Cipher.java

import java.util.Random;
import java.util.stream.IntStream;

public class Cipher {

    private static final String ALPHABET = "abcdefghijklmnopqrstuvwxyz";
    private static final Random random = new Random();

    public String key;

    public Cipher() {
        this.key = IntStream.range(0, 100)
                .map(x -> ALPHABET.toCharArray()[random.nextInt(ALPHABET.length())])
                .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
                .toString();
    }

    public Cipher(String key) {
        if (!isValidKey(key)) {
            throw new IllegalArgumentException("Invalid key");
        }

        this.key = key;
    }

    private static boolean isValidKey(String key) {
        return key.matches("^[a-z]+$");
    }

    public String getKey() {
        return key;
    }

    public String encode(String plainText) {
        StringBuilder ciphertext = new StringBuilder(plainText.length());

        for (int index = 0; index < Math.min(plainText.length(), key.length()); index++) {
            ciphertext.append(encodeCharacter(plainText, index));
        }

        return ciphertext.toString();
    }

    private char encodeCharacter(String plainText, int index) {
        int alphabetIdx = ALPHABET.indexOf(plainText.toCharArray()[index]) + ALPHABET.indexOf(key.toCharArray()[index]);

        if (alphabetIdx >= ALPHABET.length()) {
            alphabetIdx -= ALPHABET.length();
        }

        return ALPHABET.toCharArray()[alphabetIdx];
    }

    public String decode(String cipherText) {
        StringBuilder plainText = new StringBuilder(cipherText.length());

        for (int i = 0; i < cipherText.length(); i++) {
            plainText.append(decodeCharacter(cipherText, i));
        }

        return plainText.toString();
    }

    private char decodeCharacter(String cipherText, int index) {
        int alphabetIdx = ALPHABET.indexOf(cipherText.toCharArray()[index]) - ALPHABET.indexOf(key.toCharArray()[index]);

        if (alphabetIdx < 0) {
            alphabetIdx += ALPHABET.length();
        }

        return ALPHABET.toCharArray()[alphabetIdx];
    }
}