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InputMismatchException in Java replacing while loop

In my task I need to put InputMismatchException when user tries to enter some values. User get some numbers from 1 to some number (simptoms.lenght).
int number=0;
do{
System.out.printf("Choose %d simptoms: \n", number+1);
for(int j=0; j< simptomi.length;j++){
System.out.printf("%d. %s %s\n", j + 1, simptoms[j].getName(),
simptoms[j].getValue());
}
System.out.print("Choose: ");
while(!scanner.hasNextInt()){
System.out.println("Please enter number!");
scanner.next();
}
number=scanner.nextInt();
scanner.nextLine();
if(number<0 || number> simptoms.length){
System.out.println("Error, choose again");
}
}while(number<0 || number> simptoms.length);
After this code I tried to do this:
instead of while(!scanner.hasNextInt()) I tried with try and I get this message:
Declaration, final or effectively final variable expected.
Is this the right way of replacing while loop or I should try to add something else.
I'm thinking about boolean = false and somehow try with that but I don't understand how to implement it properly.
I tried this:
try{
number=scanner.nextInt();
scanner.nextLine();
}
catch (InputMismatchException ex){
System.out.println("Please, enter number!");
}

Try it out! hope it helps!
try {
do {
number = scanner.nextInt();
if (!Character.isAlphabetic(number)) {
if (number > simptoms.length) {
System.out.println("Error, choose again");
System.out.println("Please enter number!");
}
}
} while (number != -1);
} catch (InputMismatchException e) {
System.out.println("Enter number");
}

Why does my try/catch block keep looping?

When I run this, the code skips over input.nextInt(); and goes in circles:
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
// Link user to programs (ToC)
int loop = 1;
do {
try {
System.out.println("Please choose a number: ");
System.out.println("0. Exit");
System.out.println("1. Calculator");
int numChoice = input.nextInt();
if (numChoice == 0) {
System.exit(0);
} else if (numChoice == 1) {
System.out.println("Going to Calculator...");
new Calculator();
} else {
System.out.println("Not a valid choice.");
}
}
catch (Exception e) {
System.out.println("Please input a number!");
}
} while (loop == 1);
}
It seems to be skipping int numChoice for whatever reason. Also, please don't be too technical. I just code for my leisure.

Your loop goes in circles for two reasons:
You try for nextInt, but you do not clear out the input buffer on failure, and
Even if you did clear input in the catch, your loop would still go on, because there are no assignments of the loop variable which is supposed to stop your loop.

Endless loop with try and catch JAVA

so what I am trying to do is have the user input a valid coordinate in a matrix, that is an INT which is greater than -1,
Scanner scanner = new Scanner (System.in);
int coordinates[] = new int[2];
coordinates[0]=-1;
coordinates[1]=-1;
boolean check = true;
while (((coordinates[0]<0)||(coordinates[0]>R)) && check) {
System.out.print("Please enter a valid row number:\t");
try {
coordinates[0]=scanner.nextInt();
break;
}
catch (InputMismatchException e) {
}
}
while (((coordinates[1]<0)||(coordinates[1]>C)) && check) {
System.out.print("Please enter a valid col number:\t");
try {
coordinates[1]=scanner.nextInt();
break;
}
catch (InputMismatchException e) {
}
}
the problem is that it loops endlessly after entering a not valid input
int R is the size of the row
int C is the size of the collumn

Your problem is that you're not handling the error you're catching.
If you'll provide wrong format of number for the nextInt() method, then the InputMismatchException will be thrown. Then because the catch does nothing, the loop will continue (start from begining) and the scanner will read the same incorrect value, and so on...
So instead of this:
catch (InputMismatchException e) {
}
Try this:
catch (InputMismatchException e) {
System.out.println("Wrong number entered.");
scanner.nextLine();
}
This way you'll force scanner to move past the last incorrect input.
EDIT:
You're loop is also broken because you do break after reading the input. In that case if you'll put the negative number you'll break as well and won't check the loop condition. Remove the break statement and it will work as expected:
while (((coordinates[0]<0)||(coordinates[0]>R)) && check) {
System.out.print("Please enter a valid row number:\t");
try {
coordinates[0]=scanner.nextInt();
}
catch (InputMismatchException e) {
System.out.println("That's not a valid number.");
scanner.nextLine();
}
}
EDIT2:
public static void main(final String args[])
{
int maxRowsNumber = 10;
int maxColsNumber = 10;
Scanner scanner = new Scanner (System.in);
int coordinates[] = new int[2];
coordinates[0]=-1;
coordinates[1]=-1;
boolean check = true;
while (((coordinates[0]<0)||(coordinates[0]>maxRowsNumber)) && check) {
System.out.print("Please enter a valid row number:\t");
try {
coordinates[0]=scanner.nextInt();
}
catch (InputMismatchException e) {
System.out.println("That's not a valid number.");
scanner.nextLine();
}
}
while (((coordinates[1]<0)||(coordinates[1]>maxColsNumber)) && check) {
System.out.print("Please enter a valid col number:\t");
try {
coordinates[1]=scanner.nextInt();
}
catch (InputMismatchException e) {
System.out.println("That's not a valid number.");
scanner.nextLine();
}
}
System.out.println("Inserted RowsNumber: " + coordinates[0]);
System.out.println("Inserted RowsNumber: " + coordinates[1]);
}
Output:
Please enter a valid row number: 11
Please enter a valid row number: 22
Please enter a valid row number: 10
Please enter a valid col number: 11
Please enter a valid col number: 2
Inserted RowsNumber: 10
Inserted RowsNumber: 2

