#279 Поиск первого неповторяющегося символа в строке

Найти в строке первую неповторяющуюся букву:

public class First {
    //"cba" => c
    //"abc" => a
    //"abca" => b
    public static void main(String[] args) {
        checkResult("cba", 'c');
        checkResult("abc", 'a');
        checkResult("abca", 'b');
        checkResult("abcab", 'c');
        checkResult("abcabcd", 'd');
    }

    public static Character getFirstUnique(String st) {
        ...
    }


    private static void checkResult(String input, Character expectedResult) {
        Character actualCharacter = getFirstUnique(input);
        if (Objects.equals(expectedResult, actualCharacter)) {
            System.out.println(actualCharacter + " OK");
        } else {
            System.out.println("Check failed! Input '" + input + "'; Result'" + actualCharacter + "'; Expected '" + expectedResult + "'!");
        }
    }
}