I am not unable to find the reason why my program send exception

Question

I am trying to get the username after logged in from the text field and pass to another scene by instantiated a new object of the "logincontroller" class, but it shows the "NullPointerException" error. In the first class "Logincontroller" I managed to print the username by calling the method "CurrentUsername()" , the same method is called in another class "AddDocument" but doesn't work . First , is it correct to store the username in from the username text field and access it in another scence ? is there a better way to access username in another scence after logged in ? I wonder, and I would be very gratefull if anyone can help me.

public class LoginController implements Initializable {
@FXML
    private TextField txtUsername;
    @FXML
    private TextField txtPassword;
    @FXML
    .
    .
    .
     @FXML
    public void loginaction (ActionEvent event){
        try {

            if (this.loginmodel.isLogin(this.txtUsername.getText(),this.txtPassword.getText(),((Option)this.RoleSelection.getValue()).toString())){

                Stage stage = (Stage)this.btnLogin.getScene().getWindow();
                stage.close();

                switch (((Option)this.RoleSelection.getValue()).toString()){

                    case "Employee" :
                        EmployeeLogin ();
                        break;
                    case "Student":
                       StudentLogin();
                        break;
                    case "Admin":
                        AdminLogin();
                        System.out.println(CurrentUsername()); // Here it has value and shows the username 
                        break;
                }

            }

            else {this.loginStatus.setText("Der Benutzername oder das Kenntwort ist nicht Korrekt");}
        }
        catch (Exception localException){
            // System.out.print("test failed");
        }
    }


   public String CurrentUsername() {
             String username = txtUsername.getText();
              return  username;
    }

}



   public Class AddDocument {

        @FXML
        private TextField DocumentId;
        @FXML
        private TextField DocumentName;
        @FXML
        private TextField Date;
          .
          .
          .
        // for the username I created a new object from the class where there is the text field "txtUsername.getText()" 

    public static void main(String[] args) {
           LoginController loginController = new LoginController ();

          String User = loginController.CurrentUsername();

          System.out.println(User); // here it it doesn't show the value,  but an exception Caused by: java.lang.NullPointerException

        }

       }

Answer 1

I feel txtUsername is null due to which you are facing NPE.

How and where you are setting txtUsername?

Try changing your method CurrentUsername() to,

public String CurrentUsername() {
    if (txtUsername != null) {
        return txtUsername.getText();
    else {
        return null;
    }
}

This way at least you won't have NPE. Then see where and how to set txtUsername.

Answer 2

You haven't instantiated either of the TextField objects, therefore when you call a method on the object 'txtUsername' from within the CurrentUsername method, a NullPointerException is thrown.

One solution to your problem could be to instantiate the declared objects in a constructor for the LoginController class, such as:

public LoginController() {
    txtUsername = new TextField();
    txtPassword = new TextField();
}

Answer 3

You are calling the controller, and then inmediately calls for the user, wich is null.

 LoginController loginController = new LoginController ();

 String User = loginController.CurrentUsername();

Edit:

I would just create a User class with the attributes, getters and setters and instantiate it from the Main class

public class Person {
    private String userName;
    private String someUsefulValue;

    public String getUserName() {
        return userName;
    }
    public void setUserName(String userName) {
        this.userName = userName;
    }
    public String getSomeUsefulValue() {
        return someUsefulValue;
    }
    public void setSomeUsefulValue(String someUsefulValue) {
        this.someUsefulValue = someUsefulValue;
    }
    @Override
    public String toString() {
        return "Person [userName=" + userName + ", someUsefulValue=" + someUsefulValue + "]";
    }

Then, in your main class, use it:

Person user = new Person();

    public String getCurrentUser() {
        return user.getUserName();
    }

    public void setUser(Person p) {
        this.user = p;
    }

Inside your Login controller, you need to have a reference to the mainApp, something like:

  // Reference to the main application.
    private MainApp mainApp;


     /**
     * Called to give a reference back to itself.
     */
    public void setMainApp(MainApp mainApp) {
        this.mainApp = mainApp;
    }

When you are loading the Controller and the FXML, you set the mainApp reference:

LoginController controller = loader.getController();
controller.setMainApp(this);

Finally, when you log in your app, if it's OK, just store the User in the mainApp User:

    Person loginUser = new Person();
    loginUser.setUserName(txtUserName.getText());
    loginUser.setSomeUsefulValue(txtBlablabla.getText());
    mainApp.setUser(loginUser);

That's it, now you can call the User data from anywhere using the getCurrentUser() in the main Class.

I hope this can help you.