you have to create the object before you can use it
for (int i = 0; i < data.size(); i++) {
marksReads[i] = new MarksReads();
marksReads[i].setDescription(data(i).get("description"));
marksReads[i].setMark(Integer.valueOf(data.get(i).get("mark")));
marksReads[i].setType(Integer.valueOf(data.get(i).get("type")));
}