How to close current stage when clicking a button to display new stage

Question

So I'm working on a password management application and it is my first attempt at a GUI in Java. I the application to close the login window once the user has clicked the Login button and just display the main application window. Whenever I try to do so my setPrevStage method is storing a NULL value and the prevStage.close() method causes the program to crash with a NullPointerException. Not sure what I am doing wrong so any advice is greatly appreciated. Here is my .java file for this operation.

    package pwmanager;

    import java.io.IOException;
    import java.net.URL;
    import java.util.ResourceBundle;
    import javafx.event.ActionEvent;
    import javafx.event.EventType;
    import javafx.fxml.FXML;
    import javafx.fxml.FXMLLoader;
    import javafx.fxml.Initializable;
    import javafx.scene.Parent;
    import javafx.scene.Scene;
    import javafx.scene.control.*;
    import javafx.scene.layout.AnchorPane;
    import javafx.scene.layout.GridPane;
    import javafx.stage.Stage;


    /**
     *
     * @author 176878
     */
    public class FXMLDocumentController implements Initializable {

    @FXML
    private Button loginButton;


    @FXML
    Stage prevStage;
    Stage currentStage;
    public void setPrevStage(Stage stage){
        prevStage = stage;
        if (stage == null){
            System.out.println("Stage NULL");
        }

       }

    @FXML
    public void getPrevStage(Stage stage){
        currentStage = prevStage;

    }

    @FXML
    public void loginButtonAction(ActionEvent event) throws IOException {
       System.out.println("You clicked me, logging in!");
        setPrevStage(prevStage);

        Stage stage = new Stage();


        FXMLLoader loader = new FXMLLoader(getClass().getResource("MainScreen.fxml"));

        Parent mainScreen = (Parent)loader.load();
        Scene scene = new Scene(mainScreen);

        stage.setScene(scene);
        stage.setTitle("Password Manager");
        prevStage.close();
        stage.show();
    }    

    @Override
    public void initialize(URL url, ResourceBundle rb) {
        // TODO
    }    

}

Answer 1

if in prevStage there is the "old" stage then you try it:

@FXML
public void loginButtonAction(ActionEvent event) throws IOException {
   System.out.println("You clicked me, logging in!");

    FXMLLoader loader = new FXMLLoader(getClass().getResource("MainScreen.fxml"));

    Parent mainScreen = (Parent)loader.load();
    Scene scene = new Scene(mainScreen);

    prevStage.setScene(scene);
    prevStage.setTitle("Password Manager");
    prevStage.show();
}  

If not ... in your Start.class you define static Stage stage and set in this your primarystage read this example:

public class Start extends Application {

    static Stage stage;

    @Override
    public void start(Stage primarystage) {
        stage=primarystage;

        ...

         stage.show();
    }

}

and the code is now this:

@FXML
public void loginButtonAction(ActionEvent event) throws IOException {
   System.out.println("You clicked me, logging in!");

    prevStage=Start.stage;   // Start.class

    FXMLLoader loader = new FXMLLoader(getClass().getResource("MainScreen.fxml"));

    Parent mainScreen = (Parent)loader.load();
    Scene scene = new Scene(mainScreen);

    prevStage.setScene(scene);
    prevStage.setTitle("Password Manager");
    prevStage.show();
}    

Answer 2

i didn't get your question.but i am giving my solution, here's the code may be this can help you

Stage st = (Stage) loginbutton.getScene().getWindow();
            st.hide();
            try 
                {
                  showFxml(filex);
                } 
            catch (Exception exp)
                {
                  System.out.println("file loading problem "+exp);
                }

and here's the showFxml()

public void showFxml(String filen)throws Exception
 {
   FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource(""+filen+""));
   Parent root1 = (Parent) fxmlLoader.load();
   Stage stage = new Stage();
   stage.initModality(Modality.APPLICATION_MODAL);
   stage.setTitle("Password Manager");
   stage.setScene(new Scene(root1)); 
   stage.show();
 }

by using this you can hide your previous stage and bring in front to your current stage.