CompletableFuture to collect results to hashmap

I am reading a list of pdf files from DB and parsing them and performing some tasks with them.
When I am reading this list of pdf's , I have seen extracting images from PDF is taking more time and I don't to block my main thread for reading images. So I want to execute extracting images in a separate thread.
I want to read images from one pdf after another, without loading all the pdfs into memory at once (due to memory concern). So I just want 2 threads; one should be main thread (which reads some text from pdf and does some other stuff) and other should be a thread which extracts images and return set of image objects.

One caveat here is that the images from pdfs can be same in their content , so I want to remove duplicate images using checksum or some other means before collecting their result.
I don't want to hold images in memory until all tasks are getting completed, I want to remove duplicates as on when I get the result of one pdf

So the real question is , I need to submit multiple tasks to a thread pool of size 1 and need to remove duplicates as on when I get the result so that I don't need to hold the image in memory for longer time.

Below is the idea of what have I tried.
I have removed unnecessary things from the code like Images and its content and converted code to String based problem.

public static void main(String[] args) throws InterruptedException, ExecutionException {
		Map<String, Integer> uniqueImages = new HashMap<>();
		 ExecutorService newFixedThreadPool = Executors.newFixedThreadPool(1);
    	List<CompletableFuture<String>> futureList = new ArrayList<>();
    	for(int i = 0; i<20000; i++) {
	    	CompletableFuture<String> obj = CompletableFuture.supplyAsync(()->{
	    		//Assume lot of duplicates
	    		return UUID.randomUUID().toString();
			}, newFixedThreadPool).thenApply((x)->{
				if(uniqueImages.containsKey(x)) {
					int val = uniqueImages.get(x);
					uniqueImages.put(x, val+1);
				}
				else {
					uniqueImages.put(x, 1);
				}
				return x;
			});
	    	futureList.add(obj);
    	}
    	
    	for(CompletableFuture<String> future: futureList) {
    		future.get();
    	}
    	System.out.println(uniqueImages.size());
	}

I'm worried if this code really works or throws ConcurrentModification Exception.

• Does uniqueImages map really contains unique images and their counts?
• Are there any hidden issues?
• Is there better way to solve my use case?

As long as you are only using thread pool of size 1, there is no problem.

However, if there are multiple threads, then the code is indeed not thread-safe and is prone to data race.
We can demonstrate using the following scenario:

Consider this code:

if(uniqueImages.containsKey(x)) {
    int val = uniqueImages.get(x);
    uniqueImages.put(x, val+1);
}
else {
    uniqueImages.put(x, 1);
}

Imagine thread 1 and thread 2 both return the same string and reach at line if(uniqueImages.containsKey(x)) together.
if will return false in both the threads, and uniqueImages.put(x, 1); will be called in both the threads.
Either a ConcurrentModification exception will be thrown or you will get a wrong count (1 instead of 2).

If you plan to use multiple threads, then you must use a ConcurrentHashMap.