Use event listeners as Java 8 stream sources

Fast cleaning

Can a traditional event listener be refactored into a Java 8 stream so that the listener event becomes a stream source?

A long story

The client submits any job and listens for the result:

Client client = new JobClient()
client.addTaskListener(this)
client.submitJobAsync( new MultiTaskJob())  //returns void,important (see below)

public void onTaskResult(TaskResult result){
  if(result.isLastResult())
    aggregateJobResults(result)
  else
    processResult(result)
}

problem

For any job submitted, the client receives n results, but it does not know how many results it will receive (it uses islastresult() to determine when to stop and aggregate)

target

I want to refactor the listener into a "vendor" or something like that so that ontaskresult () is the stream source:

supplier<TaskResult> taskResultsupplier = 
    () -> Stream.of( .. )   //onTaskResult() Feeds this
            .map(result -> {
               if(result.isLastResult())
                 //logic here
             });

Something like this; If I can do this without customers knowing how many results will be, I am golden; Now, submitjobasync () returns void. I want to keep this way, but I'm also open to options

Alternatives

After Tomasz nurkiewicz read a similar situation on completable futures, assuming that minor changes have been made to the customer, there are alternative options:

List<CompletableFuture<TaskResult>> taskFutures = 
  client.submitJobAsync( new MultiTaskJob())

Here, the customer gets the list of completable futures < taskresult >, so we need to collect the results of futures upon completion:

//processes all task result futures 
List<TaskResult> = taskFutures.stream()
  .map(taskResult ->
     taskResult.thenApply(this::processResult))
  .collect(Collectors.<TaskResult>toList());

This article also explains the use of completable future allOf(..) To perform the final processing, but only after all futures are completed (it is very smooth); This is where aggregation happens in my case No code can be shown here, although this article explains it well (I have a total of n00b streams, but if I get it to work, I will release the code: - D)

Solution

You can build a stream around taskresults Look at this example:

import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

/**
 * Created for https://stackoverflow.com/q/27670421/1266906.
 */
public class AsyncToStream {
    public static void main(String[] args) {

        System.out.println("Unbuffered Test:");
        AsyncTaskResultIterator<TaskResult> taskListener1 = new AsyncTaskResultIterator<>();
        new TaskResultGenerator(taskListener1,5).start();
        taskListener1.unbufferedStream().forEach(System.out::println);

        System.out.println("Buffered Test:");
        AsyncTaskResultIterator<TaskResult> taskListener2 = new AsyncTaskResultIterator<>();
        new TaskResultGenerator(taskListener2,5).start();
        taskListener2.bufferedStream().forEach(System.out::println);
    }

    /**
     * This class wraps a sequence of TaskResults into an iterator upto the first TaskResult where {@code }isLastResult()} returns {@code true}
     */
    public static class AsyncTaskResultIterator<T extends TaskResult> implements Iterator<T>,TaskListener<T> {

        /**
         * This acts as an asynchronous buffer so we can easily wait for the next TaskResult
         */
        private final BlockingQueue<T> blockingQueue;
        /**
         * Becomes {@code true} once {@code TaskResult.isLastResult()} is received
         */
        private       boolean          ended;

        public AsyncTaskResultIterator() {
            blockingQueue = new LinkedBlockingQueue<>();
        }

        /**
         * Waits on a new TaskResult and returns it as long as the prevIoUs TaskResult did not specify {@code isLastResult()}. Afterwards no more elements can be retrieved.
         */
        @Override
        public T next() {
            if (ended) {
                throw new NoSuchElementException();
            } else {
                try {
                    T next = blockingQueue.take();
                    ended = next.isLastResult();
                    return next;
                } catch (InterruptedException e) {
                    throw new IllegalStateException("Could not retrieve next value",e);
                }
            }
        }

        @Override
        public boolean hasNext() {
            return !ended;
        }

        /**
         * Enqueue another TaskResult for retrieval
         */
        @Override
        public void onTaskResult(T result) {
            if (ended) {
                throw new IllegalStateException("Already received a TaskResult with isLastResult() == true");
            }
            try {
                blockingQueue.put(result);
            } catch (InterruptedException e) {
                throw new IllegalStateException("Could not enqueue next value",e);
            }
        }

        /**
         * Builds a Stream that acts upon the results just when they become available
         */
        public Stream<T> unbufferedStream() {
            Spliterator<T> spliterator = Spliterators.spliteratorUnkNownSize(this,0);
            return StreamSupport.stream(spliterator,false);
        }

        /**
         * Buffers all results and builds a Stream around the results
         */
        public Stream<T> bufferedStream() {
            Stream.Builder<T> builder = Stream.builder();
            this.forEachRemaining(builder);
            return builder.build();
        }
    }

    public static class TaskResultImpl implements TaskResult {
        private boolean lastResult;
        private String  name;

        public TaskResultImpl(boolean lastResult,String name) {
            this.lastResult = lastResult;
            this.name = name;
        }

        @Override
        public String toString() {
            return "TaskResultImpl{" +
                    "lastResult=" + lastResult +
                    ",name='" + name + '\'' +
                    '}';
        }

        @Override
        public boolean isLastResult() {
            return lastResult;
        }
    }

    public static interface TaskListener<T extends TaskResult> {
        public void onTaskResult(T result);
    }

    public static interface TaskResult {
        boolean isLastResult();
    }

    private static class TaskResultGenerator extends Thread {
        private final TaskListener<TaskResult> taskListener;
        private final int                      count;

        public TaskResultGenerator(TaskListener<TaskResult> taskListener,int count) {
            this.taskListener = taskListener;
            this.count = count;
        }

        @Override
        public void run() {
            try {
                for (int i = 1; i < count; i++) {
                    Thread.sleep(200);
                    taskListener.onTaskResult(new TaskResultImpl(false,String.valueOf(i)));
                }
                Thread.sleep(200);
                taskListener.onTaskResult(new TaskResultImpl(true,String.valueOf(count)));
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

You didn't provide your taskresult and tasklistener definitions, so I made them up myself Asynctaskresultiterator is only applicable to a single taskresult sequence If taskresult next() with islastresult() = = true is not provided, unbuffered stream and buffered stream generation will wait endlessly