I wrote this Caesar cipher and decipher just for the sake of practising OOP (previous answers to my java questions all seem to point to OOPness).

What it is:

The Caesar cipher is one of the earliest known and simplest ciphers. It is a type of substitution cipher in which each letter in the plaintext is 'shifted' a certain number of places down the alphabet.

Currently I have a Cipher interface and a CaesarCipher class: I am planning to add a Viginere Cipher soon.

Code:

Cipher.java

public interface Cipher {
    
    public String cipher(String text);
    
    public String decipher(String cipherText);
    
    public char getCipheredChar(char textChar);
    
    public char getDecipheredChar(char cipherChar);

}

CaesarCipher.java

public class CaesarCipher implements Cipher {

    private static final int LETTERS = 'Z' - 'A' + 1;

    private int shift;

    public CaesarCipher(int shift) {
        if (shift < -LETTERS || shift > LETTERS) {
            throw new IllegalArgumentException("The value of shift: " + shift
                    + " is invalid.");
        }
        this.shift = shift;
    }
    
    public int getShift() {
        return shift;
    }

    @Override
    public String cipher(String text) {
        char[] result = text.toCharArray();
        for (int i = 0; i < result.length; i++) {
            result[i] = getCipheredChar(result[i]);
        }
        return new String(result);
    }

    @Override
    public String decipher(String cipherText) {
        this.shift = -shift;
        String result = cipher(cipherText);
        this.shift = -shift;
        return result;
    }

    @Override
    public char getCipheredChar(char textChar) {
        if (textChar > 'A' && textChar < 'z') {
            char result = (char) (textChar + shift);
            char compare = Character.isUpperCase(textChar) ? 'Z' : 'z';
            char lowCompare = Character.isUpperCase(textChar) ? 'A' : 'a';
            return (char) (result > compare ? result - 26
                    : (result < lowCompare ? result + 26 : result));
        }
        return textChar;
    }

    @Override
    public char getDecipheredChar(char cipherChar) {
        this.shift = -shift;
        char result = getCipheredChar(cipherChar);
        this.shift = -shift;
        return result;
    }

}

And tests!

import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class TestCipher {

    @Test
    public void testShift1() {
        CaesarCipher cipher = new CaesarCipher(1);
        assertEquals("AaBbCc", cipher.decipher("BbCcDd"));
        assertEquals("Bzdrzq", cipher.decipher("Caesar"));
        assertEquals("Bzdrzq Bhogdq...", cipher.decipher("Caesar Cipher..."));
    }
    
    @Test
    public void testShift5() {
        CaesarCipher cipher = new CaesarCipher(5);
        assertEquals("AaBbCc", cipher.decipher("FfGgHh"));
        assertEquals("Xvznvm", cipher.decipher("Caesar"));
        assertEquals("Xvznvm Xdkczm...", cipher.decipher("Caesar Cipher..."));
    }

}

Concerns:

1. Are my tests fine?
2. Is my OOP structure good?
3. And as usual, anything else?