The solution to counting the characters in a string (including special characters such as #, $, and %) implies taking each character and comparing them with the rest. During the comparison, the counting state is maintained via a numeric counter that's increased by one each time the current character is found.

There are two solutions to this problem.

The first solution iterates the string characters and uses Map to store the characters as keys and the number of occurrences as values. If the current character was never added to Map, then add it as (character, 1). If the current character exists in Map, then simply increase its occurrences by 1, for example, (character, occurrences+1). This is shown in the following code:

public Map<Character, Integer> countDuplicateCharacters(String str) {

  Map<Character, Integer> result = new HashMap<>();

  // or use for(char ch: str.toCharArray()) { ... }
  for (int i = 0; i<str.length(); i++) {
    char ch = str.charAt(i); 

    result.compute(ch, (k, v) -> (v == null) ? 1 : ++v);
  }

  return result;
}

Another solution relies on Java 8's stream feature. This solution has three steps. The first two steps are meant to transform the given string into Stream<Character>, while the last step is responsible for grouping and counting the characters. Here are the steps:

1. Call the String.chars() method on the original string. This will return IntStream. This IntStream contains an integer representation of the characters from the given string.
2. Transform IntStream into a stream of characters via the mapToObj() method (convert the integer representation into the human-friendly character form).
3. Finally, group the characters (Collectors.groupingBy()) and count them (Collectors.counting()).

The following snippet of code glues these three steps into a single method:

public Map<Character, Long> countDuplicateCharacters(String str) {

  Map<Character, Long> result = str.chars()
    .mapToObj(c -> (char) c)
    .collect(Collectors.groupingBy(c -> c, Collectors.counting()));

  return result;
}