在 ArrayList 线程安全上移动 object 的最佳方法是什么？

Question

I'm using an external library in Android that has an ArrayList of objects that is accessed from another thread. My problem is that I'd like to move some objects from this AarrayList in a way that does not cause null exception on the other thread.

So, what I want to do is: I have an ArrayList ABCDEFGH, and I want to move F from its actual position to the first position, so the ArrayList will become FABCDEGH. Note that I can't use swap for that because every other object is shifted right.

A naive implementation is:

list.remove(F);
list.add(0,F);

But this implementation cause null exception on the other thread because the other thread is looping the list by hand-written counted loop, so because of the race condition the thread loop can run between the removal and addition of F. I can't make the ArrayList tread safe because I do not have access to the external library.

Is there a better way to make this movement (from any position to any position of the Arraylist) without the race condition? Note that the size of the ArrayList will not be modified.

Answer 1

If you own the loop and the swap code, you could synchronize both operations on the list.

// code for loop
for (int i = 0; i < length; i++) {
  synchronized(list) {
    letter = list.get(i);
  }
  // whatever else
}

// code for swap
 synchronized(list) {
   list.remove(F);
   list.add(0,F);
 }

Update

I am assuming that you need the loop thread to see the changes you make in your swap/loop code. Is that correct?

If not, they you could simply clone/copy the list for your own use.

List copy = new ArrayList(list);
copy.remove(F);
copy.add(0,F);

Answer 2

Reorder the objects in the ArrayList, but without using add() , remove() or any other method that may alter the size.

It can be done, rather inelegantly, as follows:

Object temp = F;

for (int i = list.indexOf(F); i > 0; i--) {
    list.set(i, list.get(i - 1));
}

list.set(0, temp);

Note, however, that the loop in the other thread may encounter the objects in the wrong order, or the same object at different positions. And if the other thread adds or removes items, then this will not work.

I think if you cannot synchronize the list, you perhaps shouldn't be trying to modify it from your thread.