简单的Java程序无法正常工作-while循环

Question

So I'm fairly new to programming, and I'm working on a simple practice program that finds a letters order in the alphabet. This should have been pretty easy...but for some reason I get a StringIndexOutOfBoundsException when I add a while loop. So the program does do what it should the first time...but will not allow me to test again with out re-running the program. I tested the while loop with nothing but a simple print statement inside and it worked, so I'm confused as to why the while loop isn't working with my alphabet program.

Any help would be greatly appreciated thanks!

import java.io.*;
public class test {


    public static void main(String[] args) throws IOException 
    {
        BufferedReader in;
        in = new BufferedReader (new InputStreamReader (System.in));
        boolean again=true;
        String response;

        while (again)
        {

            System.out.println("Enter a letter to find it's order in the alphabet");

            char theLetter = (char) in.read();

            System.out.println(theLetter + " is the " + convertLetter(theLetter) + "th letter of the alphabet");

            System.out.println("want to play again?");

            response = in.readLine();

            if (response.charAt(0)=='n')
            {
                again=false;
            }
        }

        System.out.println("end program");
    }


    public static int convertLetter(char TheLetter)
    {
        //number value 'a'=97
        //number value 'b'=98
        //number value 'c'=99

        //subtracting 'a' from any other number will reveal how many places away that number is from the start
        //thus finding it's chronological place in the alphabet

        int NumberValue= (int)TheLetter;
        int a = 'a';

        int CalulateOrder = (NumberValue - a) + 1; 

        return CalulateOrder;

    }
}

Answer 1

 if (response.charAt(0)=='n') 

如果String为空"" ，则位置0处将没有字符。在执行charAt()之前进行检查

Answer 2

When you hit enter for the original char, that newline is still in the buffer since you only call read() and only get 1 character, leaving the newline in the buffer from hitting enter. So when you call readLine it simply hits that newline and returns an empty string.

You can test this by typing something with more than one character when it first asks you for a character, and it will go for a second loop since the readLine will return a non-empty string.

To fix this change your original read() to readLine() so that it gets the newline caused from you hitting enter, then just grab the first character from the string.

This should fix it:

    import java.io.*;

public class test {

    public static void main(String[] args) throws IOException {
        BufferedReader in;
        in = new BufferedReader (new InputStreamReader (System.in));
        boolean again=true;
        String response;

        while (again) {
            System.out.println("Enter a letter to find it's order in the alphabet");
            response = in.readLine();
            if (response.length() > 0) {
                char theLetter = response.charAt(0);

                System.out.println(theLetter + " is the " + convertLetter(theLetter) + "th letter of the alphabet");
                System.out.println("want to play again?");

                response = in.readLine();
                if (response.length() > 0 && response.charAt(0)=='n') {
                    again=false;
                }
            }
        }
        System.out.println("end program");
    }

    public static int convertLetter(char TheLetter) {
        return (TheLetter - 'a') + 1;
    }

}

Answer 3

is the length of response null or "" ? if it is you will not be able to get the char at index 0

Answer 4

The only place you access a string index is at if (response.charAt(0) == 'n') so that is most likely your problem area.

if(response.length() > 0 && response.charAt(0) == 'n')

Should do the trick.

Edit: As @TreySchroeder points out, there is another problem with your program, in that you don't read the full line at first. Put in.readLine(); after your initial theLetter = (char) in.read(); , and use this fix for the other issue.

Answer 5

I bet the culprit is that you're hitting "enter" at the want to play again? prompt. The in.readLine(); returns the line without the trailing newline (see the javadocs ), which means that if you only press "enter" it will return an empty string, thus the StringOutOfBoundException while checking the first char.

Check for the empty string before checking for the char:

if(response.length() > 0 && response.charAt(0) == 'n')

Answer 6

if (response.isEmpty() && response.charAt(0)=='n')

将避免异常。