Java Scanner 阅读领先空间

Question

I am trying to have a menu that takes a character input for a switch case and loops till the input is q but after the loop run once i get this error:

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0 at java.lang.String.charAt(Unknown Source) at Menu.main(Menu.java:19)

Which means its getting a null input right? So i added the .trim() but i still get the error, it doesn't even wait for an input i just get the error.

Sorry if this has been answered before but i can't find it anywhere. I've also tried adding keyboard.skip("(\\r\\n|[\\n\\r\
\
\…])?"); which doesn't work as well.

Input given for the program with error log:

Enter Option (a,b,c,d,e,f,q):

c

Enter 2 Numbers

First Number:

1

Second Number:

10

1, 2, 3, 4, 5,
6, 7, 8, 9, 10

Enter Option (a,b,c,d,e,f,q):

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0
        at java.lang.String.charAt(Unknown Source)
        at Menu.main(Menu.java:19)

CODE:

import java.io.*;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

    Scanner keyboard = new Scanner (System.in);
    char ch; int m,n; String y;
    do
    {
        System.out.println("Enter Option (a,b,c,d,e,f,q):");
        ch = keyboard.nextLine().toLowerCase().charAt(0);
        switch (ch)
        {
        case 'c':
            
            System.out.println("Enter 2 Numbers");
            System.out.println("First Number: ");
            m = keyboard.nextInt();
            System.out.println("Second Number: ");
            n = keyboard.nextInt(); 
            for(int i = 1; i <= n - m + 1; i++)
            {
                if (i % 5 == 0)
                {
                    System.out.print(i);
                    if (i != n - m + 1)
                    {
                    System.out.println(", ");
                    }
                }else
                {
                    System.out.print(i);
                    if (i != n - m + 1)
                    {
                    System.out.print(", ");
                    }
                }
                
            }
            System.out.println("");
            break;
        }
    }while (ch != 'q');

}
 
} 

Answer 1

Try the line ch = keyboard.next().charAt(0); switch(ch)

import java.io.*;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

    Scanner keyboard = new Scanner (System.in);
    char ch; int m,n; String y;
    do
    {
        System.out.println("Enter Option (a,b,c,d,e,f,q):");
        ch = keyboard.next().charAt(0);

        switch (ch)
        {
        case 'c':
            
            System.out.println("Enter 2 Numbers");
            System.out.println("First Number: ");
            m = keyboard.nextInt();
            System.out.println("Second Number: ");
            n = keyboard.nextInt(); 
            for(int i = 1; i <= n - m + 1; i++)
            {
                if (i % 5 == 0)
                {
                    System.out.print(i);
                    if (i != n - m + 1)
                    {
                    System.out.println(", ");
                    }
                }else
                {
                    System.out.print(i);
                    if (i != n - m + 1)
                    {
                    System.out.print(", ");
                    }
                }
                
            }
            System.out.println("");
            break;
        }
    }while (ch != 'q');

}
 
} 

Happy coding!

Answer 2

Here's the updated code Please check:

public static void main(String[] args)
{
    

    Scanner keyboard = new Scanner (System.in);
    char ch;
    do
    {
        System.out.println("Enter Option (a,b,c,d,e,f,q):");
        ch = keyboard.nextLine().toLowerCase().charAt(0);   
        switch (ch)
        {
        //case statements for a,b,c,d,e and f
        }
    }while (ch != 'q');

}

Answer 3

Main thing here that scanner works with input right just after you hit "enter". So if your input would be entering '\ ' (whitespaces) even several times on line 19 you're doing trim to empty line and according to javadoc charAt .