Question

The situation occurs when getting some data from the database to compose a report that is grouped by various fields. For example, grouped by month, then type and then actual vs prediction. The database query returns simply a List< Object[] >. But it is boring to directly render it as an HTML table. What I do is to group the data inside a Map<Integer, Map<String, Map<Type, Integer>>>. It can be done in a straightforward way and if I use TreeMap‘s, the data is automatically sorted for me. The resulting JSP code is much simpler.

Is there any problem with this approach? (memory, speed, or something else?)

Answer 1

The HTML renderer shouldn’t have to know the implementation details of you DAO.

I think for you DAO that returning List<Object[]> is more simple than returning Map<Integer, Map<String, Map<Type, Integer>>>. Every class interface should be easy to understand, so I would use List<Object[]>.

If it’s more straightforward to render the HTML table using Map<Integer, Map<String, Map<Type, Integer>>> maybe is better to create a method to convert List<Object[]> to Map<Integer, Map<String, Map<Type, Integer>>>.

This way you keep your UI detached from your database classes.

You also asked about memory and speed. I think the best approach is to test both situations and use a profiler to measure if this is really an issue.