防止扫描程序读取Java中剩余的输入

Question

I am wondering how I can stop the scanner from reading extra erroneous input from the user.

For example, I would like to read in the user input 2 one as 2 into the variable diam , which the program below achieves.

The issue is then, that the next loop automatically detects the one leftover from the input, and executes the if statement accordingly.

I was able to work around this by creating two scanners, but unfortunately this is not allowed for this particular assignment. In addition, we are required to use .hasNextInt() in our program.

How do I prevent this "spill-over" using only one scanner? I have to assume this question has been posed before, but I did not have much luck finding an answer.

    Scanner scnr = new Scanner(System.in);

    System.out.print("Enter the diameter of a "
            + "cylinder (in centimeters): ");
    // BEGIN: diameter input verification loop      
    for (;;) {
        if (!scnr.hasNextInt()) {
            System.out.print("Please enter an integer value "
                    + "(less than 2,147,483,648) as decimal digits: ");
            scnr.nextLine(); 
            continue;
        }
        diam = scnr.nextInt();
        if (diam >= 0) {
           //null statement
        } 
        else {
          System.out.print("Please enter a positive integer value: ");
          continue;
        }
    break;
    }
    //END: diameter input verification loop

    //prompts user for container height     
    System.out.print("Enter the height of a "
            + "cylinder (in centimeters): ");

    // BEGIN: height input verification loop        
    for (;;) {
        if (!scnr.hasNextInt()) {
            System.out.print("Please enter an integer value "
                    + "(less than 2,147,483,648) as decimal digits: ");
            scnr.nextLine(); 
            continue;
        }
        height = scnr.nextInt();
        if (height >= 0) {
           //null statement
        } 
        else {
          System.out.print("Please enter a positive integer value: ");
          continue;
        }
    break;
    }
    //END: height input verification loop`

Answer 1

One option would be to just read the entire line of input from the Scanner , and then retain only the first word. For example, for the diameter of a cylinder you could use:

System.out.print("Enter the diameter of a cylinder (in centimeters): ");
String input = scnr.nextLine();
try {
    int diam = Integer.parseInt(input.split(" ")[0]);
}
catch (NumberFormatException e) {
    System.out.print("Please enter an integer value "
                + "(less than 2,147,483,648) as decimal digits: ");
}

Answer 2

I can think of a couple of approaches:

• As Tim notes, you can use readLine() to read a complete line from the user, then parse the line. You could use split , or create a new Scanner to parse the line, or various other approaches.

• You can stick with a single Scanner and call nextLine() to discard unconsumed characters up to and including the next end-of-line. Obviously, you need to do this after calling nextInt() . For example:

   height = -1; while (height < 0) { if (!scnr.hasNextInt()) { System.out.print("Please enter an integer value " + "(less than 2,147,483,648) as decimal digits: "); } else { height = scnr.nextInt(); if (height < 0) { System.out.print("Please enter a positive integer value: "); } } scanner.nextLine(); } 

  (The above version of your code has restructured things to get rid of the break and continue . The restructuring also allows me to put the readLine as an unconditional last statement for the loop. I think it makes the logic easier to understand ...)

Answer 3

// BEGIN: height input verification loop
        for (;;) {

            scnr.nextLine(); /* read fresh input, deleting the left over input */

            if (!scnr.hasNextInt()) {
                System.out.print("Please enter an integer value " + "(less than 2,147,483,648) as decimal digits: ");
                scnr.nextLine();
                continue;
            }
            height = scnr.nextInt();
            if (height >= 0) {
                // null statement
            } else {
                System.out.print("Please enter a positive integer value: ");
                continue;
            }
            break;
        }

You can simply add a scnr.nextLine() to remove the "old" scnr left over from the previous input. This fixes the spill-over problem.