CODEWITHSUNDEEP
javajava.util.scanner
slhck
slhck

Reputation: 38691

How do I gracefully make a java.util.Scanner wait for input again?

I have the following part of a program, which emulates a very basic menu.

while (true) {
    int selection;
    try {
        selection = scanner.nextInt();
    } catch (Exception e) {
        selection = -1;
    }
    switch (selection) {
    case 0:
        System.exit(0);
    default:
        System.out.println("No valid selection!");
    }
}

Now, whenever I enter not an integer, the selection is set to -1 and the error message is printed. However, the loop continues endlessly, with the Scanner not waiting for any input.

How do I tell it to wait again? How do I fail more gracefully on malformed user input here?

Upvotes: 0

Views: 2971

Answers (3)

Jean Logeart
Jean Logeart

Reputation: 53829

Not sure throwing and catching an exception is relevant in your case.

Try:

boolean isValid = false;
int selection;
while(!isValid) {
    isValid = scanner.hasNextInt();
    if(!isValid) {
        System.out.println("No valid selection!");
        scanner.next();
    } else {
        selection = scanner.nextInt();
    }
}
if(selection == 0) {
    System.exit(0);
}

Upvotes: 1

Etienne de Martel
Etienne de Martel

Reputation: 36851

When a Scanner fails to read something, the offending data is not removed from the stream, which means any subsequent read will fail again until the data is cleared.

To fix this, you could, on failure, just read something and ignore the result:

try {
    selection = scanner.nextInt();
} catch (Exception e) {
    selection = -1;
    scanner.next(); // discard the input
}

Upvotes: 6

Mechkov
Mechkov

Reputation: 4324

Make some user input exit out/break the while loop. Like if a user enters "Exit" while loop stops.

Besides that you can do something like:

catch (Exception e) {
    selection = -1;
}
switch (selection) {
case 0:
    System.exit(0);
default:
    System.out.println("No valid selection!");
    System.out.println("Try again!");
    selection = scanner.nextInt();        
}

Upvotes: -3

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