public class Console {

    String sql;
    Statement stmt;
    Connection conn;
    ResultSet rs;

    //Category Total
    public static double num;

    public Console(){
            try{
            Class.forName("com.mysql.jdbc.Driver");
            System.out.println("Connecting to database...");
            conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/database?autoReconnect=true&useSSL=false","user","password");
            stmt = conn.createStatement();
            System.out.println("Connected database successfully...");

            sql = "SELECT sum(a) FROM table";
            while(rs.next()) {
                rs = stmt.executeQuery(sql);
                num = rs.getDouble("sum(a)");
            }
        } catch(Exception e) {
            e.printStackTrace();
        }
    }

    public static void main(String args[]) {
        System.out.println(num);
    }

rs = stmt.executeQuery(...);
while (rs.next()) {
  // and process your results row by row here...
}

sql = "SELECT sum(a) FROM table";
PreparedStatement preparedStatement = connection.prepareStatement(sql);
ResultSet rs = preparedStatement.executeQuery();
while (rs.next()) {
   num = rs.getDouble("sum(a)");
}