用户角色中的JFrame LOg

Question

我是Java新手，我很难理解登录用户角色的工作原理。这个概念和所有..我写了下面的代码，但它一直直接捕捉错误..如果有人能让我通过，我将不胜感激。

public void actionPerformed(ActionEvent arg0) {
  try {
    String query = "Select * from userdetails where username=? and password=? and role=?";
    PreparedStatement pst = conn.prepareStatement(query);
    pst.setString(1, txtusername.getText());
    pst.setString(2, passwordField.getText());
    pst.setString(3, cmbselect.getSelectedItem().toString());

    ResultSet rs = pst.executeQuery();

    int count = 0;
    while (rs.next()) {
      count = count + 1;
    }
    String role = (cmbselect.getSelectedItem().toString());
    if (role == "ADMIN") {
      JOptionPane.showMessageDialog(null, "WELCOME ADMIN");
      frame.dispose();
      AdminHome admhm = new AdminHome();
      admhm.setVisible(true);
    } else if (role == "NURSE") {
      JOptionPane.showMessageDialog(null, "WELCOME NURSE");
      ManagePatient mp = new ManagePatient();
      mp.setVisible(true);
    } else if (role == "DOCTOR") {
      JOptionPane.showMessageDialog(null, "WELCOME Doctor");
      Main m = new Main();
      m.setVisible(true);
    } else {
      JOptionPane.showMessageDialog(null, "Username and password is INVALID");
    }
  } catch (Exception e) {
    JOptionPane.showMessageDialog(null, e);

  }
}