public Database(String db, String server, String username, String password) throws Exception{
        String url = "jdbc:mysql://"+server+"/"+db;
        Class.forName("com.mysql.jdbc.Driver");
        connection = DriverManager.getConnection(url,username,password);

        if (connection!=null){
            System.out.println("Conexión a base de datos "+db+" OK\n");
        }
    }