Returning a ResultSet

You should never pass a ResultSet around through public methods. This is prone to resource leaking because you're forced to keep the statement and the connection open. Closing them would implicitly close the result set. But keeping them open would cause them to dangle around and cause the DB to run out of resources when there are too many of them open.

Map it to a collection of Javabeans like so and return it instead:

public List<Biler> list() throws SQLException {
    Connection connection = null;
    PreparedStatement statement = null;
    ResultSet resultSet = null;
    List<Biler> bilers = new ArrayList<Biler>();

    try {
        connection = database.getConnection();
        statement = connection.prepareStatement("SELECT id, name, value FROM Biler");
        resultSet = statement.executeQuery();

        while (resultSet.next()) {
            Biler biler = new Biler();
            biler.setId(resultSet.getLong("id"));
            biler.setName(resultSet.getString("name"));
            biler.setValue(resultSet.getInt("value"));
            bilers.add(biler);
        }
    } finally {
        if (resultSet != null) try { resultSet.close(); } catch (SQLException ignore) {}
        if (statement != null) try { statement.close(); } catch (SQLException ignore) {}
        if (connection != null) try { connection.close(); } catch (SQLException ignore) {}
    }

    return bilers;
}

Or, if you're on Java 7 already, just make use of try-with-resources statement which will auto-close those resources:

public List<Biler> list() throws SQLException {
    List<Biler> bilers = new ArrayList<Biler>();

    try (
        Connection connection = database.getConnection();
        PreparedStatement statement = connection.prepareStatement("SELECT id, name, value FROM Biler");
        ResultSet resultSet = statement.executeQuery();
    ) {
        while (resultSet.next()) {
            Biler biler = new Biler();
            biler.setId(resultSet.getLong("id"));
            biler.setName(resultSet.getString("name"));
            biler.setValue(resultSet.getInt("value"));
            bilers.add(biler);
        }
    }

    return bilers;
}

By the way, you should not be declaring the Connection, Statement and ResultSet as instance variables at all (major threadsafety problem!), nor be swallowing the SQLException at that point at all (the caller will have no clue that a problem occurred), nor be closing the resources in the same try (if e.g. result set close throws an exception, then statement and connection are still open). All those issues are fixed in the above code snippets.

If you don't know what you want of the ResultSet on retrieving time I suggest mapping the complete thing into a map like this:

    List<Map<String, Object>> resultList = new ArrayList<Map<String, Object>>();
    Map<String, Object> row = null;

    ResultSetMetaData metaData = rs.getMetaData();
    Integer columnCount = metaData.getColumnCount();

    while (rs.next()) {
        row = new HashMap<String, Object>();
        for (int i = 1; i <= columnCount; i++) {
            row.put(metaData.getColumnName(i), rs.getObject(i));
        }
        resultList.add(row);
    }

So basically you have the same thing as the ResultSet then (without the ResultSetMetaData).

Well, you do call rs.close() in your finally-block.

That's basically a good idea, as you should close all your resources (connections, statements, result sets, ...).

But you must close them after you use them.

There are at least three possible solutions:

1. don't close the resultset (and connection, ...) and require the caller to call a separate "close" method.

   This basically means that now the caller needs to remember to call close and doesn't really make things easier.

2. let the caller pass in a class that gets passed the resultset and call that within your method

   This works, but can become slightly verbose, as you'll need a subclass of some interface (possibly as an anonymous inner class) for each block of code you want to execute on the resultset.

   The interface looked like this:

   public interface ResultSetConsumer<T> {
     public T consume(ResultSet rs);
   }
   

   and your select method looked like this:

   public <T> List<T> select(String query, ResultSetConsumer<T> consumer) {
     Connection con = null;
     Statement st = null;
     ResultSet rs = null;
   
       try {
         con = DriverManager.getConnection(url, user, password);
         st = con.createStatement();
   
         rs = st.executeQuery(query);
         List<T> result = new ArrayList<T>();
         while (rs.next()) {
             result.add(consumer.consume(rs));
         }
       } catch (SQLException ex) {
         // logging
       } finally {
         try {
           if (rs != null) {
               rs.close();
           }
           if (st != null) {
               st.close();
           }
           if (con != null) {
               con.close();
           }
         } catch (SQLException ex) {
           Logger lgr = Logger.getLogger(MySQL.class.getName());
           lgr.log(Level.WARNING, ex.getMessage(), ex);
         }
       }
     return rs;
   }
   

3. do all the work inside the select method and return some List as a result.

   This is probably the most widely used one: iterate over the resultset and convert the data into custom data in your own DTOs and return those.