A VigenereCypher in Java, handling only with Alphabetical Letters and Spaces.

VigenereCypher.java

import java.util.Arrays;
import java.util.List;

public class VigenereCypher {
  
  private static List<Character> alphabet = Arrays.asList('A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z');
  
  public static void main(String[] args) {
    System.out.println(alphabet.indexOf('X') + " " + alphabet.indexOf('K') + " " +alphabet.indexOf('H'));
    System.out.println(encodeString("X", "K"));
    System.out.println(decodeString("H", "K"));
  }
  
  public static String encodeString(String input, String key) {
    // --- Init ---
    System.out.println("Encoding...");
    input = cleanUpString(input.toUpperCase());
    key = cleanUpString(key.toUpperCase());
    System.out.println("Input: " + input); // Print in console for debug
    System.out.println("Key: " + key);
    int indexInKey = 0;
    String output = "";
    // --- Encode ---
    for (Character inputChar : input.toCharArray()) {
      output += encodedChar(inputChar, key.charAt(indexInKey));
      indexInKey++;
      if (indexInKey >= key.length()) indexInKey = 0;
    }
    return output;
  }
  
  public static String decodeString(String input, String key) {
    // --- Init ---
    System.out.println("Decoding...");
    input = cleanUpString(input.toUpperCase());
    key = cleanUpString(key.toUpperCase());
    System.out.println("Input: " + input); // Print in console for debug
    System.out.println("Key: " + key);
    int indexInKey = 0;
    String output = "";
    // --- Decode ---
    for (Character inputChar : input.toCharArray()) {
      output += decodedChar(inputChar, key.charAt(indexInKey));
      indexInKey++;
      if (indexInKey >= key.length()) indexInKey = 0;
    }
    return output;
  }
  
  public static char decodeChar(Character inputChar, Character key) {
    if (Character.isSpaceChar(inputChar)) return inputChar;
    int index = alphabet.indexOf(inputChar) - alphabet.indexOf(key);
    if (index < 0) index += 26;
    return alphabet.get(index % 26);
  }
  
  public static char encodeChar(Character inputChar, Character key) {
    return Character.isSpaceChar(inputChar) ? inputChar : alphabet.get(((alphabet.indexOf(inputChar)
        + alphabet.indexOf(key)) % 26));
  }
  
  private static String cleanUpString(String string) {
    String output = "";
    for (char c : string.toCharArray())
      if (Character.isLetter(c) || Character.isSpaceChar(c)) output += c;
    return output;
  }
}