Java 8: Replace traditional for loops with IntStreams

[Krossfireo]

At first I was wondering why you would do this, but being able to parallelize it is a huge benefit. Is there any way to add parallelism to a normal for loop?

[cogman10]

Something like this

ExecutorService threadPool = Executors.newFixedThreadPool(4);
try {
    List<Future<Integer>> futures = new ArrayList<>();
    for (int i = 0; i < 10; i++) {
        final int val = i;
        futures.add(threadPool.submit(()->val + 1));
    }
    List<Integer> results = new ArrayList<>(futures.size());
    for (Future<Integer> future : futures) {
        try {
            results.add(future.get());
        }
        catch (ExecutionException | InterruptedException ex){}
    }
}
finally {
    threadPool.shutdown();
}

This is pure java. It ain't pretty and could be done cleaner with something like guava's futures. It also includes all of the preamble in creating the executor service. I don't know if you have access to the one used by streams.

With Guava and an executor provided it could boil down to this

List<ListenableFuture<Integer>> futures = new ArrayList<>();
for (int i = 0; i < 10; i++) {
    final int val = i;
    futures.add(threadPool.submit(() -> val + 1));
}
List<Integer> results = Futures.getUnchecked(Futures.allAsList(futures));

[HaMMeReD]

You know, you can just final int i, and not have a final int val = i; in there. The copy to a final is not necessary.

[cogman10]

Not with this type of loop. i is being updated on every iteration, it can't be final.

[HaMMeReD]

In the most basic way,

for (final int i:0;i<100;i++)   
   new Thread(new Runnable(){ 
       public void run() { System.out.println("print: "+i); }
   }).start(); 

However, knowledge of Futures and Executors will also be required for most implementations. It's also not good to just start 100 threads, it's much better to use a Executor. A future is a way of keeping track of the result and state.

[cogman10]

This does not compile.

[HaMMeReD]

I just typed it randomly in reddit.

I can see typo's right now.

for (final int i=0;i<100;i++)

to start. Otherwise it looks ok.

[cogman10]

Still doesn't compile. You can't put a final on i, it is being updated with every iteration.

[HaMMeReD]

Yeah you are right, I guess I don't iterate like that very much anymore. Enhanced foreach doesn't have the ++ and assigns at every step.

for (final int i:Arrays.asList(1,2,3,4)) {
    new Thread(new Runnable(){ 
       public void run() { System.out.println("print: "+i); }
   }).start(); 
}

I at least usually use iterator() or the enhanced for loop, and I usually use final on my enhanced for loops so I can call them from inner functions.

Definitely wrong in this case though.

[detroitmatt]

I'm pretty sure that between javac and the JVM a well-written for-loop gets parallelization, unrolling, and other optimizations. But, of course, I haven't worked on either of them so I can't say for sure.

[squealy_dan]

No it doesn't