How to execute this paralell task in Java8

Question

I'm new in Java concurrency, so i ask what is the best way to perform action like this:

I have a static method which matches a sub image within an image. It looks like that:

public static Point match(final BufferedImage subimage, final BufferedImage image)

The method returns null if nothing was matched, else it returns the Point of the match.

Now I have 40 different sub images for one (big) image, which I want to match in parallel. Each second I will get a new (big) image in which I need to search those 40 smaller images over and over again. I need the return values of each call to to match method at the end of the match task in my main task, so I can analyze it. Furthermore I need to use as many CPU cores as possible for this task.

How can I accomplish that? I have read a lot about ExecutorService, Task, Runnable and so on. Most examples only show how to print something on the console in paralles. I am really confused which way I should go in my scenario: How to pass the values and how to get the results? How should the layout of the class(es) look like? Also I have no idea what is the way to go if I create 40 tasks a second (it will take some time to setup the task, right?)

Code would be great to explain it :)

Answer 1

Use a CompletionService, more likely a ExecutorCompletionService.

class Matcher {
  ExecutorService threadPool = Executors.newCachedThreadPool();
  private List<BufferedImage> subimages; // populate it yourself
  public static Point match(BufferedImage subimage, BufferedImage image) {
    // Your implementation
  }
  public List<Point> match(BufferedImage image) {
    CompletionService<Point> completionService = new ExecutorCompletionService(threadPool);
    int size = subimages.size();
    List<Point> results = new ArrayList<>(size);
    for (BufferedImage subimage: subimages) {
      completionService.submit(()->match(subimage, image));
    }
    for (int i = 0; i < size; i++) {
      Point point = completionService.take().get();
      if (point != null) {
        results.add(point);
      }
    }
    return results;
  }
}

If you want to use all the CPU, you probably want to change your ExecutorService to Executors.newWorkStealingPool(). Be careful with this though!