I have read user input that must be only of type int, the problem comes when the user enters letter instead of a int. I know how to handle the exception, but I would like to return the scanner read where the user has made a mistake. How can I do?
I already tried with an infinite loop, but it does not work.
try{
System.out.print("enter number: ");
value = scanner.nextInt();
}catch(InputMismatchException e){
System.err.println("enter a number!");
}
While other answers give you correct idea to use loop you should avoid using exceptions as part of your basic logic. Instead you can use hasNextInt from Scanner to check if user passed integer.
System.out.print("enter number: ");
while (!scanner.hasNextInt()) {
scanner.nextLine();// consume incorrect values from entire line
//or
//tastiera.next(); //consume only one invalid token
System.out.print("enter number!: ");
}
// here we are sure that user passed integer
int value = scanner.nextInt();
A loop is the right idea. You just need to mark a success and carry on:
boolean inputOK = false;
while (!inputOK) {
try{
System.out.print("enter number: ");
numAb = tastiera.nextInt();
// we only reach this line if an exception was NOT thrown
inputOK = true;
} catch(InputMismatchException e) {
// If tastiera.nextInt() throws an exception, we need to clean the buffer
tastiera.nextLine();
}
}
Related
I write a code to let the user input cruise id first and then enter the ship name.
At first, I want to detect whether the user input integer type, if not, the user has to re-enter the first question again.
But in my code, it will directly print the second question instead of go back to the first question and ask again. Same, for the second question, I also want it return back and ask user to input again if the input is wrong
Please help me for that. Thanks!!
try{
System.out.println("Input Cruise ID:");
sc1 = sc.nextInt();
}catch(Exception e){
System.out.println("Please Enter integer:");
sc.nextLine();
}
System.out.println("Input ship name :");
try{
sc2 = sc.next();
}catch(Exception e){
if( sc2 != "Sydney1" || sc2 !="Melmone1"){
System.out.println("Oops!! We don't have this ship!! Please enter the ship name : Sydney1 or Melbone1");
}
}
I write a code to let the user input cruise id first and then enter the ship name. At first, I want to detect whether the user input integer type, if not, the user has to re-enter the first question again.
What you need is an input validation. try-catch block itself will not create an endless loop to reprompt the user should the input is not an integer. What you need is a while loop.
You can use a do-while loop as follows so that it runs first before performing a check:
String input = ""; //just for receiving inputs
do{
System.out.println("Input Cruise ID:");
input = sc.nextInt();
}while(!input.matches("[0-9]+")); //repeat if input does not contain only numbers
int cruiseID = Integer.parseInt(input); //actual curiseID in integer
To perform validation for your second input (i.e, your shipName, you need another while loop which encloses your prompt for input).
try-catch block are mainly used to handle exceptional cases. Try not to misuse it as a control statement for your implementations.
You can add more checks inside the while loop itself. For example, checking if the number is a negative number or zero etc. For example
while (true) {
try {
System.out.println("Input Cruise ID:");
cruiseId = sc.nextInt();
if(cruiseId <=0){
System.out.println("Please Enter integer:");
sc.nextLine();
}
break; // break when no exception happens till here.
} catch (Exception e) {
System.out.println("Please Enter integer:");
sc.nextLine();
}
}
I have read user input that must be only of type int, the problem comes when the user enters letter instead of a int. I know how to handle the exception, but I would like to return the scanner read where the user has made a mistake. How can I do?
I already tried with an infinite loop, but it does not work.
try{
System.out.print("enter number: ");
value = scanner.nextInt();
}catch(InputMismatchException e){
System.err.println("enter a number!");
}
While other answers give you correct idea to use loop you should avoid using exceptions as part of your basic logic. Instead you can use hasNextInt from Scanner to check if user passed integer.
