Ceasar Cypher

Caesar.java

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;
    }
}

CaesarTest.java

import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;

class CaesarTest {

    @Test
    void caesarTesd() {
        assertEquals("BCD", Caesar.caesar("ABC", 1));
        assertEquals("BCD", Caesar.caesar("ABC", 27));
        assertEquals("ZAB", Caesar.caesar("ABC", -1));
        assertEquals("ZAB", Caesar.caesar("ABC", -27));
        assertEquals("ABC", Caesar.caesar("1A2B3C$#Ä+", 0));
        assertEquals("ABC", Caesar.caesar("1A2B3C$#Ä+", 26));
        assertEquals("YZA", Caesar.caesar("XYZ", 1));
        assertEquals("XYZABCDEFGHIJKLMNOPQRSTUVW", Caesar.caesar("ABCDEFGHIJKLMNOPQRSTUVWXYZ", 23));

        assertEquals("bcd", Caesar.caesar("abc", 1));
        assertEquals("bcd", Caesar.caesar("abc", 27));
        assertEquals("zab", Caesar.caesar("abc", -1));
        assertEquals("zab", Caesar.caesar("abc", -27));
        assertEquals("abc", Caesar.caesar("1a2b3c$#ä+", 0));
        assertEquals("abc", Caesar.caesar("1a2b3c$#ä+", 26));
        assertEquals("yza", Caesar.caesar("xyz", 1));
        assertEquals("xyzabcdefghijklmnopqrstuvw", Caesar.caesar("abcdefghijklmnopqrstuvwxyz", 23));
    }
}