public class ObjectToJson {
    public static void main(String[] args) throws IOException {
        ObjectMapper objectMapper = new ObjectMapper();
        ClassRoom classRoom1 =  new ClassRoom("Math 101", Arrays.asList(new Person("Bill", 12), new Person("John", 15), new Person("Matt", 10)));
        ClassRoom classRoom2 =  new ClassRoom("English 101", Arrays.asList(new Person("Bob", 13), new Person("Alex", 16), new Person("Matt", 10)));
        ClassRoom classRoom3 =  new ClassRoom("French 101", Arrays.asList(new Person("Jill", 9), new Person("John", 15), new Person("John", 15)));

        //write it
        String classRoomJson = objectMapper.writeValueAsString(new ClassRoom[]{classRoom1, classRoom2, classRoom3});
        System.out.println(classRoomJson);
    }
}