If by "not valid input" you mean "not any kind of integer", then your scanner will fail each time it tries to read another integer, so you'll hit your catch, and do nothing to stop the loop. Maybe you intended to set check to false in such circumstances? Or maybe you meant to put the break in each catch?
Using a break when a valid integer is read isn't right, because it might be a negative integer, which your loop guard says you don't want.

This is basically the same as what your doing, I just tried to improve it a little bit by removing hardcoded values, made variables more descriptive, and included input validations.
final int ROW = 0;
final int COL = 1;
int coordinates[] = new int[2];
coordinates[ROW] = -1;
coordinates[COL] = -1;
boolean isInputValid = true;
Scanner scanner = new Scanner(System.in);
do {
try {
System.out.print("Please enter a valid row number:\t");
coordinates[ROW] = Integer.parseInt(scanner.nextLine());
} catch (NumberFormatException nfe) {
isInputValid = false; //if the input is not int
}
} while (!isInputValid && (coordinates[ROW] < 0) //do this until the input is an int
|| (coordinates[ROW] > R)); //and it's also not less than 0 or greater than R
//same logic applies here
do {
try {
System.out.print("Please enter a valid col number:\t");
coordinates[COL] = Integer.parseInt(scanner.nextLine());
} catch (NumberFormatException nfe) {
isInputValid = false;
}
} while (!isInputValid && (coordinates[COL] < 0)
|| (coordinates[COL] > C));
Hope this helps.

Try and Catch User Input causes variables to be uninitialized

*EDIT: Okay after fixing the try catch error I get a problem in the catch {.. when it prints.
*, Basically when I say I want to play again it continues the game as it should but it also prints the first catch and then asks for an input at line 23.
if (decision.equalsIgnoreCase("yes"))
{
ai = (int)(Math.random()*101);
System.out.println("From 0 to 100, what number do you think I have generated?");
tryCatch = true;
loop = true;
rtrn = true;
while (tryCatch == true)
{
while (loop == true)
{
try
{
guess = Integer.parseInt(iConsole.nextLine());
if (guess >= 0)
{
loop = false;
}
}
catch (NumberFormatException e)
{
System.out.println("Invalid input. Please try again.");
}
catch (InputMismatchException e)
{
System.out.println("Invalid input. Please try again!");
}
}
Hi this is my first post so if I get the code formatting on the forum wrong I'll edit it.
Right now I'm coding a game in java eclipse where the cpu generates a number and the user has to guess it. I am using the scanner class for most of this. What I am having trouble doing is creating a try catch to check the user input if it is a valid Integer.
What ends up happening is that the code block below it doesn't recognize the already-initialized variable.
package ics3U;
import java.util.*;
import java.io.*;
public class highLow
{
static public void main (String args[]) throws IOException
{
String name;
String decision;
String decision2;
int ai;
int guess;
int counter = 1;
boolean fullGame = true;
boolean tryCatch = true;
boolean rtrn = true;
Scanner iConsole = new Scanner(System.in);
System.out.println("Hello! Welcome to HiLo!");
System.out.println("What is your full name?");
name = iConsole.nextLine();
System.out.println("Hello " + name + "! Would you like to play?");
decision = iConsole.nextLine();
while (fullGame == true)
{
if (decision.equalsIgnoreCase("yes"))
{
ai = (int)(Math.random()*101);
System.out.println("From 0 to 100, what number do you think I have generated?");
tryCatch = true;
rtrn = true;
while (tryCatch == true)
{
try
{
guess = Integer.parseInt(iConsole.nextLine());
}
catch (Exception e)
{
System.out.println("Invalid input. Please try again.");
}
while (guess != ai)
{
if (guess < ai)
{
System.out.println("Too low!");
guess = iConsole.nextInt();
}
else if (guess > ai)
{
System.out.println("Too high!");
guess = iConsole.nextInt();
}
counter = counter + 1;
}
System.out.println("Correct! You guessed it after " + counter + " tries!");
counter = ((counter - counter)+1);
System.out.println("Would you like to play again?");
while (rtrn == true)
{
decision2 = iConsole.next(); //finally..
if (decision2.equalsIgnoreCase("yes"))
{
fullGame = true;
tryCatch = false;
rtrn = false;
break; //do-while may be needed, have to bypass catch, 'break' works after restating value of tryCatch & rtrn
}
else if (decision2.equalsIgnoreCase("no"))
{
System.out.println("Goodbye.");
fullGame = false;
tryCatch = false;
rtrn = false;
iConsole.close();
}
else
{
System.out.println("Sorry?");
}
}
/*catch (Exception e)
{
System.out.println("Invalid input. Please try again.");
}
catch (NumberFormatException e)
{
System.out.println("Invalid input. Please try again.");
}
//More specific Exceptions, turn this on later
catch (InputMismatchException e)
{
System.out.println("Invalid input. Please try again!");
}*/
}
}
else if (decision.equalsIgnoreCase("no"))
{
System.out.println("Goodbye.");
fullGame = false;
tryCatch = false;
rtrn = false;
iConsole.close();
}
else
{
System.out.println("Sorry?");
decision = iConsole.nextLine();
}
}
}
}

