How to switch letter in a String

Question

I am working on a project and the problem asks me to switch specific letters in an array. For example, a goes to z, b to y, c to x etc...

I wrote this:

import java.util.Scanner;
public class Project1
{
public static void main( String [] args )
{
    System.out.println("This program helps to encode or decode some text, the concept is that an A turns into a Z, B into a Y etc.");
    Scanner reader = new Scanner ( System.in );
    System.out.print("What is your message (encoded or not): ");
    String Sentence = reader.nextLine();

    System.out.println();

    System.out.print("Press 1 to encode and 2 to decode: ");
    int Number = reader.nextInt();

    if ( Number == 1)
    {
        Encode (Sentence);
    }
    else if ( Number == 2)
    {
        Decode (Sentence);
    }   
    else
    {
        System.out.println("The number has to be 1 or 2.");
    }
}

public static void Encode (String a)
{
    a = a.replace( 'a' , 'z' );
    a = a.replace( 'b' , 'y' );
    a = a.replace( 'c' , 'x' );
    a = a.replace( 'd' , 'w' );
    a = a.replace( 'e' , 'v' );
    a = a.replace( 'f' , 'u' );
    a = a.replace( 'g' , 't' );
    a = a.replace( 'h' , 's' );
    a = a.replace( 'i' , 'r' );
    a = a.replace( 'j' , 'q' );
    a = a.replace( 'k' , 'p' );
    a = a.replace( 'l' , 'o' );
    a = a.replace( 'm' , 'n' );
    a = a.replace( 'n' , 'm' );
    a = a.replace( 'o' , 'l' );
    a = a.replace( 'p' , 'k' );
    a = a.replace( 'q' , 'j' );
    a = a.replace( 'r' , 'i' );
    a = a.replace( 's' , 'h' );
    a = a.replace( 't' , 'g' );
    a = a.replace( 'u' , 'f' );
    a = a.replace( 'v' , 'e' );
    a = a.replace( 'w' , 'd' );
    a = a.replace( 'x' , 'c' );
    a = a.replace( 'y' , 'b' );
    a = a.replace( 'z' , 'a' );
    System.out.println("Encoded message is: " + a);
}

public static void Decode ( String s)
{
    s = s.replace( 'z' , 'a' );
    s = s.replace( 'y' , 'b' );
    System.out.println("Decoded message is: " + s);
}

}

However, when I put in a word with an a it is turned into a z and then back into and a again. I was wondering if there was any other way to switch from one letter to another like this so that the problem does not happen again.

Answer 1

I think the easiest way is to iterate through your String, changing each character individually. It may be easiest to do this in a char[], which you can convert back into a String at the end.

Also, you don't need separate encode and decode methods, since encoding and decoding a String is exactly the same transformation.

public String encode(String toEncode) {
    char[] characters = toEncode.toCharArray();
    for (int index = 0; index < characters.length; index++) {
        if (characters[index] >= 'a' && characters[index] <= 'z') {
            characters[index] = (char)('a' + 'z' - characters[index]);
        }
    }
    return new String(characters);
}

Notice the char arithmetic in the middle there, which will translate a to z, b to y and so on. The if condition ensures that you don't replace any characters that aren't lower case letters.