How speed up for loop without increasing memory (easy and simple way)?

Question

I have simple loop:

   List<String> results = new ArrayList<String>();
            for (int i = 0; i < 100000000; i++) {
               results.add("a" + i + "b");
            }

    return results;

must be simple idea- because if I will start to create thread pool, I will spend memory for additional objects, also without java8.

How can I implement simple parallel loop ?

Possible duplicate of ["Parallel.For" for Java?](http://stackoverflow.com/questions/4010185/parallel-for-for-java) — tnw, Oct 08 '15 at 18:56
What is the type of `results`? I _strongly_ doubt parallelism will work for you in this case, or that there's any better way to do it. — Louis Wasserman, Oct 08 '15 at 18:56
Out of curiosity, why do you need 100000000 identical strings? — Joekomino, Oct 08 '15 at 19:01
could you use 1 "ab" string and refer to it as and when you need it? — Joekomino, Oct 08 '15 at 19:04
@MaxUsanin It would help if you added more info. What are you doing with the results? What's your actual loop creating? — Kayaman, Oct 08 '15 at 19:22

score 2 · Answer 1 · answered Oct 08 '15 at 19:09

2

The easiest way to speed this up is to create an ArrayList with the size that you need ahead of time. I have no idea why you would need a list like this, but I can only assume that your program is slowing down everytime it has to allocate and copy another array because it ran out of space.

List<String> results = new ArrayList<String>(10000000);

answered Oct 08 '15 at 19:09

John E.

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I upvoted but looking back, this does not populate the array – Joekomino Oct 08 '15 at 19:11
No it allocates it ahead of time with initial capacity. – John E. Oct 08 '15 at 19:13
yes it is fresh idea but it can not speed up to ten times – Max Usanin Oct 08 '15 at 19:15

score 1 · Accepted Answer · 2015-10-08T19:25:48.490

1

This can't be done parralel for a simple reason: Java doesn't provide an efficient List implementation for concurrent access.

The simplest solution would be to implement an own LinkedList and link several LinkedLists into one, which can be done a lot faster and in parralel. The merge-operation can afterwards be done in O(1).

edited Oct 08 '15 at 19:25

answered Oct 08 '15 at 19:12

and perform these operations in parallel ? after to merge the LinkedList(s)? – Max Usanin Oct 08 '15 at 19:18
@MaxUsanin no fill several LinkedLists in parrallel and merge them afterwards. You'll have to implement the list on your own. – Oct 08 '15 at 19:23
@Kayaman the only list is `CopyOnWriteArrayList` and as for the efficiency: the underlying array is copied on each write, nothing more to be said. – Oct 08 '15 at 19:25
You're right (after the edit) about no `List` implementation besides `CopyOnWriteArrayList`. However considering how vague the question is, he might be able to use for example [ConcurrentLinkedDeque](https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ConcurrentLinkedDeque.html). – Kayaman Oct 08 '15 at 19:40
I will thumb up for your suggestion, because it sounds like map reduce approach. Thank you. – Max Usanin Oct 08 '15 at 19:44

marstran · Answer 3 · 2015-10-08T20:27:47.617

0

Using Java 8 parallell streams you can do this:

IntStream.range(0, 100000000)
         .parallel()
         .mapToObj(i -> new StringBuilder()
                               .append("a")
                               .append(i)
                               .append("b")
                               .toString())
         .collect(Collectors.toList());

First, IntStream.range(0, 100000000) creates an IntStream with the values 0 to 100000000 (exclusive). Then, the IntStream is made parallel using the parallel method. Then, I use mapToObj in order to make the IntStream into a Stream<String>. At last, the stream will be collected in a List. I also use a StringBuilder to make the string concatenation a bit faster.

edited Oct 08 '15 at 20:27

answered Oct 08 '15 at 20:22

marstran

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He said "without java8" – Jean Jung Oct 13 '15 at 20:30
Still useful for others who might stumble upon this question. – marstran Oct 14 '15 at 06:43

How speed up for loop without increasing memory (easy and simple way)?

3 Answers3