This question already has answers here:
try/catch with InputMismatchException creates infinite loop [duplicate]
(7 answers)
Closed 7 years ago.
I am writing a simple java console game. I use the scanner to read the input from the console. I am trying to verify that it I ask for an integer, I don't get an error if a letter is entered. I tried this:
boolean validResponce = false;
int choice = 0;
while (!validResponce)
{
try
{
choice = stdin.nextInt();
validResponce = true;
}
catch (java.util.InputMismatchException ex)
{
System.out.println("I did not understand what you said. Try again: ");
}
}
but it seems to create an infinite loop, just printing out the catch block. What am I doing wrong.
And yes, I am new to Java
nextInt() won't discard the mismatched output; the program will try to read it over and over again, failing each time. Use the hasNextInt() method to determine whether there's an int available to be read before calling nextInt().
Make sure that when you find something in the InputStream other than an integer you clear it with nextLine() because hasNextInt() also doesn't discard input, it just tests the next token in the input stream.
Try using
boolean isInValidResponse = true;
//then
while(isInValidResponse){
//makes more sense and is less confusing
try{
//let user know you are now asking for a number, don't just leave empty console
System.out.println("Please enter a number: ");
String lineEntered = stdin.nextLine(); //as suggested in accepted answer, it will allow you to exit console waiting for more integers from user
//test if user entered a number in that line
int number=Integer.parseInt(lineEntered);
System.out.println("You entered a number: "+number);
isInValidResponse = false;
}
//it tries to read integer from input, the exceptions should be either NumberFormatException, IOException or just Exception
catch (Exception e){
System.out.println("I did not understand what you said. Try again: ");
}
}
Because of common topic of avoiding negative conditionals https://blog.jetbrains.com/idea/2014/09/the-inspection-connection-issue-2/
Related
This question already has answers here:
How do I compare strings in Java?
(23 answers)
Closed 6 years ago.
I'm doing some basic homework, and it's honestly all stuff I know. But I decided to jazz it up a little, and something I'm missing is causing an unexpected error.
The idea is to use a while loop which asks if the user would like to do something. So long as they say yes, the loop continues the operations that it holds (in this case rounding a decimal to an int). However, as soon as I enter yes, or anything really, the loop breaks there and won't continue on to the rounding portion.
public class DecimalRounder {
//Main class method that starts and ends the program. It is prepared to throw an IO exception if need be.
public static void main(String args[])throws IOException
{
//Main initializes a new reader to take input from System.in
InputStreamReader rawInput = new InputStreamReader(System.in);
//Main then initializes a new buffer to buffer the input from System.in
BufferedReader bufferedInput = new BufferedReader(rawInput);
//Main initializes other used variable
double castInput = 0.0;
String contin = "yes";
//Program then sets up loop to allow user to round numbers till content
while (contin == "yes")
{
//Program asks user if they'd like to round.
System.out.println("Would you like to round? Type yes to continue... ");
contin = bufferedInput.readLine();
//If user says yes, rounding begins. ERROR FOUND HERE?
if (contin == "yes") //INPUT "yes" DOESN'T MATCH?
{
//Program then requests a decimal number
System.out.println("Please enter a decimal number for rounding: ");
String givenLine = bufferedInput.readLine();
//rawInput is worked on using a try catch statement
try {
//givenLine is first parsed from String into double.
castInput = Double.parseDouble(givenLine);
//castInput is then rounded and outputted to the console
System.out.println("Rounded number is... " + Math.round(castInput));
//Branch then ends restarting loop.
}catch(Exception e){
//If the data entered cannot be cast into a double, an error is given
System.err.println("That is not a roundable number: " + e.getMessage());
//And branch ends restarting loop.
}
}
}
System.out.println("Have a nice day!");
}
}
Use .equals instead of == to compare strings in JAVA.
Try this :
contin.equals("yes")
I am making a basic application where it trains your math skills. I have this code:
while (true)
{
try
{
int userAnswer;
System.out.println("Type quit to exit to the menu!");
int randInt = r.nextInt(num2);
System.out.println(num1 + " + " + randInt + " =");
userAnswer = in.nextInt();
if(userAnswer == num1 + randInt) System.out.println("Correct!");
else System.out.println("Wrong!");
break;
}
catch(Exception e)
{
}
}
When someone prints out a d or something in the answer, the try catch goes. But, then it goes to the while loop and repeatedly spams Type quit to exit to the menu and then something like 1 + 2 = infinitely... I think I know what's wrong, userAnswer has been assigned already as something that throws an exception that goes to the catch and it just keeps printing those and goes to the catch and goes back because userAnswer is already assigned. I think this is what is happening, I could be wrong. Please help!
