Question

I’m new to Java, and I need some help working on this program. This is a small part of a large class project, and I must use multithreading.

Here’s what I want to do algorithmically:

while (there is still input left, store chunk of input in <chunk>)
{
    if there is not a free thread in my array then
      wait until a thread finishes

    else there is a free thread then
       apply the free thread to <chunk> (which will do something to chunk and output it).  
              Note: The ordering of the chunks being output must be the same as input
}

So, the main things I don’t know how to do:

• How can I check whether or not there’s a free thread in the array? I know that there is a function ThreadAlive, but it seems super inefficient to poll every single thread every time in my loop.
• If there is no free thread, how can I wait until one has finished?
• The ordering is important. How can I preserve the ordering in which the threads output? As in, the order of the output needs to match the order of the input. How can I guarantee this synchronization?
• How do I even pass the chunk to my thread? Can I just use the Runnable interface to do this?

Any help with these four bullets is greatly appreciated. Since I’m a super noob, code samples would help significantly.

(side-note: Making an array of threads was just an idea of mine to handle the user defined number of threads. If you have a better way to handle this you’re welcome to suggest it!)

Answer 1

Sounds like you basically have a producer/consumer model and can be solved with an ExecutorService and BlockingQueue. Here is a similar question with a similar answer:

producer/consumer work queues