Your query doesn't look right to me.
String query = "select sum(Entree) from Argent where date between \"strdatedebut\" and \"strdatefin\" ;";
This looks like it will send the two literal strings strdatedebut and strdatefin rather than inserting variables. Try this?
String query = "select sum(Entree) from Argent where date between '" + strdatedebut + "' and '" + strdatefin + "' ;";
Note that's single quote around the insertion of the variables holding the date.
BTW one way that you can debug these things is to copy the contents of query to the clipboard and paste it into a query window. When you try to run it you might get an error in the results window.