public final class Caesar {
    private static final int NUM_LETTERS = 26;

    public static String caesar(final String s, final int shift) {
        return s.chars()
                .filter(Caesar::isEnglishLetter)
                .map(character -> getIntUnaryOperator((char) character, shift))
                .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
                .toString();
    }

    private static boolean isEnglishLetter(final int i) {
        return (i >= 'A' && i <= 'Z') || (i >= 'a' && i <= 'z');
    }

    private static int getIntUnaryOperator(final char character, final int shift) {
        if (Character.isUpperCase(character)) {
            return safeShift(character, shift, 'A');
        }

        if (Character.isLowerCase(character)) {
            return safeShift(character, shift, 'a');
        }

        throw new IllegalArgumentException("Character has to be an english letter, was " + character);
    }

    private static int safeShift(final char character, final int shift, final char startOffset) {
        return (character - startOffset + NUM_LETTERS + (shift % NUM_LETTERS)) % NUM_LETTERS + startOffset;
    }
}