JavaFx 程序应用程序启动方法中出现异常，使用多个布局构建我的程序 |网格 Pane 和边框 Pane

Question

我知道这个论坛上发布了很多问题，但我仍然无法追踪我的程序的错误。

我的程序的功能与下面发布的代码并不完全相似，但我只是删除了不需要显示的代码。我刚刚包含了 gui 代码，我相信它是导致错误的代码。

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

public class Main extends Application{

    Label username;
    Label password;
    Button submit;
    TextField user;
    PasswordField pass;

    Label numObj;
    Label tri;
    TextField numOpt;
    Button sub;



    public static void main(String[] args){
        launch(args);
    }

    public void start(Stage primaryStage) throws Exception {
        primaryStage.setTitle("Authentication");
        GridPane layout = new GridPane();
        layout.setPadding(new Insets(10,10,10,10));
        layout.setAlignment(Pos.CENTER);
        layout.setVgap(8);
        layout.setHgap(10);
        Scene scene = new Scene(layout, 400,200);
        primaryStage.setScene(scene);
        primaryStage.show();

        username = new Label("username: ");
        GridPane.setConstraints(username, 0,0);
        password = new Label("password: ");
        GridPane.setConstraints(password, 0,1);
        submit = new Button("submit");      
        GridPane.setConstraints(submit, 1,2);
        user = new TextField();
        GridPane.setConstraints(user, 1,0);
        pass = new PasswordField();
        GridPane.setConstraints(pass, 1,1);
        layout.getChildren().addAll(username,password,submit,user,pass);


        BorderPane layout2 = new BorderPane();
        layout2.setPrefSize(500, 350);
        layout2.setPadding(new Insets(10,10,10,10));
        Scene secondScene = new Scene(layout2);

        submit.setOnAction(e -> {
            primaryStage.setScene(secondScene);     
        });

        HBox atTop = new HBox();

        numObj = new Label("Number of Object/s: ");
        numOpt = new TextField("5");
        sub = new Button("submit");

        atTop.getChildren().addAll(numObj, numOpt, sub);
        layout2.getChildren().add(atTop);
        layout2.setTop(atTop);




    }


}

并显示错误

Exception in Application start method
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:389)
    at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.launcher.LauncherHelper$FXHelper.main(Unknown Source)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$155(LauncherImpl.java:182)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.IllegalArgumentException: Children: duplicate children added: parent = BorderPane@5ac4cb88[styleClass=root]
    at javafx.scene.Parent$2.onProposedChange(Parent.java:454)
    at com.sun.javafx.collections.VetoableListDecorator.add(VetoableListDecorator.java:206)
    at javafx.scene.layout.BorderPane$BorderPositionProperty.invalidated(BorderPane.java:680)
    at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:111)
    at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:146)
    at javafx.scene.layout.BorderPane.setTop(BorderPane.java:278)
    at Main.start(Main.java:74)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$162(LauncherImpl.java:863)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$175(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null$173(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$174(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
    ... 1 more
Exception running application Main

Answer 1

删除这个“layout2.getChildren().add(atTop);