Iterate through List of single character strings using indexOf()

Question

I'm making a program that will take an input string and decode it using the Rot13 encryption method. This takes the alphabet, and rotates it by 13.

I'm having a hard time getting the index of a letter in the list, and every time I run it it gives me -1 as if the item is not in the list. I looked in the java documentation, and indexOf() asks for an object. I tried explicitly typing my input as an object but that didn't work either.

This is the code I have so far:

package rot13;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.List;

/**
 *
 * @author andrewjohnson
 */
public class CipherKey {

    List<String> alpha = Arrays.asList("A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", " ");
    List<String> alphaRev = Arrays.asList("Z", "Y", "X", "W", "V", "U", "T", "S", "R", "Q", "P", "O", "N", "M", "L", "K", "J", "I", "H", "G", "F", "E", "D", "C", "B", "A", " ");

    public String codeDecode(String s) {
        System.out.println(s);

        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            //System.out.println(ch);
            int x = alpha.indexOf(ch);
            //System.out.println(x);
            String y = alphaRev.get(x);
            System.out.print(y);
        }

        return null;
    }

    public static String readInput() throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        System.out.println("Enter message to be encoded or decoded");

        String s = br.readLine().toUpperCase();
        //System.out.println(s);

        return s;

    }
}

And my main():

/**
 *
 * @author andrewjohnson
 */
public class Rot13 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws IOException {
        CipherKey x = new CipherKey();


        x.codeDecode(x.readInput());
    }

}

I'm not sure why it is not working, but I've narrowed it down to the line:

 int x = alpha.indexOf(ch);

Not being able to find ch in alpha. I'm new to Java and I've tried everything that I can think of. Thanks for your suggestions!

Answer 1

The problem is here:

char ch = s.charAt(i);

int x = alpha.indexOf(ch); // <-------- HERE

You are searching for a char in a String array. Which of course doesn't exist. Hense the -1

Change it to int x = alpha.indexOf("" + ch);

or int x = alpha.indexOf(Character.toString(ch));

or int x = alpha.indexOf(String.valueOf(ch));

Any of these will do.

Answer 2

ch is of type char and your list contains String . List may accept Object for indexOf , but the type still counts.

Change int x = alpha.indexOf(ch); to int x = alpha.indexOf(String.valueOf(ch)); to fix it.

Example:

System.out.println(alpha.indexOf('D'));
System.out.println(alpha.indexOf(String.valueOf('D')));

will print

-1
3

Answer 3

1. That is not the rot13 algorithm - you just appear to be reversing the alphabet. rot13 maps the range A - M to N - Z , and vice versa. Two invocations of rot13 give you back the original text.

2. ASCII letters follow a numerical sequence. Instead of performing a linear search through a list to find the matching index, it's far faster to just calculate the difference between the current letter and A , and then use that difference to offset into a second array of characters (or a string).

ie

static String map = "NOPQRSTUVWXYZABCDEFGHIJKLM";  // for rot13

function rot13(String input) {
    StringBuffer output;
    for (char ch : input) {
        int index = ch - 'A';
        if (ch >= 0 && ch < 26) {
            output.append(map.charAt(index));
        } else {
            output.append(ch);
        }
    }
    return output.toString();
}

NB: untested, may not compile, E&OE etc