TDD - recruitment task implementation

gistfile1.txt

class FrequencyImpl implements Frequency {

    @Override
    public Map<Character, Double> analyze(String text) {
        if (text == null) {
            throw new IllegalArgumentException();
        }
        text = text.toLowerCase();
        Map<Character, Double> characterFrequency = new HashMap<>();
        long distintChars = text.chars().distinct().count();
        for (Character character: text.toCharArray()) {
            characterFrequency.putIfAbsent(character, 1.0 / distintChars);
        }
        return characterFrequency;
    }

}