Add a continue statement in your catch block. That way, if the user enters something that's not an integer and parsing fails, it will immediately try again rather than trying to run the rest of the loop.
try
{
guess = Integer.parseInt(iConsole.nextLine());
}
catch (Exception e)
{
System.out.println("Invalid input. Please try again.");
continue; // jump to beginning of loop
}

Try moving all your code after the catch block (in the loop) inside the try block after this line
guess = Integer.parseInt(iConsole.nextLine());
As you currently have it, anytime there is an exception in the parseInt, it will still try to process the unassigned guess instead of restarting the loop.

Since the statements are in a try block there's a chance that they will fail, and your program has a chance of trying to use a non-initialized variable. The solution is to initialize the variables to a default value that makes sense, i.e.,
int guess = -1; // some default value
You should also wrap the while loop around the try/catch block. Don't let the program progress until inputted data is valid.
boolean validGuess = false;
while (!validGuess) {
// prompt user for input here
try {
guess = Integer.parseInt(iConsole.nextLine());
if (/* .... test if guess is valid int */ ) {
validGuess = true;
}
} catch (NumberFormatException e) {
// notify user of bad input, that he should try again
}
}
You could even encapsulate all of this into its own method if you need to do similar things throughout the program.

A try-catch method in while loop?

I have this code, and I want to put the try-catch inside a while loop. The logic would be, "while there is an input error, the program would keep on asking for a correct input". How will I do that? Thanks in advance.
public class Random1 {
public static void main(String[] args) {
int g;
Scanner input = new Scanner(System.in);
Random r = new Random();
int a = r.nextInt(10) + 1;
try {
System.out.print("Enter your guess: ");
g = input.nextInt();
if (g == a) {
System.out.println("**************");
System.out.println("* YOU WON! *");
System.out.println("**************");
System.out.println("Thank you for playing!");
} else if (g != a) {
System.out.println("Sorry, better luck next time!");
}
} catch (InputMismatchException e) {
System.err.println("Not a valid input. Error :" + e.getMessage());
}
}

Here I have used break and continue keyword.
while(true) {
try {
System.out.print("Enter your guess: ");
g = input.nextInt();
if (g == a) {
System.out.println("**************");
System.out.println("* YOU WON! *");
System.out.println("**************");
System.out.println("Thank you for playing!");
} else if (g != a) {
System.out.println("Sorry, better luck next time!");
}
break;
} catch (InputMismatchException e) {
System.err.println("Not a valid input. Error :" + e.getMessage());
continue;
}
}

boolean gotCorrect = false;
while(!gotCorrect){
try{
//your logic
gotCorrect = true;
}catch(Exception e){
continue;
}
}

You can add a break; as the last line in the try block. That way, if any execption is thrown, control skips the break and moves into the catch block. But if not exception is thrown, the program will run down to the break statement which will exit the while loop.
If this is the only condition, then the loop should look like while(true) { ... }.

You could just have a boolean flag that you flip as appropriate.
Pseudo-code below
bool promptUser = true;
while(promptUser)
{
try
{
//Prompt user
//if valid set promptUser = false;
}
catch
{
//Do nothing, the loop will re-occur since promptUser is still true
}
}

In your catch block write 'continue;' :)