When to CompletionService over an ExecutorService

CompletionService is a useful interface in the Java concurrency framework that allows you to efficiently process the results of asynchronous tasks as they become available. You can use CompletionService when you need to execute multiple tasks asynchronously and process their results as soon as they are available, rather than waiting for all tasks to complete before processing the results.

Here are some situations where you might want to use CompletionService:

1. Parallel processing of independent tasks: If you have a large number of independent tasks that need to be executed in parallel, you can use CompletionService to execute the tasks asynchronously and process their results as soon as they are available.
2. Time-critical processing: If you need to process the results of asynchronous tasks as quickly as possible, CompletionService can help you achieve this by allowing you to process the results as soon as they are available, rather than waiting for all tasks to complete before processing the results.
3. Resource efficiency: If you have limited resources, such as memory or processing power, you can use CompletionService to manage the execution of multiple tasks in a way that minimizes resource usage and maximizes efficiency.

Here’s a simple example that demonstrates how to use CompletionService to execute tasks asynchronously and process their results as they become available:

import java.util.concurrent.*;

public class CompletionServiceExample {
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        int numTasks = 10;
        ExecutorService executorService = Executors.newFixedThreadPool(numTasks);
        CompletionService<String> completionService = new ExecutorCompletionService<>(executorService);

        for (int i = 0; i < numTasks; i++) {
            completionService.submit(new Task(i));
        }

        for (int i = 0; i < numTasks; i++) {
            Future<String> resultFuture = completionService.take();
            String result = resultFuture.get();
            System.out.println("Result of task " + i + " is " + result);
        }

        executorService.shutdown();
    }

    private static class Task implements Callable<String> {
        private final int taskId;

        public Task(int taskId) {
            this.taskId = taskId;
        }

        @Override
        public String call() throws Exception {
            Thread.sleep(1000);
            return "Result of task " + taskId;
        }
    }
}

In this example, we create a CompletionService by wrapping an ExecutorService with an ExecutorCompletionService. We then submit numTasks tasks to the CompletionService using the submit method.

The take method is then used to retrieve the results of the tasks as they become available. When a result is available, we retrieve it using the get method of the Future object returned by the take method.

The Task class simply sleeps for a fixed amount of time before returning a result. In a real-world scenario, the Task class would perform some meaningful work.

In summary, you can use CompletionService to execute tasks asynchronously and process their results as soon as they become available. This can be useful in situations where you have a large number of independent tasks that need to be executed in parallel or where time-critical processing of results is important.