StreamsCharSummarizer.java

final var totalDigits = string.chars()
                        .filter(Character::isDigit)
                        .count();

final var totalUppercaseChars = string.chars()
                                .filter(Character::isUpperCase)
                                .count();

final var totalLowercaseChars = string.chars()
                                .filter(Character::isLowerCase)
                                .count();

final var totalInvalidChars = string.chars()
                              .filter(character -> !Character.isLetterOrDigit(character))
                              .count();