System.out.print("enter number: ");
while (!scanner.hasNextInt()) {
scanner.nextLine();// consume incorrect values from entire line
//or
//tastiera.next(); //consume only one invalid token
System.out.print("enter number!: ");
}
// here we are sure that user passed integer
int value = scanner.nextInt();
A loop is the right idea. You just need to mark a success and carry on:
boolean inputOK = false;
while (!inputOK) {
try{
System.out.print("enter number: ");
numAb = tastiera.nextInt();
// we only reach this line if an exception was NOT thrown
inputOK = true;
} catch(InputMismatchException e) {
// If tastiera.nextInt() throws an exception, we need to clean the buffer
tastiera.nextLine();
}
}
I have a method that a wrote. This method just scans for a user entered integer input. If the user enters a character value it will throw an input mismatch exception, which is handled in my Try-Catch statement. The problem is that, if the user inputs anything that is not a number, and then an exception is thrown, I need the method to loop back around to ask the user for input again. To my understanding, a Try catch statement automatically loops back to the Try block if an error is caught. Is this not correct? Please advise.
Here is my method (it's pretty simple):
public static int getMask() {
//Prompt user to enter integer mask
Scanner keyboard = new Scanner(System.in);
int output = 0;
try{
System.out.print("Enter the encryption mask: ");
output = keyboard.nextInt();
}
catch(Exception e){
System.out.println("Please enter a number, mask must be numeric");
}
return output;
}//end of getMask method
Here is how the method is implemented into my program:
//get integer mask from user input
int mask = getMask();
System.out.println("TEMP mask Value is: " + mask);
/***********************************/
Here is my updated code. It creates an infinate loop that I can't escape. I don't understand why I am struggling with this so much. Please help.
public static int getMask() {
//Prompt user to enter integer mask
Scanner keyboard = new Scanner(System.in);
int output = 0;
boolean validInput = true;
do{
try {
System.out.print("Enter the encryption mask: ");
output = keyboard.nextInt();
validInput = true;
}
catch(InputMismatchException e){
System.out.println("Please enter a number, mask must be numeric");
validInput = false;
}
}while(!(validInput));
return output;
/********************/FINAL_ANSWER
I was able to get it finally. I think I just need to study boolean logic more. Sometimes it makes my head spin. Implementing the loop with an integer test worked fine. My own user error I suppose. Here is my final code working correctly with better exception handling. Let me know in the comments if you have any criticisms.
//get integer mask from user input
int repeat = 1;
int mask = 0;
do{
try{
mask = getMask();
repeat = 1;
}
catch(InputMismatchException e){
repeat = 0;
}
}while(repeat==0);
To my understanding, a Try catch statement automatically loops back to the Try block if an error is caught. Is this not correct?
No this is not correct, and I'm curious as to how you arrived at that understanding.
You have a few options. For example (this will not work as-is but let's talk about error handling first, then read below):
// Code for illustrative purposes but see comments on nextInt() below; this
// is not a working example as-is.
int output = 0;
while (true) {
try{
System.out.print("Enter the encryption mask: ");
output = keyboard.nextInt();
break;
}
catch(Exception e){
System.out.println("Please enter a number, mask must be numeric");
}
}
Among others; your choice of option usually depends on your preferred tastes (e.g. fge's answer is the same idea but slightly different), but in most cases is a direct reflection of what you are trying to do: "Keep asking until the user enters a valid number."
Note also, like fge mentioned, you should generally catch the tightest exception possible that you are prepared to handle. nextInt() throws a few different exceptions but your interest is specifically in an InputMismatchException. You are not prepared to handle, e.g., an IllegalStateException -- not to mention that it will make debugging/testing difficult if unexpected exceptions are thrown but your program pretends they are simply related to invalid input (and thus never notifies you that a different problem occurred).
Now, that said, Scanner.nextInt() has another issue here, where the token is left on the stream if it cannot be parsed as an integer. This will leave you stuck in a loop if you don't take that token off the stream. To that end you actually want to use either next() or nextLine(), so that the token is always consumed no matter what; then you can parse with Integer.parseInt(), e.g.:
int output = 0;
while (true) {
try{
System.out.print("Enter the encryption mask: ");
String response = keyboard.next(); // or nextLine(), depending on requirements
output = Integer.parseInt(response);
break;
}
catch(NumberFormatException e){ // <- note specific exception type
System.out.println("Please enter a number, mask must be numeric");
}
}
Note that this still directly reflects what you want to do: "Keep asking until the user enters a valid number, but consume the input no matter what they enter."
To my understanding, a Try catch statement automatically loops back to the Try block if an error is caught. Is this not correct?
It is indeed not correct. A try block will be executed only once.