EDIT: I forgot to make this clear, but I want the question to be re-printed again, exiting out of the loop goes to a menu where you can't get the question back, I want it to redo what's in the try catch...
You should never catch an Exception without handling it.
catch(Exception e)
{
System.out.println("An error has occured");
break;
}
This should stop your program from looping infinitely if an Exception occurs.
If user input comes as letter it will get an exception because you are trying to read(parse) as integer. So your catch clause is in the loop you have to write break in there to go out from loop.
Still i will suggest you to getline as string and than compare with your cli commands (quit in your case) than you can try to parse it as an integer and handle loop logic.
You're not breaking the while loop if there is a mismatch
while(true)
{
try
{
}
catch(InputMisMatchException e)//I suggest you to use the exact exception to avoid others being ignored
{
System.out.println("Thank you!");
break;//breaks the while loop
}
}
Yoy're not breaking the loop in case of Exception occurs.
Add break; statement in the catch block to run your program without going to infinite loop, in case exception occurs.
Since the given answers don't match your requirement I'll solve that "riddle" for you.
I guess what you didn't knew is that the scanner won't read the next token if it doesn't match the expectation. So, if you call in.nextInt() and the next token is not a number, then the scanner will throw an InputMismatchException and keeps the reader position where it is. So if you try it again (due to the loop), then it will throw this exception again. To avoid this you have to consume the erroneous token:
catch (Exception e) {
// exception handling
in.next();
}
This will consume the bad token, so in.nextInt() can accept a new token. Also there is no need to add break here.
Mind that in.next() reads only one token, which is delimited by a whitespace. So if the user enters a b c, then your code will throw three exception and therefore generate three different question befor the user can enter a number. You can avoid that by using in.nextLine() instead. But this can lead into another problem: Scanner issue when using nextLine after nextXXX, so pay attention to that :).
This question already has answers here:
try/catch with InputMismatchException creates infinite loop [duplicate]
(7 answers)
Closed 7 years ago.
I am currently working on some Java code in Eclipse and trying to use a try-catch statement inside of a do-while statement. My current code is as follows:
import java.util.Scanner;
import java.util.InputMismatchException;
import java.util.Random;
public class GG_HighLowInvalid{
public static void main(String[] args){
Scanner cg = new Scanner(System.in);
//Assign and define variables
int guess, rand;
guess = 0;
rand = 10;
//Create loop
do
try{
guess = 0;
//Ask for a guess
System.out.print("Enter your guess: ");
//Record the guess
guess = cg.nextInt();
}
catch(InputMismatchException exception){
System.out.println("Your guess must be an integer.");
}
while (guess != rand);
}
}
When I put in any number, the code works fine and will loop to ask for another input and when 10 is entered, the code stops as it is supposed to (because guess becomes equal to rand). However, if I put in anything that is not an integer (such as "No"), an infinite loop occurs where the output prints the following:
"Your guess must be and integer."
"Enter your Guess: Your guess must be an integer."
"Enter your Guess: Your guess must be an integer."
"Enter your Guess: Your guess must be an integer."
repeating forever until the program is externally terminated.
Since the while statement is (guess != rand), why is a non-integer causing this infinite loop? Shouldn't the manual input under the try-statement be called again? Any assistance in understanding this would be greatly appreciated. Also, I am pretty new to Java, so sorry in advance if I am having simple issues.
When a scanner throws an InputMismatchException, the scanner will not
pass the token that caused the exception, so that it may be retrieved
or skipped via some other method.
Currently, your Scanner is not moving ahead to read the next input but reading the same continuously. You have to explicitly call some method which can read this incorrect value which was not expected. For example, scanner.next() call in catch block can avoid this infinite loop.
Use the following code:
catch(InputMismatchException exception){
cg.next();
System.out.println("Your guess must be an integer.");
}
After you have unsuccessfully read buffer its value isn't emptied and next time when it came to cg.nextInt() it tries to read same wrong value, and you went to loop. You need "to empty buffer", so next time it will read correct value.
