[java] Thread problem: "java.util.ConcurrentModificationException"

Wherever you are accessing the ArrayList do this (where theList is your ArrayList):

...
synchronized( theList )
{
//loop through the list
}
...

...
synchronized( theList )
{
//Add an element to the list
}
...

etc.

That way, only one section of code will be allowed to use the ArrayList at a time.

If responsiveness for adding and removing items is a priority, then you will have to clone the list in a sychronized block. After you have cloned the list, release the lock and print it out so adding & removing can continue elsewhere.

Cheers,
Brett

If I'm not mistaken, you can also create a synchronized List using a Collections' class method, synchronizedList().

You should look into and understand the need for the java.util.concurrent package.

Lizard

Thanks dudes!
I'll take a look at these three answers...

Basically you can not add/remove when you are iterating over a collection that is not synchronized such as the ArrayList.

You can use an Iterator instance to do the removal:

Iterator it = list.iterator();while ( it.hasNext() ) {  it.remove();}

and with a ListIterator you can add...

Quote:

Original post by Son of Cain You can use an Iterator instance to do the removal: Iterator it = list.iterator();while ( it.hasNext() ) { it.remove();}

But what happens if I try to "it.remove()" while my thread is reading the arraylist?

You'll want to put a synchronized block around your iteration loop. E.g.

synchronized (list){  for (Iterator it = list.iterator(); it.hasNext();)  {    it.remove();  }}

because modifying the list from another thread will invalidate the iterator.