按钮未出现在GridPane JavaFX8中

Question

I am declaring buttons prior to using them inside the GridPane so I can use the button.setOnAction() function to switch scenes, but the buttons don't appear.

When I create the button inside the gridpane.add() function it works just fine. I've tried using BorderPanes instead and doing the non-lambda version of the EventHandler.

import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.GridPane;
import javafx.stage.Stage;

public class Main extends Application {

    public static Scene scene1;
    public static Scene scene2;
    public static Scene scene3;
    public static Scene[] scenes = new Scene[]{null, scene1, scene2, scene3};

    @Override
    public void start(Stage primaryStage) {
        primaryStage.setTitle("Scene Switcher");

        GridPane gp1 = new GridPane();
        GridPane gp2 = new GridPane();
        GridPane gp3 = new GridPane();

        Button button1 = new Button("scene 1");
        button1.setOnAction(event -> primaryStage.setScene(scene1));
        Button button2 = new Button("scene 2");
        button2.setOnAction(event -> primaryStage.setScene(scene2));
        Button button3 = new Button("scene 3");
        button3.setOnAction(event -> primaryStage.setScene(scene3));

        //
        // SCENE 1
        //
        Label l1 = new Label("this is scene 1");
        BorderPane.setAlignment(l1, Pos.CENTER);
        gp1.add(l1, 0, 0);
        gp1.add(button2, 0, 2);
        gp1.add(button3, 0, 3);
        scene1 = new Scene(gp1, 500, 500);

        //
        // SCENE 2
        //
        Label l2 = new Label("this is scene 2");
        BorderPane.setAlignment(l2, Pos.CENTER);
        gp2.add(l2, 0, 0);
        gp2.add(button1, 0, 2);
        gp2.add(button3, 0, 3);
        scene2 = new Scene(gp2, 500, 500);

        //
        // SCENE 3
        //
        Label l3 = new Label("this is scene 3");
        BorderPane.setAlignment(l3, Pos.CENTER);
        gp3.add(l3, 0, 0);
        gp3.add(button1, 0, 2);
        gp3.add(button2, 0, 3);
        scene3 = new Scene(gp3, 500, 500);

        primaryStage.setScene(scene1);
        primaryStage.show();
    }
}

The buttons don't show at all.

Answer 1

Your Button nodes can only belong to one GridPane at a time. Here, you're adding them to gp2 and gp3 after adding them to gp1 .

You'll need to rethink your design for this Stage as the way you're attempting to do it is not possible.

You can confirm this by commenting out all the code for scenes 2 & 3. The buttons will appear as they should.