IteratorForCombination.java

class CombinationIterator {

    private Queue<String> queue;
    
    public CombinationIterator(String characters, int combinationLength) {
        queue = new LinkedList<>();
        generateCombinations(new char[combinationLength], 0, 
                             characters.toCharArray(), 0, combinationLength, queue);
    }
    
    public String next() {
        return queue.poll();
    }
    
    public boolean hasNext() {
        return !queue.isEmpty();
    }
    
    private void generateCombinations(char[] chars, int index, char[] base, int start, int len, Queue<String> combinations) {
        if (index == len) {
            combinations.offer(new String(chars, 0, len));
            return;
        }
        for (int i = start; i < base.length; i++) {
            chars[index++] = base[i];
            generateCombinations(chars, index, base, i + 1, len, combinations);
            index--;
        }
    }
    
}