public class CombinationsGenerator implements Iterator {
    private int n, k;
    private int[] array;

    public CombinationsGenerator(int n, int k) {
      this.n = n;
      this.k = k;

      array = new int[k];

      for (int i = 0; i < k; i++)
        array[i] = n - i;
      array[k - 1]++; // Fix for the first next() call
    }

    @Override
    public boolean hasNext() {
      return array[0] > k;
    }

    @Override
    public int[] next() {
      for (int i = k - 1; i >= 0; i--) {
        if (array[i] > k - i - 1) {
          array[i]--;
          for (int j = i + 1; j < k; j++) {
            array[j] = array[j - 1] - 1;
          }
          break;
        }
      }

      return array;
    }
  }