Remove Duplicates in a String Java

Used a HashSet:

public class RemoveCharacters {

    public static void main(String[] args) {

        String testOne = "Remove Duplicate Characters";

        System.out.println(removeDuplicateCharacters(testOne));

    }

    public static String removeDuplicateCharacters(String str) {

        Set<Character> strUnique = calculateUniqueCharacters(str);

        return strUnique.toString();

    }

    public static Set<Character> calculateUniqueCharacters(String str) {

        Set<Character> uniqueCharacterSet = new HashSet<>();

        str = str.toLowerCase();

        str = str.replaceAll("\\s+", "");

        for (int i = 0; i < str.length(); i++) {

            uniqueCharacterSet.add(str.charAt(i));

        }

        return uniqueCharacterSet;

    }

}