Java Concurrency & Tasks

The consensus when it comes to multithreading and concurrency in Java-land is to isolate the code you want multithreaded as a “task” and send that task to some kind of executor service or thread pool.

I am wondering:

• Could the task be any method of any object, or should it be something special (if so, what)?
• How do you determine if this task is a processor, an IO binding, or another? What are some dead giveaways or other crucial factors?

Thanks in advance for any clarity!