Mülakat sorusu olarak soruldu. Geleneksel Yöntem, Collection Yöntem, Lambda Yöntemi.

CharacterOccurences.java

class StringUtil {
    // "baba" a:2, b:2 tane var gibi mesela.
    public static HashMap<Character, Integer> countOccurencesWithHashMapWay(String str)
    {
        char [] strArray = str.toCharArray();

        HashMap<Character, Integer> hm = new HashMap<>();

        for (char ch : strArray)
            if (hm.containsKey(ch))
                hm.put(ch, hm.get(ch) + 1);
            else
                hm.put(ch, 1);
            
        return hm;
    }
}

public static HashMap<Character, Integer> countOccurencesWithHashMapWayLambda(String str)
{

    HashMap<Character, Integer> hm = new HashMap<>();

    str.chars().mapToObj(val -> (char) val).collect(Collectors.toList())
            .stream().forEach(character -> {

                if (hm.containsKey(character))
                    hm.put(character, hm.get(character) + 1);
                else
                    hm.put(character, 1);

    });

    return hm;

}

 public static int [] countOccurencesWithFrequencyArrayTradionatalWay(String str)
{
        int [] result = new int['z' + 1];

        char [] chars = str.toCharArray();

        for (char ch : chars)
            result[ch - 'A']++;

        return result;
}