch block which variable caused the problem (you could then call a generic method with a specific error message to avoid code duplication)
You could use reference types for your variables; if you use Double / Integer instead of double / int ... you could check which of the two variables is still null
You put in a little boolean variable, like billAmountIsValid. Initially that variable is false, you turn it to true after the call to nextDouble(). Then you can easily check in your try block whether you got a valid billAmount.
After some more thinking: you really want a combination of 1 + 2: you see; when the users enters a correct billAmount; why would you want to forget about that value when the second value gives a bad second value? No - you should be looping for each variable, until you receive a valid input. And only then you start asking for the next value!

The variable isn't throwing the exception, the evaluation of the right hand side of the variable assignment is, and so there is no information in the exception to say which variable it was about to assign that to had it succeeded.
What you could consider instead is a new method that encompasses the prompting messages and retries:
billAmount = doubleFromUser(sc, "What is the bill? ", "bill");
Where doubleFromUser is:
static double doubleFromUser(Scanner sc, String prompt, String description){
while(true) { //until there is a successful input
try {
System.out.print(prompt); //move to before the loop if you do not want this repeated
return sc.nextDouble();
} catch (InputMismatchException e1) {
System.out.println("Please enter a valid number for the " + description);
}
}
}
You will need a different one for int and double, but if you have more prompts, you will save in the long run.

Guessing Game Help :)

Ok, so my computer teacher has asked us to make a simple game that asks the user to guess a radomly generated number, but I want to take it one step further and make it so that it display error messages when the user tries certain things. The problem here is that I am new to booleans and well, I am having a bit of trouble using java.util.Scanner and booleans. So, if anyone could take a quick look at this I would appreciate it.
import java.util.Scanner;
import java.util.Random;
public class MoreGuessing{
//Instantiation
Scanner reader = new Scanner(System.in);
Random number = new Random();
//Variables
int randomnumber = number.nextInt(10) + 1;
int cntr = 1;
static String decimalguessed;
String error1 = "Error001: Decimal found, please enter a whole number between 1-10." + "\n" + "Program terminated......";//Decimal portion error.
String error2 = "Please enter a positive number." + "\n" + "Program terminated......"; //Negative number error.
String error3 = "Unknown character entered." + "\n" + "Program terminated......"; //Unknown character error.
//Verifier
public static boolean verifyLetters() {
if (decimalguessed.matches("[a-zA-Z]+")){
return true;
}else{
return false;
}
}
public static void main(String [] args){
//Input and display
System.out.print("Please enter a whole number between 1-10: ");
decimalguessed = reader.nextLine();
//Process and Errors
while (decimalguessed != randomnumber) {
if (verifyLetters() != false){
System.out.println(error3);
System.exit(1);}
if (decimalguessed % 1 != 0) {
System.out.println(error1);
System.exit(1);}
if (decimalguessed < 0) {
System.out.println(error2);
System.exit(1);}
if (randomnumber != decimalguessed){
System.out.println("You've lost, please make another attempt.");}
System.out.print("Please enter a whole number between 1-10: ");
decimalguessed = reader.nextDouble();
cntr++;
}
if (cntr == 1) {System.out.println("Congratulations! You've guessed the number on your first attempt!");;
}
else {System.out.println("Congratulations! You've guessed the number, it took you " + cntr + " tries");}
}
}

You need to parse your input. decimalguessed is a string, and so you can't do comparisons like decimalguessed % 1.
You can convert it to an integer like this:
int guess = 0;
try {
guess = Integer.parseInt(decimalguessed);
} catch (NumberFormatException e) {
System.out.println("Your guess was not an integer: " + e.getMessage());
System.exit(1);
}
This will handle both cases where decimalguessed contains letters, and where it contains decimal points/fractions. decimalguessed is still a string, but guess now contains the integer version of it, so you can compare it to randomnumber properly. (Your loop would have never exited before, because a string is never == an integer)
Some other notes:
You should never have:
if (condition) {
return true;
} else {
return false;
}
This can always be simply replaced with
return condition;

