JavaFX ObservableList remove element in listener throws exception

Question

When I attach listener to observable list and in that listener I try to remove some element, in some cases it passes and in some cases it crashes.

Scenario: Item is removed from a list. It triggers listener, and in that listener I try to delete another item.

1. If in listener I try to remove element that is not just next to initially removed, it works OK.
2. If in listener I try to remove element that is JUST NEXT to initially removed, it crashes with UnsupportedOperationException!!!

Did anybody had similar problem? Do you have any tips, suggestions or workaround?

I know that you can delete both at the same time, but problem is that in listener I need to detect which items I need to remove also so I delete i there.

Is this a bug in ObservableList?

I would expect that it always work, or at least to always crash.

Here is code example:

import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.collections.ListChangeListener;
import javafx.collections.ObservableList;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class ListRemoveFromListener extends Application {

    boolean changing = false;

    @Override
    public void start(Stage primaryStage) throws Exception {
        VBox vbox = new VBox();

        Button buttonSuccess = new Button("remove success");
        buttonSuccess.setOnAction(e -> {
            removeSuccess();
        });

        Button buttonBreak = new Button("Remove breaks");
        buttonBreak.setOnAction(e -> {
            removeBreaks();
        });

        vbox.getChildren().addAll(buttonSuccess, buttonBreak);

        Scene scene = new Scene(vbox);

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

    /**
     * If we try in listener to remove element just next to one that was
     * initially removed, exception is thrown.
     */
    private void removeBreaks() {
        ObservableList<String> list = FXCollections.observableArrayList();
        list.add("first");
        list.add("second");
        list.add("third");
        list.add("fourth");
        list.add("fifth");
        list.add("sixth");

        list.addListener(new ListChangeListener<String>() {

            @Override
            public void onChanged(Change<? extends String> c) {
                list.remove("second");
            }
        });

        list.remove("third");
    }

    /**
     * If we try in listener to remove element that is not just next to initially
     * removed element, element is removed and all works O.
     */
    private void removeSuccess() {
        ObservableList<String> list = FXCollections.observableArrayList();
        list.add("first");
        list.add("second");
        list.add("third");
        list.add("fourth");
        list.add("fifth");
        list.add("sixth");

        list.addListener(new ListChangeListener<String>() {

            @Override
            public void onChanged(Change<? extends String> c) {
                list.remove("fifth");
            }
        });

        list.remove("third");
    }

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

}

Answer 1

ListChangeListener.Change documentation states:

the source list cannot be modified inside the listener

You can work around this using a Platform.runLater call to schedule the additional change to be performed at some time in the future:

list.addListener((ListChangeListener<String>) c -> {
    if (list.contains("second")) {
        Platform.runLater(() -> list.remove("second"));
    }
});

be careful when doing so that you don't cause a cascading infinite loop of changes.