I have a similar approach but a little different:

static int makeAnagram(String a, String b) { HashMap map = new HashMap<>(); int count = 0; String str; if(a.length()

        else map.put(c, 1);
    }

    for(char c : b.toCharArray()){
        if(!map.containsKey(c))count++;
        else map.put(c, map.get(c)-1);
    }


    ArrayList<Integer> vals = new ArrayList<>(map.values());
    for(int i=0; i<vals.size(); i++){
        if(!vals.get(i).equals(0))count=count+Math.abs(vals.get(i));
    }
     return count;

}

     return count;

}