Scanner class hasNextInt() method seems to allow a decimal to get through, or maybe it's caused by something else.

Question

Scanner choice = new Scanner(System.in);
    while(!choice.hasNextInt()) {
        System.out.println("Invalid input");
        choice.next();
    }

    // Carry out appropriate method relating to user choice
    boolean done = false; // Loop is not finished
    while (!done) {
        int i = choice.nextInt(); // Save the user's choice as int i
        /*
         * A switch statement here would probably be more elegant but this works too
         * Problem: If the user inputs a non-integer number e.g. 2.3 the program explodes :(
         */
        if (i == 1) {
            newGame(); // Call newGame method
        } else if (i == 2) {
            playGame(); // Call playGame method
        } else if (i == 3) {
            viewResults(); // Call viewResults method
        } else if (i == 4) {
            done = true; // If user quits, the loop is done
            quitGame(); // Call quitGame method
        } else { 
            System.out.println("Invalid input");
        }
    }

The only valid input for this needs to be the numbers 1, 2, 3 and 4. If I enter a string it doesn't accept it. If i enter a number larger than 4 it doesn't accept it. However if I was to enter 2.3 or something the program crashes. I can't see what it causing this as 2.3 is not an integer and I don't know how it gets past the hasNextInt() method in Scanner. Anyone shed some light?

Can you post the stacktrace? – Jean Logeart Apr 21 '14 at 09:27 — Jean Logeart, Apr 21 '14 at 09:27

Jean Logeart · Answer 1 · 2014-04-21T09:47:23.200

1

For input 2., first int is 2, so playGame() is executed but done is still false so because of the while(!done) loop choice.nextInt() is called again on . which is not an int.

Hence the exception.

edited Apr 21 '14 at 09:47

answered Apr 21 '14 at 09:29

Jean Logeart

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If the input is really `2.3` to start with, `hasNextInt()` will return `false`. – tom Apr 21 '14 at 09:36
To correct it, move your ``while(!choice.hasNextInt())`` loop inside the ``while(!done)`` loop. – Jean Logeart Apr 21 '14 at 09:51
@Vakh, with `2.` it also returns `false`. – tom Apr 21 '14 at 09:54

score 0 · Answer 2 · answered Apr 21 '14 at 09:40

You only check that choice.hasNextInt() for the first input. The second time round it is not checked. Move the check into the while loop so that it looks like this:

boolean done = false;
while (!done) {
    while(!choice.hasNextInt()) {
        System.out.println("Invalid input");
        choice.nextLine(); // drop entire line, not just next token
    }
    int i = choice.nextInt();
    // ...
}

Scanner class hasNextInt() method seems to allow a decimal to get through, or maybe it's caused by something else.

2 Answers2