Java accepting only numbers from user with Scanner

Question

I am trying to understand how to only accept numbers from the user, and I have attempted to do so using try catch blocks, but I would still get errors with it.

    Scanner scan = new Scanner(System.in);

    boolean bidding;
    int startbid;
    int bid;

    bidding = true;

    System.out.println("Alright folks, who wants this unit?" +
            "\nHow much. How much. How much money where?" );

    startbid = scan.nextInt();

try{
    while(bidding){
    System.out.println("$" + startbid + "! Whose going to bid higher?");
    startbid =+ scan.nextInt();
    }
}catch(NumberFormatException nfe){

        System.out.println("Please enter a bid");

    }

I am trying to understand why it is not working.

I tested it out by inputting a into the console, and I would receive an error instead of the hopeful "Please enter a bid" resolution.

Exception in thread "main" java.util.InputMismatchException
at java.util.Scanner.throwFor(Scanner.java:909)
at java.util.Scanner.next(Scanner.java:1530)
at java.util.Scanner.nextInt(Scanner.java:2160)
at java.util.Scanner.nextInt(Scanner.java:2119)
at Auction.test.main(test.java:25)

Answer 1

尝试捕获引发的异常类型，而不是NumberFormatException （ InputMismatchException ）。

Answer 2

The message is pretty clear: Scanner.nextInt() throws an InputMismatchException , but your code catches NumberFormatException . Catch the appropriate exception type.

Answer 3

While using Scanner.nextInt() , it causes some problems. When you use Scanner.nextInt() , it does not consume the new line (or other delimiter) itself so the next token returned will typically be an empty string. Thus, you need to follow it with a Scanner.nextLine() . You can discard the result.

It's for this reason that I suggest always using nextLine (or BufferedReader.readLine() ) and doing the parsing after using Integer.parseInt() . Your code should be as follows.

        Scanner scan = new Scanner(System.in);

        boolean bidding;
        int startbid;
        int bid;

        bidding = true;

        System.out.print("Alright folks, who wants this unit?" +
                "\nHow much. How much. How much money where?" );
        try
        {
            startbid = Integer.parseInt(scan.nextLine());

            while(bidding)
            {
                System.out.println("$" + startbid + "! Whose going to bid higher?");
                startbid =+ Integer.parseInt(scan.nextLine());
            }
        }
        catch(NumberFormatException nfe)
        {
            System.out.println("Please enter a bid");
        }