Aufruf:

Student std = (Student) tableview.getSelectionModel().getSelectedItem();
        // Dialog zum Ändern
EditStage editstage = new EditStage(stage, std);
editstage.showAndWait();
if (editstage.retcode) {
}


public class EditStage extends Stage {

   private Student std=null;
   public boolean retcode=false;

   private   TextField tLastname = new TextField("") ;
   private   TextField tFirstname = new TextField("") ;

   public EditStage (Stage parent, Student std) {
     this.std=std;
     setGUI();
     setResizable(true);
     this.initOwner(parent);
     initModality(Modality.WINDOW_MODAL);   // javafx.stage.Modality
     this.setTitle("edit student");        

   }


   private void setGUI() {
            GridPane gridpane = new GridPane();       
            gridpane.setVgap(25);
            gridpane.setHgap(15);
            gridpane.setGridLinesVisible(true);
            gridpane.setPadding(new Insets(10) );  // TRBL

           ColumnConstraints column1 = new ColumnConstraints();
           //column1.setPercentWidth(0);
           ColumnConstraints column2 = new ColumnConstraints();
           column2.setPercentWidth(100);
           gridpane.getColumnConstraints().addAll(column1, column2); 

            Label label = new Label("Bearbeiten eines Studenten") ;
            label.setFont(new Font("Courier New",32));
            label.setTextFill(Color.BLUE);
            gridpane.add(label, 0, 0,2,1); // column=1 row=0


            label = new Label("Nachname") ;
            label.setFont(new Font("Courier New",22));
            gridpane.add(label, 0, 1); // column=1 row=0

            gridpane.add(tLastname, 1, 1); // column=1 row=0
            gridpane.setHgrow(tLastname, Priority.ALWAYS);

             // -----------------------------------------------------

            label = new Label("Vorname") ;
            gridpane.add(label, 0, 2); // column=1 row=0

            tFirstname.setFont(new Font("Courier New",22));
            gridpane.add(tFirstname, 1, 2); // column=1 row=0
            gridpane.setHgrow(tFirstname, Priority.ALWAYS);


             // -----------------------------------------------------

            FlowPane boxpane = new FlowPane(20,20);    
            boxpane.setBorder(new Border(new BorderStroke(Color.RED, 
            BorderStrokeStyle.SOLID, CornerRadii.EMPTY, BorderWidths.DEFAULT)));
            boxpane.setAlignment(Pos.CENTER);
            boxpane.setMaxWidth(Double.POSITIVE_INFINITY);
            gridpane.add(boxpane, 0, 5,2,1); // column=1 row=0
            gridpane.setHalignment(boxpane, HPos.CENTER);


            Button bnOk = new Button("Ok") ;
            bnOk.setFont(new Font("Courier New",22));            
            bnOk.setMaxWidth(Double.POSITIVE_INFINITY);
            bnOk.setOnAction(e->okClick());
            boxpane.getChildren().add(bnOk);
            boxpane.setMargin(bnOk, new Insets(0, 10, 0, 10) );  // TRBL

            Button bnEsc = new Button("Esc") ;
            bnEsc.setFont(new Font("Courier New",22));            
            bnEsc.setMaxWidth(Double.POSITIVE_INFINITY);
            bnEsc.setOnAction(e->escClick());
             boxpane.getChildren().add(bnEsc);
            boxpane.setMargin(bnEsc, new Insets(0, 10, 0, 10) );  // TRBL


            tLastname.setText(std.getLastname());
            tFirstname.setText(std.getFirstname());
            Scene scene= new Scene(gridpane, 600, 410);
            this.setScene(scene);        
      }

      private void okClick() {
         String lastname = tLastname.getText().trim();
         if (lastname.length()==0) {
           Basis.errorBox("Bitte geben Sie einen Nachnamen ein","Hinweis");
           tLastname.requestFocus();
           return;
         }
         String firstname = tFirstname.getText().trim();
         if (firstname.length()==0) {
           Basis.errorBox("Bitte geben Sie einen Vornamen ein","Hinweis");
           tFirstname.requestFocus();
           return;
         }

         retcode=true;
         close();
      } // bnOk
 
      private void escClick() {
         close();
      }

}