It feels like you're very new to this. Welcome to programming!
So first, in Java generally you're not going to have all of that instantiation and variables stuff outside of your main function, unless you're going to make everything static. I would move all of that into your main function, un-static the decimalguessed variable and setup your verifyLetters function to take an argument of String decimalguessed. It may also be wise to check if the value is a number, rather than seeing if it is not a letter. There a lot of non-number, non-letter characters.
Once you've figured out that the guess is a number, you need to tell java it is one (cast it) to a decimal, then do you further comparisons against that decimal.
Darth Android also makes some good points, especially about booleans. You should never have the only result of an if/else be to return a boolean, just return the boolean. Also avoid comparisons to true/false, just do the if on the function/variable alone, or negate it with an '!' to check for false.
Good luck!

How to convert JOptionPane.showInputDialog to int to use in programme?

So basically have an Array and am using the Joption input system so the user has to input a int between 1 and 3. Basically how do I use this int I to divide the number of objects in my array? Also not necessary but it would help if you could advise me how do I stop people inputting a number less than 1 and greater than 5.
Rain[] drops = new Rain [3000]; // WANT TO DIVIDE INT INTO ARRAY
import javax.swing.*;
void setup() {
noCursor();
JOptionPane.showInputDialog( "Please enter a number between one and three","2");
}
So can I use the output from this to adjust the number of objects created by my array ?

Java cannot convert between strings and number by itself, you have to use specific functions, just use:
int ans = Integer.parseInt(JOptionPane.showInputDialog(...))
For your other question you may wanna do something like the following:
boolean inputAccepted = false;
while(!inputAccepted) {
try {
int answer = Integer.parseInt(JOption....
// do some other validation checks
if (answer < 1 || answer > 3) {
// tell them it's still a bad number
} else {
// a good value
inputAccepted = true;
}
} catch(NumberFormatException e) {
// input is bad. popup something
// some communication
// saying what you expect the user to enter.
}
... do stuff with good input value

Since you tagged this with processing, I'll assume you're using the Processing language.
Processing has an int() function that converts from a String to an int.
float f = 65.0;
int i = int(f);
println(f + " : " + i); // Prints "65.0 : 65"
char c = 'E';
i = int(c);
println(c + " : " + i); // Prints "E : 69"
More info on this and related functions can be found in the Processing reference.

You could try this
String ans_s = JOptionPane.showInputDialog( "Please enter a number between one and three","2");
int ans = Integer.parseInt(ans_s);
Please wrap this in try{ }catch(NumberFormatException nfe){}

Get user to input integers

I want to make a program which keeps prompting the user to input integers(from CUI) until it receives a 'X' or 'x' from the user.
The program then prints out the maximum number, minimum number and average value of the input numbers.
I did manage to get the user to input numbers until someone types 'X', but I can't seem to get it to stop if someone types 'x' and the second bit.
This is the code that I have managed to work out:
Scanner in = new Scanner(System.in);
System.out.println("Enter a number")
while(!in.hasNext("X") && !in.hasNext("x"))
s = in.next().charAt(0);
System.out.println("This is the end of the numbers");
Any hints on how I proceed further?

