Purpose

This problem comes from this dailyProgrammer subreddit challenge.

The task is to take an input of C / cs and V / vs and to replace Cs with random consonants and Vs with random vowels (English alphabet) while keeping the case the same. Y is not considered a vowel in this case.

Example:

• CcVvv => BgAoi
• ccVVV => zqUUE

Implementation

For this case, I've put all methods in the same class - in reality, I might split these methods out into different classes.

public class RandomCharacterReplacer {
  public static final char C = 'C';
  public static final char V = 'V';
  public static final List<Character> CONSONANTS = new ArrayList<>(
      Arrays.asList('B', 'C', 'D', 'F', 'G', 'H', 'J', 'K', 'L', 'M', 'N', 'P', 'Q', 'R', 'S', 'T', 'V', 'W', 'X', 'Y', 'Z')
  );
  public static final List<Character> VOWELS = new ArrayList<>(
      Arrays.asList(
          'A', 'E', 'I', 'O', 'U'
      )
  );

  private static boolean isValidCharacter(final char c) {
    final char upperCaseChar = Character.toUpperCase(c);
    return upperCaseChar == C || upperCaseChar == V;
  }

  public static AlphabetCharacterCase returnAlphabetCharacterCase(final char c) {
    if (!Character.isAlphabetic(c)) {
      throw new RuntimeException("character is non-alphabetic");
    }

    if (Character.isUpperCase(c)) {
      return AlphabetCharacterCase.UPPER;
    }

    if (Character.isLowerCase(c)) {
      return AlphabetCharacterCase.LOWER;
    }

    throw new RuntimeException("unexpected character case");
  }

  public static AlphabetCharacterType returnAlphabetCharacterType(final char c) {
    if (!Character.isAlphabetic(c)) {
      throw new RuntimeException("character is non-alphabetic");
    }

    final char upperCaseChar = Character.toUpperCase(c);
    if (upperCaseChar == C) {
      return AlphabetCharacterType.CONSONANT;
    }

    if (upperCaseChar == V) {
      return AlphabetCharacterType.VOWEL;
    }

    throw new RuntimeException("unexpected character type");
  }

  public static char returnRandomCharacter(final List<Character> characterList) {
    if (characterList.isEmpty()) {
      throw new IllegalArgumentException("character list must be non-empty");
    }
    final Random random = new Random();
    return characterList.get(random.nextInt(characterList.size() - 1));
  }

  public static List<Character> returnAlphabetCharacterTypeList(final AlphabetCharacterType characterType) {
    switch (characterType) {
      case CONSONANT: {
        return CONSONANTS;
      }

      case VOWEL: {
        return VOWELS;
      }

      default: {
        throw new RuntimeException("unexpected character type");
      }
    }
  }


  public static char generateRandomCharacter(final AlphabetCharacterCase alphabetCharacterCase, final AlphabetCharacterType characterType) {
    final List<Character> characters = returnAlphabetCharacterTypeList(characterType);
    switch (alphabetCharacterCase) {
      case UPPER: {
        return returnRandomCharacter(characters);
      }

      case LOWER: {
        return Character.toLowerCase(returnRandomCharacter(characters));
      }

      default: {
        throw new RuntimeException("unexpected character case");
      }
    }
  }

  public static String generateRandomString(final String input) {
    final char[] chars = input.toCharArray();
    final StringBuilder stringBuilder = new StringBuilder();
    for (final char c : chars) {
      if (isValidCharacter(c)) {
        final AlphabetCharacterCase alphabetCharacterCase = returnAlphabetCharacterCase(c);
        final AlphabetCharacterType alphabetCharacterType = returnAlphabetCharacterType(c);
        stringBuilder.append(generateRandomCharacter(alphabetCharacterCase, alphabetCharacterType));
      }
    }

    return stringBuilder.toString();
  }
}