You dont need to use a try catch statement. You just have to check if it is an integer or not with the hasNextInt() method of your object scanner. This is an example, it will solve your problem:
public static void main(String[] args) {
Scanner cg = new Scanner(System.in);
boolean valid = false;
//Assign and define variables
int guess, rand;
guess = 0;
rand = 10;
//Create loop
do{
System.out.println("Enter your guess: ");
if(cg.hasNextInt()){
guess = cg.nextInt();
valid = true;
}else{
System.out.println("Your guess must be an integer.");
cg.next();
}
}while (!valid || guess != rand);
}
Try resetting your variable "guess = 0" in catch block.
This question already has answers here:
Endless while loop problem with try/catch
(2 answers)
Closed 7 years ago.
Below code,
import java.util.Scanner;
public class Dummy {
static Scanner sc = new Scanner(System.in);
public static int getIntegerInput(String prompt){
int choice = 0;
for(;;){
System.out.print(prompt);
try{
choice = sc.nextInt();
break;
}catch(java.util.InputMismatchException ex){
System.out.print("What??? ");
}
}
return choice;
}
public static void main(String[] args) {
int choice = getIntegerInput("Enter a number: ");
} //end main
}
does not stop for next user input, if the first user input raised an exception.
How do I understand this problem in the above code? placing sc.next() in catch resolves the problem. But I'm still not clear what is going on under the hood? What is the right approach to resolve this problem?
When nextXYZ() fails to consume a token it leaves it in the InputStream. Here, you are looping over the same input endlessly - each iteration, you attempt to consume this token, throw an exception if it isn't an integer, catch it, and try reading it again - forever.
EDIT:
In order to work around this, you could use next() to consume that token and move on to the next one:
for(;;) {
System.out.print(prompt);
try{
choice = sc.nextInt();
break;
} catch(java.util.InputMismatchException ex) {
sc.next(); // here
}
}
The problem with Scanner next() are they will not advances if the match is not found. And the character for which it failed remain in the stream. Hence its very important to advance the scanner if you found non intended character.
You can use next() method which actually consumes any character or you can use skip method passing skip pattern.
Use hasNext() to know whether a valid match is present or not. If not then consume that character using above said methods.
If it doesnt find an int on the next like, it throws an error. This error is then caught by your program, so the break is never hit because the error jumps over it whenever a non-int (including nothing) is found.
This question already has answers here:
try/catch with InputMismatchException creates infinite loop [duplicate]
(7 answers)
Closed 7 years ago.
I keep getting my code caught in an infinite while loop.
It is nothing to advanced, but i can not figure it out for the life of me!
Someone Please help
I have purplosely just re created the specific error without all of the if statements i have in my actual program.
package bs;
import java.util.InputMismatchException;
import java.util.Scanner;
public class bs {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
boolean continueVar = true;
while (continueVar) {
try {
System.out.println("Enter Something");
int input = sc.nextInt();
} catch (InputMismatchException i) {
System.out.println("What the f***?");
continueVar = true;
}
}
}
}
The infinite loop occurs when the Input mismatch exception is caught. I would think that it would atleast ask the user to re enter their input but instead of doing that it just continues in the loop as so:
run:
Enter Something
df
What the f***?
Enter Something
What the f***?
Enter Something
What the f***?
It acts like it is just ignoring the scanner object sc?!
No the scanner is not skipped, it's just starting at the beginning of the input. From the JavaDoc:
If the translation is successful, the scanner advances past the input that matched.
This means if the conversion isn't successfull the scanner won't advance. You'd thus have to manually skip the incorrect input using just next().
Edit: you might want to check for hasNextInt() before trying to read the input.
you loop while continueVar is true, but you never set to to false, so the loop never exits.
I think you want to set continueVar to false in the exception handler.
When a scanner throws an
InputMismatchException, the scanner
will not pass the token that caused
the exception, so that it may be
retrieved or skipped via some other
method.
The token that caused the mismatch is still in the scanner's buffer. You need to clear it before trying to scan again.
You can do that by calling next() in your catch block, like this:
catch (InputMismatchException i) {
System.out.println("What the f***?");
sc.next();
}
Also, you don't need to set continueVar to true again. You never set it to false, so it will stay true. Guessing that's an artifact of you removing this into a mini program.
Scanner does not advance when bad token is found. Look at Scanner.java, lines 2095-2096:
catch (NumberFormatException nfe) {
position = matcher.start(); // don't skip bad token