You will need to do something like this:
Scanner in = new Scanner(System.in);
System.out.println("Enter a number")
while(!(in.hasNext("X") || in.hasNext("x")))
s = in.next().charAt(0);
System.out.println("This is the end of the numbers");
Whenever you use while loop you have to use the {} in case the arguments in the while block are more than 1 line, but if they are just of a line then you can just go on without using the {}.
But the problem, you had I suppose is the use of && instead of ||. What the && (AND) operator does is execute if both the statements are true but a || (OR) Operator works if any of the conditions are true.
If you say while(!in.hasNext("X") && !in.hasNext("x")) it makes no sense as the user input is not both at the same time, but instead if you usewhile(!in.hasNext("X") || !in.hasNext("x"))` it makes sense. Understood?
And about sorry, im really new at this. but ive added the code No problem, you need not say sorry but there are a few things to keep in mind before asking a question. You must read this https://stackoverflow.com/help/how-to-ask and yeah one more thing, you should use proper English Grammar while framing your question.
Last of all, about how to calculate the average..., for that what you need to do is store all the input variables into an array and then take out the mean of that or alternatively you could think about it and code something up yourself. Like to take out mean, you could make a variable sum and then keep adding the integers the user enters and also keep a variable count which will keep the count of the number of integers entered and then at last you could divide both of them to have your answer
Update: For checking the minimum and the maximum, what you can do is make 2 new variables like int min=0, max=0; and when the user enters a new variable you can check
//Note you have to change the "userinput" to the actual user input
if(min>userinput){
min=userinput;
}
and
if(max<userinput){
max=userinput;
}
Note: At stackoverflow we are there to help you out with the problems you are facing BUT you cannot exploit this. You cannot just post your homework here. But if you are trying to code something up and are stuck at it and cannot find a answer at google/stackoverflow then you can ask a new question and in that you need to tell what all you have already tried. Welcome to SO! :D Hope you have a nice time here

This would fit your needs:
public void readNumbers() {
// The list of numbers that we read
List<Integer> numbers = new ArrayList<>();
// The scanner for the systems standard input stream
Scanner scanner = new Scanner(System.in);
// As long as there a tokens...
while (scanner.hasNext()) {
if (scanner.hasNextInt()) { // ...check if the next token is an integer
// Get the token converted to an integer and store it in the list
numbers.add(scanner.nextInt());
} else if (scanner.hasNext("X") || scanner.hasNext("x")) { // ...check if 'X' or 'x' has been entered
break; // Leave the loop
}
}
// Close the scanner to avoid resource leaks
scanner.close();
// If the list has no elements we can return
if (numbers.isEmpty()) {
System.out.println("No numbers were entered.");
return;
}
// The following is only executed if the list is not empty/
// Sort the list ascending
Collections.sort(numbers);
// Calculate the average
double average = 0;
for (int num : numbers) {
average += num;
}
average /= numbers.size();
// Print the first number
System.out.println("Minimum number: " + numbers.get(0));
// Print the last number
System.out.println("Maximum number: " + numbers.get(numbers.size() - 1));
// Print the average
System.out.println("Average: " + average);
}

Java Scanner keep asking for input until variable is a double or int, or empty, at which point is given default value

here is the pseudo for what im asking
1. Take value
2. is value double or int?
3. if so, continue in program
4. else
5. is value empty?
6. if value empty; value=0.08
7. else
8. at this stage, value is not an empty valid, or a valid double or valid int
9. user did it wrong, prompt error
10. jump to step one take value
So to me this is pretty complicated, im new to this.
Ive been trying to impliment it like so;
while ( costscan.hasNext() )
{
double costperkm = costscan.nextDouble();
if (costperkm=double){
System.out.println("Value is double");
System.out.println("FUEL COST SAVED ");
}
else {
if(costperkm=null){
costperkm=0.08;
}
else{
}
System.out.println("FUEL COST SAVED ");
}
System.out.print("\n");
System.out.print("\n");
}
My code above is the result of just playing about so at this stage it may not even make sense anymore. Hope someone can help, thanks.

The problem with hasNextDouble and nextDouble is that as long as the user just presses enter, they will both keep asking for input.
If you want to use a default value when the user simply presses enter, you should rather use Scanner.nextLine combined with Double.parseDouble, since nextLine is the only next-method that accepts empty input.
Here's a possible solution:
String input;
while(true) {
input = costscan.nextLine();
if(input.isEmpty()) {
input = "0.08";
break;
}
if(isParseable(input)) {
break;
}
System.out.println("ENTER ONLY NUMBERS [DEFAULT 0.08]");
}
double costperkm = Double.parseDouble(input);
The method isParseable looks like this:
private static boolean isParseable(String str) {
try {
Double.parseDouble(str);
return true;
} catch(NumberFormatException e) {
return false;
}
}