Problems with ListIterator and Concurrent Modification Exception

Question

I have two ArrayLists, each holding blocks of certain size: blockList, eraserList. Blocks are objects with two fields: start and end. I need to subtract one set of blocks from the other set of blocks.

I must walk through the eraserList and "erase" blocks out of the blockList where they overlap. Thus my code looks like:

 void eraseBlocks (Arrylist<Blocks> blockList, ArrayList<Blocks> eraserList) {
    ListIterator<Blocks> it = blockList.listIterator();

    for (Blocks eraser: eraserList) {
        while (it.hasNext()) {
            Blocks block= it.next();
            if ((eraser.start <= block.start) && (eraser.end >= block.end))
                 blockList.remove(block);
            else if ((eraser.start <= block.start) && (eraser.end < block.end)){
                 block.set(start, eraser.end);
            else if () {
                        ...
                 //more code for where the eraser partially erases the beginning, end, or splits the block
                 //if statements call the .add(), .set(), and remove() methods on the blockList.
                        ...
                  }
            }
        }

I don't understand why I am getting a Concurrent Modification Exception. I never modify the eraserList.

I am trying to modify the block object that is assigned in the "Block block = it.next();" statement. I am also modifying the blockList by removing or adding blocks to the list. I thought the whole point of the ListIterator was that it allowed you to modify, add, or subtract a list you are walking through.

The Failure Trace points to the Blocks eraser = it.next(); as the line drawing the exception, but I don't know what that is telling me.

Can anyone help me figure out what I am doing wrong?

Thanks!

Answer 1

Yes, ListIterator is designed to allow the modification of the list. But you are not using the remove() method of the ListIterator, but directly manipulate the underlying list itself.