Jewels and Stones

You're given strings jewels representing the types of stones that are jewels, and stones representing the stones you have. Each character in stones is a type of stone you have. You want to know how many of the stones you have are also jewels.

Letters are case sensitive, so "a" is considered a different type of stone from "A".

    public int numJewelsInStones(String J, String S) {
        int res = 0;
        Set setJ = new HashSet();
        for (char j: J.toCharArray())
            setJ.add(j);
        for (char s: S.toCharArray())
            if (setJ.contains(s)) res++;
        return res;
    }