Explore multithreading and concurrency concepts in Java, from the basics on upward.

Java for Programmers

Java Multithreading

# CompletionService Interface
<p>In the previous lesson we discussed how tasks can be submitted to executors but imagine a scenario where you want to submit hundreds or thousands of tasks. You'll retrieve the future objects returned from the submit calls and then poll all of them in a loop to check which one is done and then take appropriate action. Java offers a better way to address this use case through the <code>CompletionService</code> interface. You can use the <code>ExecutorCompletionService</code> as a concrete implementation of the interface.</p>
<p>The completion service is a combination of a blocking queue and an executor. Tasks are submitted to the queue and then the queue can be polled for completed tasks. The service exposes two methods, one <code>poll</code> which returns null if no task is completed or none were submitted and two <code>take</code> which blocks till a completed task is available.</p>
<p>Below is an example program that demonstrates the use of completion service.</p>

This lesson talks about how to batch multiple tasks together