You can use this to "work around" it (although JasonC's answer is more solid -- go with that):
boolean validInput = false;
while (!validInput) {
try {
System.out.print("Enter the encryption mask: ");
output = keyboard.nextInt();
validInput = true;
}
catch(Exception e) {
keyboard.nextLine(); // swallow token!
System.out.println("Please enter a number, mask must be numeric");
}
}
return output;
Further note: you should NOT be catching Exception but a more specific exception class.
As stated in the comments, try-catch -blocks don't loop. Use a for or while if you want looping.
This question already has answers here:
How to handle infinite loop caused by invalid input (InputMismatchException) using Scanner
(5 answers)
How to use Scanner to accept only valid int as input
(6 answers)
Closed 5 years ago.
I am trying add catch blocks to my program to handle input mismatch exceptions. I set up my first one to work inside of a do while loop, to give the user the opportunity to correct the issue.
System.out.print("Enter Customer ID: ");
int custID=0;
do {
try {
custID = input.nextInt();
} catch (InputMismatchException e){
System.out.println("Customer IDs are numbers only");
}
} while (custID<1);
As it stands, if I try to enter a letter, it goes into an infinite loop of "Customer IDs are numbers only".
How do I make this work properly?
Be aware that 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.
To avoid "infinite loop of "Customer IDs are numbers only".", You need to call input.next(); in the catch statement to to make it possible to re-enter number in Console
From
statement
catch (InputMismatchException e) {
System.out.println("Customer IDs are numbers only");
To
catch (InputMismatchException e) {
System.out.println("Customer IDs are numbers only");
input.next();
}
Example tested:
Enter Customer ID: a
Customer IDs are numbers only
b
Customer IDs are numbers only
c
Customer IDs are numbers only
11
What's happening is that you catch the mismatch, but the number "wrong input" still needs to be cleared and a .next() should be called. Edit: since you also require it to be greater than or equal to 1 per your do/while
boolean valid = false;
while(!valid) {
try {
custID = input.nextInt();
if(custID >= 1) //we won't hit this step if not valid, but then we check to see if positive
valid = true; //yay, both an int, and a positive one too!
}
catch (InputMismatchException e) {
System.out.println("Customer IDs are numbers only");
input.next(); //clear the input
}
}
//code once we have an actual int
Why not use a scanner object to read it with Scanner.readNextInt()?
I got it, this is solution you are looking for:
public class InputTypeMisMatch {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int custID=0;
System.out.println("Please enter a number");
while (!input.hasNextInt()) {
System.out.println("Please enter a number");
input.next();
}
custID = input.nextInt();
}
}
This piece of code is supposed to get an integer number from user and then finish the program. If the user inputs an invalid number, it asks user again.
After catching exception, it uses Scanner.reset() to reset the scanner, but it doesn't work. and it re-throws previous exception.
Scanner in = new Scanner(System.in);
while (true) {
try {
System.out.print("Enter an integer number: ");
long i = in.nextLong();
System.out.print("Thanks, you entered: ");
System.out.println(i);
break;
} catch (InputMismatchException ex) {
System.out.println("Error in your input");
in.reset(); // <----------------------------- [The reset is here]
}
}
I thought Scanner.reset() will reset everything and forget the exception. I put it before asking the user for a new input.
If I get the point wrong, what is the right way?
You misunderstood the purpose of the reset method: it is there to reset the "metadata" associated with the scanner - its whitespace, delimiter characters, and so on. It does not change the state of its input, so it would not achieve what you are looking for.
What you need is a call of next(), which reads and discards any String from the Scanner:
try {
System.out.print("Enter an integer number: ");
long i = in.nextLong();
System.out.print("Thanks, you entered: ");
System.out.println(i);
break;
} catch (InputMismatchException ex) {
System.out.println("Error in your input");
in.next(); // Read and discard whatever string the user has entered
}
Relying upon exceptions to catch exceptional situations is OK, but an even better approach to the same issue would be using has... methods before calling the next... methods, like this:
System.out.print("Enter an integer number: ");
if (!in.hasNextLong()) {
in.next();
continue;
}
long i = in.nextLong();
System.out.print("Thanks, you entered: ");
System.out.println(i);
break;
Per Scanner.reset() javadoc, the method only "resets" locale, radix and delimiter settings. It does not do anything to the data it already read.