使用Java中的Do-while循环的简单菜单

Question

I am trying to make a simple menu using a do-while loop in java. My code looks like this:

int choice;
    Scanner scanChoice = new Scanner(System.in);
    do {
        System.out.println("Pick an option. 1 2 or 3.");
        System.out.println("1. Apple");
        System.out.println("2. Pear");
        System.out.println("3. Pineapple");

        choice = scanChoice.nextInt();
    } while (choice < 1 || choice > 3);

    System.out.println("You picked " + choice);

The problem is, every time I try to run it, it throws "java.util.NoSuchElementException". The full error is below:

Exception in thread "main" java.util.NoSuchElementException
at java.util.Scanner.throwFor(Scanner.java:862)
at java.util.Scanner.next(Scanner.java:1485)
at java.util.Scanner.nextInt(Scanner.java:2117)
at java.util.Scanner.nextInt(Scanner.java:2076)
at mainPackage.Main.fruitMenu(Main.java:135)
at mainPackage.Main.main(Main.java:103)

I know that this is because scanChoice.hasNextInt() returns false, but I'm not sure how to fix this. When I add an if statement ( if (scanChoice.hasNextInt()) ), the method scanChoice.hasNextInt() still returns false, so it just passes over the line that initializes the variable choice , and that variable never gets initialized.

Anyone know how to fix this?

EDIT: The problem is that it does not wait for the user to input another integer. The function scanchoice.nextInt() , and the function scanChoice.nextLine() , both immediately return no value, without waiting for the user to input a value. Any way to make it wait for input?

Answer 1

It seems to work for me, consistently. For any valid integers it works as expected, either accepting the input or goes through a loop, and when typing invalid input like "abcd" it throws InputMismatchException , that is the expected behavior anyway.

Online Java compiler IDE

Answer 2

I made your code more robust. It catches alphanumeric characters and displays the menu again, rather than abending with a InputMismatchException.

Here's a test run.

Pick an option. 1 2 or 3.
1. Apple
2. Pear
3. Pineapple
x
Pick an option. 1 2 or 3.
1. Apple
2. Pear
3. Pineapple
asdf
Pick an option. 1 2 or 3.
1. Apple
2. Pear
3. Pineapple
2
You picked 2

And here's the code. I called the Scanner nextLine method.

package com.ggl.testing;

import java.util.Scanner;

public class MenuTest {

    public static void main(String[] args) {
        int choice;
        Scanner scanChoice = new Scanner(System.in);

        do {
            System.out.println("Pick an option. 1 2 or 3.");
            System.out.println("1. Apple");
            System.out.println("2. Pear");
            System.out.println("3. Pineapple");

            String input = scanChoice.nextLine();
            choice = convertToInteger(input.trim());
        } while (choice < 1 || choice > 3);

        System.out.println("You picked " + choice);
        scanChoice.close();
    }

    private static int convertToInteger(String input) {
        try {
            return Integer.valueOf(input);
        } catch (NumberFormatException e) {
            return Integer.MIN_VALUE;
        }
    }

}