ADDITIONAL SYSTEM INFORMATION :
openjdk 13.0.1 2019-10-15
OpenJDK Runtime Environment (build 13.0.1+9)
OpenJDK 64-Bit Server VM (build 13.0.1+9, mixed mode, sharing)

javafx.version=13.0.1
javafx.runtime.version=13.0.1+1
javafx.runtime.build=1


A DESCRIPTION OF THE PROBLEM :
The VetoableListDecorator class does not override the default implementation of List#sort(Comparator). The default implementation copies the elements into an array, sorts the array, and then calls List#set in a loop to sort the List **without removing the old elements first**. This leads to the VetoableListDecorator intercepting the call to #set and seeing a "duplicate child" being added.

Since sorting a list does not add or remove elements the VetoableListDecorator throwing the IllegalArgumentException does not make sense. Sorting the children of a Parent should be supported.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Add some children to a Parent.
2. Sort the children by an attribute (e.g. the text of a Label).

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The children list is sorted without any problems.
ACTUAL -
An exception is thrown -> Caused by: java.lang.IllegalArgumentException: Children: duplicate children added: parent = ...

---------- BEGIN SOURCE ----------
import javafx.application.Application;
import javafx.scene.control.Label;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

import java.util.Comparator;

public class App extends Application {

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

  @Override
  public void start(Stage primaryStage) {
    Label label1 = new Label("1");
    Label label2 = new Label("2");

    HBox box = new HBox(5, label2, label1);

    box.getChildren()
        .sort(Comparator.comparing(node -> ((Label) node).getText()));
  }

}
---------- END SOURCE ----------

FREQUENCY : always