Converting char "a" to number 0 using java function

So I'm learning about functions and methods, and trying to create a function that would allow me to replace a Letter with a Number, thus "a" would be 0, "b" would be 1, so on and so forth. I don't know ascii at all, and have only run into creating a very long if, else statement, but I don't even know if I'm on the right track. I'm trying to find a way to create a function without having to make a long conditional statement and use less line of code.

This is the new code I have written with suggestions:

public class CaesarCipher {

/*
 * create function that converts a letter to a number
 * ex. a -> 0, b -> 1, etc...
 */
static char letterToNumber (char firstLetter){
    if (firstLetter < 'a' || firstLetter > 'z') {
}
    return firstLetter;
}
static int numberToLetter (int firstNumer){
    if (firstNumber < '0' || firstNumber '25'){ 
    }
    return firstNumber;
}
public static void main(String[] args) {
    char a = 0;
    // TODO Auto-generated method stub
    System.out.println (letterToNumber (a)); //suppose to compile to convert a -> the number 0
    System.out.println(numberToLetter (1)); //compile to convert 1 -> the letter b
}

}

Jon Skeet
people
quotationmark

The simplest approach is just to subtract the literal 'a'... which will implicitly convert both your input letter and the 'a' to int:

public int convert(char letter) {
    if (letter < 'a' || letter > 'z') {
        throw new IllegalArgumentException("Only lower-case ASCII letters are valid");
    }
    return letter - 'a';
}

The nice thing about this solution is that it's reasonably "obviously correct" (with the assumption that the letters 'a' to 'z' are consecutive in UTF-16). You don't need to include any magic integer values.

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