Building a Caesar Cipher (a Character-Shifting Cipher)

This lesson covers implementing a Caesar cipher in Java, so you understand the basics of a character-shifting encryption scheme. It's for anyone searching "Java Caesar cipher implementation."

A Caesar cipher is a simple cipher, dating back to ancient Rome, that shifts letters by a fixed number of positions. It's like a childhood game of shifting every letter of the alphabet by a few spots.

The example builds the result with a StringBuilder, checking whether each character is in the alphabet range, and computing the shifted character with (c - 'A' + shift) % 26 + 'A'. Notice non-alphabet characters are passed through unchanged.

A common early mistake is the character-code math. You need to correctly handle the case where the shifted result goes past the end of the alphabet (A–Z), using % 26.

It's simple compared to modern cryptography, but it's a perfect introductory exercise for practicing character-code arithmetic.

📖 Reference code

static String caesarEncrypt(String text, int shift) {
    StringBuilder result = new StringBuilder();
    for (char c : text.toCharArray()) {
        if (c >= 'A' && c <= 'Z') {
            char code = (char) ((c - 'A' + shift) % 26 + 'A');
            result.append(code);
        } else {
            result.append(c);
        }
    }
    return result.toString();
}

System.out.println(caesarEncrypt("HELLO", 3));

✍️ Your code

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🧪 This site can't compile or run Java directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).