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Hectal

Lesson 3.1 · Strings

Characters Are Numbers

How char values map to numbers, how to test and convert characters, and the c - 'a' trick for indexing letters.

10 min

Think of it like this

Every character has a seat number in a big table (Unicode). 'a' sits in seat 97, 'b' in 98, and so on in order. Knowing letters sit in consecutive seats lets you do arithmetic with them: 'c' − 'a' is 2 because 'c' is two seats after 'a'.

1.char arithmetic

A Java char holds a character's code. Letters a–z are consecutive (97–122), as are A–Z (65–90) and digits 0–9 (48–57). So c - 'a' gives a letter's position 0–25, and c - '0' turns a digit character into its number.

Going back, (char) ('a' + 2) is 'c'. Without the cast, 'a' + 2 is the int 99.

Chars.java
public class Main {
    public static void main(String[] args) {
        char c = 'c';
        System.out.println((int) c);          // 99
        System.out.println(c - 'a');          // 2
        System.out.println('7' - '0');        // 7
        System.out.println((char) ('a' + 25)); // z
        System.out.println('a' + 2);          // 99, an int
    }
}

Output

99
2
7
z
99

2.The Character helpers

Character.isLetterOrDigit(c), Character.isLetter(c), Character.isDigit(c), Character.isWhitespace(c), Character.toLowerCase(c) and Character.toUpperCase(c) save you from writing range checks by hand, and they handle non-English letters too.

Helpers.java
public class Main {
    public static void main(String[] args) {
        String s = "A man, a plan!";
        StringBuilder clean = new StringBuilder();
        for (char c : s.toCharArray()) {
            if (Character.isLetterOrDigit(c)) clean.append(Character.toLowerCase(c));
        }
        System.out.println(clean);
    }
}

Output

amanaplan

Quick check

How do you convert the string "472" to the number 472 without Integer.parseInt?

Remember

  • c - 'a' maps a lowercase letter to 0–25; c - '0' maps a digit to its value.
  • Cast back with (char) when you need a character.
  • Use Character helpers for letter, digit and case checks.

Common mistakes

  • Printing 'a' + 1 and expecting 'b' (it prints 98).
  • Assuming only lowercase letters when the problem allows uppercase or digits.