Iterator for Combination

class CombinationIterator {
    Queue<String> q = new LinkedList<>();
    public CombinationIterator(String characters, int combinationLength) {
        combos(characters, "",0 ,combinationLength);
    }

    public void combos(String chars, String build, int s, int ix) {
        if (ix == 0) {
            q.add(build);
            return;
        }
        for(int i = s; i < chars.length(); i++) {
            combos(chars,build + chars.charAt(i), i + 1, ix - 1);
        }
    }
    
    public String next() {
        return q.poll();
    }
    
    public boolean hasNext() {
        return !q.isEmpty();
    }
}

/**
 * Your CombinationIterator object will be instantiated and called as such:
 * CombinationIterator obj = new CombinationIterator(characters, combinationLength);
 * String param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */