Achieving task ordering with the Executor framework can be done in a few ways:

Using a SingleThreadExecutor:

If you create an instance of SingleThreadExecutor, it will execute tasks sequentially in the order they are submitted. This means that if you submit multiple tasks to the executor, they will be executed one after the other in the order they were submitted.

ExecutorService executor = Executors.newSingleThreadExecutor();


Using a ThreadPoolExecutor with a LinkedBlockingQueue:

You can create a ThreadPoolExecutor with a LinkedBlockingQueue as the work queue. This will ensure that tasks are executed in the order they are submitted to the executor.

ExecutorService executor = new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>());


Using a CompletionService:

You can use a CompletionService to execute tasks in parallel, but still maintain the order of their completion. The CompletionService allows you to submit tasks to an executor and retrieve their results in the order they complete.

ExecutorService executor = Executors.newFixedThreadPool(5);
CompletionService<Integer> completionService = new ExecutorCompletionService<>(executor);


for (int i = 0; i < 3; i++) {
    Future<Integer> result = completionService.take();
    // process result in order of completion

In the above example, task1, task2, and task3 are submitted to the executor using the CompletionService. The take() method of the CompletionService is then used to retrieve the completed tasks in the order they finish. This ensures that the results are processed in the order of completion.