Java - Set generic table with SQL results

Below you can find the code that I'm using to retrieve a table from a generic SQL statement (the SQL code is inputed by the user in another part of the code). Since I'll be storing more than one table in the future, and the retrieved table will have some format functions applied to it, I'm storing the values in a:

Map<String, Map<String,List<Object>>> tables = new HashMap<String, Map<String,List<Object>>>();

The first Map is a Table, the second is a Column and finally the list hold each line of data.

The column names/order in:

Map<String, TreeMap<Integer, String>> order = new HashMap<String, TreeMap<Integer, String>>();

The Map is tha table, the TreeMap are the column number - column name

The code to create de generic table is:

PreparedStatement stmt = conn.prepareStatement(sql);
ResultSet rs = stmt.executeQuery();
ResultSetMetaData rsmd = rs.getMetaData();
for(int i = 1; i <= rsmd.getColumnCount(); i++){
    String name = rsmd.getColumnLabel(i); //Label vs name. Label is also what's defined by the user in: "as VENDA"
    answer.put(name, null);
    singleOrder.put(i, name);
}

while(rs.next()){
    Iterator answerIt = answer.entrySet().iterator();
    while (answerIt.hasNext()) { //Get Columns in index order to compare with body HashMap (unordered)
        Map.Entry columnNameValue = (Map.Entry)answerIt.next(); //key = columnName, value = List
        String columnName = (String) columnNameValue.getKey();


        List<Object> tmp = answer.get(columnName);
        if (tmp == null) {
            tmp = new ArrayList<Object>();
            answer.put(columnName, tmp);
        }

        Object item = rs.getObject(columnName);
        if(item instanceof Integer){
            item =  ((Integer) item).doubleValue();
        } else if (item instanceof Long){
            item =  ((Long) item).doubleValue();
        } else if (item instanceof BigDecimal){
            item =  ((BigDecimal) item).doubleValue();
        }
        tmp.add(item);

    }

}

Efficiency will be key for this part of my code. How can I improve the table creation? (I think this is a important question for Java coders, so a clear response can serve a pattern for future programmers that have the same difficulty that I'm having now)