Kevin Bear

Thank you Steve for the clarification and help.

Many thanks to Alan and Steve for the replies.

Perhaps, perhaps not. You don't show what happens with callableList in the code you provide, and we have no clue what the Callable code is doing, so it is impossible to say with the code you provided.

Sorry I made some mistake in decribing how the Callable module works. The callableList is used in the callable module (for 1000 times in the innnermost for-loop) to generate a result. What happens in the Callable(callableList) module is like this:

Step 1. callableList.add(10 new items); Step 2. Some complex calculation is done based on the new callableList and returns an Object; Step 3. callableList.remove(the last 10 items in the list);

An addtional question: I have an ArrayList of Objects (I'll name it callableList) for use in the Callable module. callableList gets some new items (as well as get some items removed) every time the program enters the innnermost for loop. It seems that this would cause concurrency problem, but the test I ran seems to be fine. I am now totally confused with all the concurrency ideas

ExecutorService exec = Executors.newCachedThreadPool(); ArrayList result = new ArrayList(); for (int i = 0; i < 1000; i++) { for (int j = 0; j < 1000; j++) { //callableList gets changes here for (int k = 0; k < 1000; k++) { result.add(exec.submit(new Callable(arg))); //Callable(arg) does some complex calculation and returns an Object; } } }

I am using an ArrayList to store some information, the code looks like this:

ExecutorService exec = Executors.newCachedThreadPool(); ArrayList result = new ArrayList(); for (int i = 0; i < 1000; i++) { for (int j = 0; j < 1000; j++) { for (int k = 0; k < 1000; k++) { result.add(exec.submit(new Callable(arg))); //Callable(arg) does some complex calculation and returns an Object; } } }

Two questions:
1. Will the order of the results in the list be guaranteed? (From what I read, it seems not. But the experiments I have done always produce nicely ordered results)
2. if I use the following code to examine the result after the for loops, will the get() method ensure that all the calculations in the for loops completed before the result is examined?

for (Future<Object> fs : results) try { // get() blocks until completion: Object result = fs.get(); } catch (InterruptedException e) { System.out.println(e); return; } catch (ExecutionException e) { System.out.println(e); } finally { exec.shutdown(); }

Thanks in advance for all the help.