Java，使用扫描仪时如何验证输入？

Question

I am currently working on Java code. Basically, the int input works. However, if I type in a character, the whole system crashes. My question is as to what needs to be changed in the below code in order for the user to receive a message stating that only an int is the valid input, and to try again if they input a character.

do {
    System.out.println("How many players would like to participate in this game?\t(2-4 players)");
    numberOfPlayers = in.nextInt();
} while(in.hasNextInt()); 

numberOfPlayers = in.nextInt();

Answer 1

I am currently working on Java code. Basically, the int input works. However, if I type in a character, the whole system crashes. My question is as to what needs to be changed in the below code in order for the user to receive a message stating that only an int is the valid input, and to try again if they input a character.

do {
    System.out.println("How many players would like to participate in this game?\t(2-4 players)");
    numberOfPlayers = in.nextInt();
} while(in.hasNextInt()); 

numberOfPlayers = in.nextInt();

Answer 2

I personally prefer to use a while loop for this sort of thing rather than the do/while . Not that there is anything wrong with the do/while , I just feel it's more readable to use the while loop.

I agree with others here, accept String digits from the User instead of Integer. In my opinion it saves you other possible problems down the road and you have no need to purposely apply a try/catch mechanism should the User supply an invalid entry. It also allows you to easily apply a mechanism to quit the application which, again IMHO, should be made available to all Console app's.

You've got your answer for carrying out the task using a do/while loop but I would like to show you another way to do this sort of thing:

Scanner in = new Scanner(System.in);
String ls = System.lineSeparator();
int numberOfPlayers = 0;
String userInput = "";

while (userInput.equals("")) {
    // The Prompt to User...
    System.out.print("How many players would like to participate in this game?" + ls 
            + "2 to 4 players only (q to quit): --> ");
    userInput = in.nextLine();

    // Did the User enter: q, quit (regardless of letter case) 
    if (userInput.toLowerCase().charAt(0) == 'q') {
        // No, the User didn't...
        System.out.println(ls + "Quiting Game - Bye Bye.");
        System.exit(0);  // Close (exit) the application.
    }

    /* Did the User supply a string representation of a numerical 
       digit consiting of either 2, 3, or 4.      */
    if (!userInput.matches("[234]")) { 
        // No, the User didn't...
        System.out.println("Invalid input! You must supply a number from 2 to 4 "
                         + "(inclusive)." + ls + "Try again..." + ls);
        userInput = "";
        continue;  // Loop again.
    }

    // Convert numerical string digit to an Ingeger value.
    numberOfPlayers = Integer.parseInt(userInput);
} 

System.out.println(ls + "The Number of players you provided is: --> " 
                 + numberOfPlayers);

You will notice that the Scanner#nextLine() method is used to accept User input as a String. This now means that we need to validate the fact that a string representation of a Integer numerical digit (2 to 4 inclusive) was supplied by that User. To do this you will notice that I used the String#matches() method along with a small Regular Expression (RegEx) which consists of the following string: "[234]" . What this does in conjunction with the String#matches() method is it checks to see if the string value in the userInput variable contains either a single "2", a single "3", or a single "4". Anything else other than any one of those three digits will display this message:

Invalid input! You must supply a number from 2 to 4 (inclusive).
Try again...

and, force the User make yet another entry.