java.util.ConcurrentModificationException in javafx timeline

Question

I haven't made Thread myself. I have one timeline that runs from the beginning to the end of the program which is as follows:

Timeline timeline = new Timeline(
        new KeyFrame(Duration.millis(TIMELINE_DELAY), event -> {
            intrudersList.forEach(Intruder::action);
            towersList.forEach(Tower::action);
            otherActivesList.forEach(Active::action);
        }));
timeline.setCycleCount(Timeline.INDEFINITE);
timeline.play();

And when the die method of the Intruder class is called, i get this Concurrent Modification Exception.

First, I don't actually understand how timeline works! Does it create new Threads itself or what? and what will happen if for example we have a timeline that does a task every 10 seconds and that task takes 15 seconds to be done! And second indeed: How can i get this fixed!?

public void die() {
    this.getCell().getContent().remove(this);
    TimeLine.getInstance().removeIntruder(this);
    System.out.println("death of intruder at: " + cell);
}

Simply wrap the content of `die` into a Platform.runLater(() -> { ... }) block. — DVarga, Jul 06 '16 at 15:11
Maybe you are trying to edit a List while you iterate over it? What is `Intruder::action` doing? — aw-think, Jul 06 '16 at 19:20
Please post the stack trace. Which line is actually throwing the exception? The `Timeline` does not create any new threads, and the event handler is executed on the (already running) FX Application Thread. There's no reason at all to believe this is a threading issue. — James_D, Jul 06 '16 at 20:18

score 1 · Answer 1 · answered Jul 07 '16 at 09:38

Check out the docs. This exceptions typically happens when a list is modified while that is not allowed.

This can occur when 2 different threads are trying to modify the same list concurrently, but more often it means you are trying to modify a list while iterating over it (which is not allowed).

score 0 · Accepted Answer · answered Jul 08 '16 at 16:39

Many thanks to @DVarga, This is what the problem was and how i fixed it: the problem was that I was modifying the intrudersList in removeIntruder , while iterating on it in KeyFrame. And I solved it by wrapping the body of the removeIntruder method in a Platform.runLater(() -> { ... }) block. I Guess what Platform.runLater does is that it waits until the list is modifyable and then modifies it.

java.util.ConcurrentModificationException in javafx timeline

2 Answers2