CopyOnWriteArrayList

Programming Questions

kachilous

in Programming Questions • 1 year ago

I've run into a major issue with my program where I occasionally get a ConcurrentModificationException.

One possible reason is that when the user clicks the erase button (which removes all point objects from the ArrayList), the program could be concurrently iterating through that same ArrayList in order to draw a point to the screen.

One fix that I have seen is to use CopyOnWriteArrayList. However, when I try creating a CopyOnWriteArrayList, I get the error that it cannot find a type or class named as this.

I even imported java.util.

Does Processing not support the use of CopyOnWriteArrayList?

Is there a way to get around this?

Also, for added detail...

Here are the possible operations that can be done on my ArrayList in my program.

Add a newly calculated point object to ArrayList

Draw point objects in ArrayList to the screen using an Iterator

Remove points from ArrayList if user does not want previous points to still appear on screen using an Iterator

User presses erase button which removes all point objects from ArrayList. This is done so using an iterator

The only time I get the ConcurrentModificationException is when I click the erase button.

And the error is highlighted in the block of code that draws a point to the screen.

Code Segment:

for(Iterator<Point> it = PointArray.iterator(); it.hasNext();)

{

Point pa = it.next(); //This is the line that is highlighted

drawPoint(pa);

}

Any suggestions in fixing this would be great.

Replies(3)

PhiLho

Re: CopyOnWriteArrayList

1 year ago

1) CopyOnWriteArrayList is in java.util.concurrent. It is useful only if you have different threads accessing the same list.
2) " removes all point objects from ArrayList. This is done so using an iterator" You can use arrayList.clear().
3) " for(Iterator<Point> it = PointArray.iterator(); it.hasNext();) Point pa = it.next();" Old fashioned. You can use: for (Point pa : PointArray) if you declared a typed array list.
I don't see how drawPoint() erases the element. Nor why you have such error.

kachilous

Re: CopyOnWriteArrayList

1 year ago

It works now. CopyOnWriteArrayList fixed the problem. :)

The reason I was getting the exception is because my program is multi-threaded. Various threads are trying to modify/access the same array list, and occasionally these threads may be trying to modify the array list at the same time. Hence why I was sometimes getting the ConcurrentModificationException.

I saw that the use of iterators could help to fix the problem, but CopyOnWriteArrayList is a solution that guarantees no concurrent reads or writes to the collection.

And also drawPoint() is a method that calls the point method to draw a point to the screen.

Thanks again.

Jesse Sco..

Re: CopyOnWriteArrayList

1 year ago

I just ran into this same problem, and CopyOnWriteArrayList works perfectly - thanks for the heads up PhiLho !

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