Question

From a sql database, I am reading a table with 2 fields: appName, user. So I may see:

+-------------+
| appA | John |
+-------------+
| appB | Mary |
+-------------+
| appC | Tom  |
+-------------+
| appA | John |
+-------------+
| appA | Mary |
+-------------+

Each of these records is stored as an object of a AppClass with appName and user.
Now, i want to list how many times the apps were run by different users. So :

+-----------------+
| appA | John | 2 |
+-----------------+
| appA | Mary | 1 | 
+-----------------+
| appB | Mary | 1 |  
+-----------------+
| appC | Tom  | 1 | 
+-----------------+

Is it possible do the counting with a HashMap with 2 keys?

Answer 1

Yes, you can have a structure like the following:

Map<String, Map<String, Integer>>
       ^           ^       ^
       appName    user    count

But it’d be much easier if you just fetch the aggregated data directly from the database:

SELECT appName, user, COUNT(*)
FROM table
GROUP BY appName, user;