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I have a HashMap of people who want to communicate with each other as follows:

{zidane=[rooney, rooney, rooney, scholes, rooney], rooney=[scholes, messi, scholes], scholes=[ronaldo], ronaldo=[rooney, messi, scholes], messi=[zidane]}

Here, each person in the key will have their own threads in which they will send a message to each person in their list and receive a response.

public class Master {
    public Map callsMap = new HashMap<String, List>();

    public static void main(String[] args) throws InterruptedException, ExecutionException {
        Master m = new Master();
        m.readFile();
        ExecutorService executorService = Executors.newFixedThreadPool(6);
        List<Future<String>> returnFutures = new ArrayList<>();
        Iterator<Map.Entry<String, List>> it = m.callsMap.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry<String, List> pair = it.next();
            String caller = pair.getKey();
            List receiverList = pair.getValue();
            returnFutures.add(executorService.submit(new SubTasks(caller, receiverList) ));
        }

        while(executorService.awaitTermination(1, TimeUnit.SECONDS));
        for(Future<String> returnFuture: returnFutures){
            System.out.println(returnFuture.get());
        }
        executorService.shutdown();
    }
}

The Master class will iteratively create a thread for every key in the HashMap. Please note that the readFile method will just read from a text file and produce the input (into callsMap variable) I have mentioned above. It is not particularly important for this context so I have omitted it here.

I have the SubTasks class which implements the Callable interface and creates messages for each interaction. This class will not create any additional threads but just operate on the caller and receiver list it received as follows:

import java.util.List;
import java.util.concurrent.Callable;

public  class SubTasks implements Callable<String> {
    private String caller;
    private List receiverList;
    private volatile String returnMessage;
    private volatile String callerMessage;
    private volatile String str = "";

    SubTasks(String s, List l){
        caller = s;
        receiverList = l;
    }

    @Override
    public String call (){
        for (int i = 0; i < receiverList.size(); i++) {
            callerMessage = receiverList.get(i)+" received intro message from "+caller +" ["+System.currentTimeMillis()+"]";
            //System.out.println(callerMessage);
            returnMessage = caller+" received reply message from "+receiverList.get(i) +" ["+System.currentTimeMillis()+"]";
            //System.out.println(returnMessage);
            str = str + callerMessage +"\n"+ returnMessage +"\n";
        }
        return null;
    }
}

If I directly print in the child threads, I get the desired result as the messages are being printed in random order:

rooney received intro message from zidane [1592165414388]
ronaldo received intro message from scholes [1592165414388]
scholes received intro message from rooney [1592165414388]
rooney received intro message from ronaldo [1592165414389]
zidane received intro message from messi [1592165414389]
scholes received reply message from ronaldo [1592165414434]
zidane received reply message from rooney [1592165414434]
...
...

But I want these messages to be printed in the main method instead of the child threads. I tried to return the messages by concatenating the returnMessage and callerMessage but since I return at the end of each thread, they appear to be printed sequentially (call-reply for each pair) as follows:

rooney received intro message from zidane [1592165584566]
zidane received reply message from rooney [1592165584614]
rooney received intro message from zidane [1592165584632]
zidane received reply message from rooney [1592165584632]
rooney received intro message from zidane [1592165584632]
zidane received reply message from rooney [1592165584632]
...
...

So, my question is how do I print each callerMessage and returnMessage in the main method instead of the child threads as soon as they are created?

Aayush Lamichhane
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1 Answers1

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It looks like a classic example of producer-consumer problem, where your SubTasks are producers, and Master is a single consumer. It can be solved in multiple different ways using low-level java concurrency constructs, but the simplest one would probably be by using some kind of thread safe intermediary Queue. You can start by taking a look at Producer/Consumer threads using a Queue.

kstrek
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