public String execute() throws SQLException {
    String returnPage = "ERROR";
    Connection connection = null;
    try {
        Class.forName("org.postgresql.Driver");
        connection = DriverManager.getConnection("jdbc:postgresql://127.0.0.1:5432/Dislocation", "user","pass");

        String sql = "SELECT username FROM User_Login WHERE";
        sql+=" username = ? AND password = ?";

        PreparedStatement stmt = connection.prepareStatement(sql);
        stmt.setString(1, this.getUsername());
        stmt.setString(2, this.getPassword());

        ResultSet resultSet = stmt.executeQuery(); // causing error

        while (resultSet.next()) {
            this.username = resultSet.getString(1);
            this.password = resultSet.getString(2);
            returnPage = "SUCCESS";
        }

    } catch (ClassNotFoundException ex) {
        returnPage = "ERROR";
        ex.getMessage();
    } finally {
        if (connection != null) {
            connection.close();
        }
    }
    return returnPage;
}