Placing HBox in BorderPane throws Exception in start method, JavaFX - javafx

Question

I am trying to make a layout consisting of a TreeView List and a couple of textfields, buttons at the bottom. I figured i'd use a BorderPane to hold my layout in place. But i am getting an Exception in Application start method and it also tells me Children: duplicate children added: parent = BorderPane#71e344e8
Do you guys have an idea of whats going on?
public class Main extends Application {
Stage window;
TextField nameInput, quantityInput;
TreeView<String> tree;
public static void main(String[] args) {
launch(args);
}
public void start (Stage primaryStage) throws Exception{
window = primaryStage;
window.setTitle("GrocerieList");
// tree view
TreeItem<String> root, food, drink;
root = new TreeItem<>();
root.setExpanded(true);
// food
food = makeBranch("Food",root);
makeBranch("Banana", food);
makeBranch("Coconut", food);
makeBranch("Eggs", food);
// drink
drink = makeBranch("Drinks",root);
makeBranch("Water", drink);
makeBranch("Fanta", drink);
makeBranch("Beer", drink);
// tree
tree = new TreeView<>(root);
tree.setShowRoot(false);
// bottom input fields
nameInput = new TextField();
nameInput.setPromptText("Productname");
nameInput.setMinWidth(100);
quantityInput = new TextField();
quantityInput.setPromptText("Quantity");
quantityInput.setMinWidth(100);
// buttons
Button addbutton = new Button("Add");
// Layout
// HBOX
HBox hBox = new HBox();
hBox.setPadding(new Insets(10));
hBox.setSpacing(10);
hBox.getChildren().addAll(nameInput, quantityInput, addbutton);
// BorderPane
BorderPane borderPane = new BorderPane();
borderPane.getChildren().addAll(tree, hBox);
borderPane.setCenter(tree);
borderPane.setBottom(hBox);
Scene scene = new Scene(borderPane, 520, 620);
window.setScene(scene);
window.show();
}
public TreeItem<String> makeBranch(String title, TreeItem<String> parent){
TreeItem<String> item = new TreeItem<>(title);
item.setExpanded(true);
parent.getChildren().add(item);
return item;
}
Complete error:
Exception in Application start method
java.lang.reflect.InvocationTargetException
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:567)
at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:464)
at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:363)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:567)
at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:1051)
Caused by: java.lang.RuntimeException: Exception in Application start method
at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:900)
at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:195)
at java.base/java.lang.Thread.run(Thread.java:830)
Caused by: java.lang.IllegalArgumentException: Children: duplicate children added: parent = BorderPane#71e344e8
at javafx.graphics/javafx.scene.Parent$3.onProposedChange(Parent.java:560)
at javafx.base/com.sun.javafx.collections.VetoableListDecorator.add(VetoableListDecorator.java:206)
at javafx.graphics/javafx.scene.layout.BorderPane$BorderPositionProperty.invalidated(BorderPane.java:692)
at javafx.base/javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:112)
at javafx.base/javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:147)
at javafx.graphics/javafx.scene.layout.BorderPane.setCenter(BorderPane.java:268)
at sample.Main.start(Main.java:70)
at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:846)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:455)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
Exception running application sample.Main

Answer 1

Your problem is here:
BorderPane borderPane = new BorderPane();
borderPane.getChildren().addAll(tree, hBox);
borderPane.setCenter(tree);
borderPane.setBottom(hBox);
The calls to setCenter and setBottom add those nodes to the children list. Since you've manually added the two nodes to the children list, those method calls lead to adding the same node to the same parent twice—that's not allowed. When it comes to layouts that provide special properties for positioning, such as BorderPane, you want to avoid interacting with the children list directly. Change your code to:
BorderPane borderPane = new BorderPane();
borderPane.setCenter(tree);
borderPane.setBottom(hBox);