Explanation for different behavior in Vector.set() and ArrayList.set()

Project background aside, I've implemented a table of custom JComboBoxes. Each row of ComboBoxes is exclusive: while each ComboBox has its own model (to allow different selections), each choice can only be selected once per row. This is done by adding a tag to the front of an item when selected and removing it again when deselected. If a user tries to select a tagged item, nothing happens.

However, this only works when using a Vector as the backing for the list of options. I can get the Vector of strings, use either set() or setElementAt(), and boom presto it works.

With an ArrayList instead of a Vector, however, this doesn't work at all. I was under the impression that ArrayLists functioned similarly in that I can retrieve an anonymous ArrayList, change its contents, and all other objects relying on the contents of that ArrayList will update accordingly, just like the Vector implementation does.

I was hoping someone could tell me why this is different, as both Vector and ArrayList implement List a开发者_如何学Pythonnd supposedly should have similar behavior.

EDIT:

Thanks for the prompt responses! All answers refer to synchronization disparities between ArrayList and Vector. However, my project does not explicitly create new threads. Is it possible that this is a synchronization issue between my data and the Swing thread? I'm not good enough with threads to know...

2nd EDIT:

Thanks again everybody! The synchronization between data and Swing answers my question readily enough, though I'd still be interested in more details if there's more to it.

I suspect the difference is due to Vector being thread-safe and ArrayList not. This affects the visibility of changes to its elements to different threads. When you change an element in a Vector, the change becomes visible to other threads instantly. (This is because its methods are synchronized using locks, which create a memory barrier, effectively synchronizing the current state of the thread's memory - including the latest changes in it - with that of other threads.) However, with ArrayList such synchronization does not automatically happen, thus the changes made by one thread may become visible to other threads only later (and in arbitrary order), or not at all.

Since Swing is inherently multithreadedd, you need to ensure that data changes are visible between different (worker, UI) threads.

Vector is synchronized. It uses the synchronized keyword to ensure that all threads that access it see a consistent result. ArrayList is not synchronized. When one thread sets an element of an ArrayList there is no guarantee that another thread will see the update.

Access to Vector elements are synchronized, whereas its not for an ArrayList. If you have different threads accessing and modifying the lists, you will see different behavior between the two.

I don't have time to test this code, and your code sample is still really light (a nice fully functional sample would be more helpful - I don't want to write a full app to test this) but I'm willing to bet that if you wrapped your call to 'setSelectDeselect' (as shown in your pastebin) like this then ArrayList would work as well as Vector:

Runnable selectRunnable = new Runnable() 
{
  public void run()
  {
    setSelectDeselect(cat, itemName, selected);
  }
};

SwingUtilities.invokeLater(selectRunnable);

You're updating your ArrayList in the middle of event processing. The above code will defer the update until after the event is complete. I suspect there's something else at play here that would be apparent from reviewing the rest of your code.