Changed the implementation for better understanding

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/AsyncTask.java

@@ -0,0 +1,29 @@
+package com.iluwatar.halfsynchalfasync;
+
+import java.util.concurrent.Callable;
+
+/**
+ * Represents some computation that is performed asynchronously. The computation is typically
+ * done is background threads and the result is posted back in form of callback.
+ * 
+ * @param <O> type of result
+ */
+public interface AsyncTask<O> extends Callable<O> {
+	/**
+	 * Is called in context of caller thread before call to {@link #call()}. 
+	 * Validations can be performed here so that the performance penalty of context
+	 * switching is not incurred.
+	 */
+	void preExecute();
+	
+	/**
+	 * Is a callback which is called after the result is successfully computed by
+	 * {@link #call()}.
+	 */
+	void onResult(O result);
+	
+	void onError(Throwable throwable);
+	
+	@Override
+	O call() throws Exception;
+}

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/AsynchronousService.java

@@ -1,7 +1,6 @@
 package com.iluwatar.halfsynchalfasync;
 
 import java.util.concurrent.BlockingQueue;
-import java.util.concurrent.Callable;
 import java.util.concurrent.Future;
 import java.util.concurrent.ThreadPoolExecutor;
 
@@ -12,40 +11,27 @@ import java.util.concurrent.ThreadPoolExecutor;
  * picks up the task and executes it in background and the result is posted back to the caller via 
  * {@link Future}.
  */
-public abstract class AsynchronousService<I, O> {
+public class AsynchronousService {
 	
 	/*
-	 * This is the synchronous layer to which request to do work is submitted.
+	 * This is the synchronous layer to which request to do work is delegated.
 	 */
-	private SynchronousLayer syncLayer = new SynchronousLayer();
+	private SynchronousLayer syncLayer;
+	
+	public AsynchronousService(QueuingLayer queuingLayer) {
+		this.syncLayer = new SynchronousLayer(queuingLayer);
+	}
 	
 	/**
-	 * Computes arithmetic sum for n
 	 * 
-	 * @return future representing arithmetic sum of n
 	 */
-	public Future<O> execute(final I input) {
+	public void execute(final AsyncTask<?> task) {
 		/*
 		 * This is the key part of this pattern where the caller thread does not block until
 		 * the result of work is computed but is delegated to the synchronous layer which
 		 * computes the task in background. This is useful if caller thread is an UI thread, 
 		 * which MUST remain responsive to user inputs.
 		 */
-		return syncLayer.submit(new Callable<O>() {
-
-			@Override
-			public O call() throws Exception {
-				return doInBackground(input);
-			}
-
-		});
+		syncLayer.execute(task);
 	}
-	
-	/**
-	 * This method is called in context of background thread where the implementation should compute
-	 * and return the result for input.
-	 * 
-	 * @return computed result
-	 */
-	protected abstract O doInBackground(I input);
 }

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/QueuingLayer.java

@@ -0,0 +1,8 @@
+package com.iluwatar.halfsynchalfasync;
+
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.LinkedBlockingQueue;
+
+public class QueuingLayer {
+	BlockingQueue<Runnable> incomingQueue = new LinkedBlockingQueue<>();
+}

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/SynchronousLayer.java

@@ -1,32 +1,54 @@
 package com.iluwatar.halfsynchalfasync;
 
-import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
-import java.util.concurrent.Future;
-import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.FutureTask;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
 
 /**
  * This represents the Queuing and Synchronous layer of Half-Sync/Half-Async pattern. 
- * The incoming requests are queued and then picked up by the background threads for execution.
+ * The {@link ThreadPoolExecutor} plays role of both Queuing layer as well as Synchronous layer
+ * of the pattern, where incoming tasks are queued if no worker is available.
  */
 public class SynchronousLayer {
 	
 	/*
-	 * This is the queuing layer where incoming work is queued 
+	 * This is the synchronous layer where background threads execute the work.
 	 */
-	private LinkedBlockingQueue<Runnable> tasks = new LinkedBlockingQueue<Runnable>();
-	/*
-	 * This is the synchronous layer where background threads execute the work
-	 */
-	private ExecutorService service = new ThreadPoolExecutor(10, 10, 10, TimeUnit.SECONDS, tasks);
+	private ExecutorService service;
 	
+	/**
+	 * Creates synchronous layer which uses queuing layer to wait for incoming tasks to execute.
+	 */
+	public SynchronousLayer(QueuingLayer queuingLayer) {
+		service = new ThreadPoolExecutor(10, 10, 10, TimeUnit.SECONDS, queuingLayer.incomingQueue);
+	}
 	/**
 	 * Submit new work for backgrounds threads to compute 
 	 * @return the result after executing the work
 	 */
-	public <T> Future<T> submit(Callable<T> work) {
-		return service.submit(work);
+	public <T> void execute(final AsyncTask<T> work) {
+		work.preExecute();
+		
+		service.submit(new FutureTask<T>(work) {
+			@Override
+			protected void done() {
+				super.done();
+				try {
+					/* called in context of background thread. There is other variant possible
+					 * where result is posted back and sits in the queue of caller thread which
+					 * then picks it up for processing. An example of such a system is Android OS,
+					 * where the UI elements can only be updated using UI thread. So result must be
+					 * posted back in UI thread.
+					 */
+					work.onResult(get());
+				} catch (InterruptedException e) {
+					// should not occur
+				} catch (ExecutionException e) {
+					work.onError(e.getCause());
+				}
+			}
+		});
 	}
 }

half-sync-half-async/src/test/java/com/iluwatar/halfsynchalfasync/AsynchronousServiceTest.java

@@ -1,10 +1,11 @@
 package com.iluwatar.halfsynchalfasync;
 
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.fail;
+
 import java.util.concurrent.ExecutionException;
-import java.util.concurrent.Future;
 
 import org.junit.Test;
-import static org.junit.Assert.*;
 
 public class AsynchronousServiceTest {
 
@@ -14,32 +15,48 @@ public class AsynchronousServiceTest {
 		 * Addition service is asynchronous layer which does not block on single request,
 		 * and is always available for listening new requests.
 		 */
-		ArithmeticSumService service = new ArithmeticSumService();
-		Future<Long> output1 = service.execute(100L);
-		Future<Long> output2 = service.execute(50L);
-		Future<Long> output3 = service.execute(200L);
-		Future<Long> output4 = service.execute(5L);
+		QueuingLayer queuingLayer = new QueuingLayer();
+		new SynchronousLayer(queuingLayer);
+		AsynchronousService service = new AsynchronousService(queuingLayer);
 		
-		assertEquals(ap(100), output1.get().longValue());
-		assertEquals(ap(50), output2.get().longValue());
-		assertEquals(ap(200), output3.get().longValue());
-		assertEquals(ap(5), output4.get().longValue());
+		service.execute(new ArithmeticSumTask(100));
+		service.execute(new ArithmeticSumTask(50));
+		service.execute(new ArithmeticSumTask(200));
+		service.execute(new ArithmeticSumTask(5));
 	}
 	
-	/*
-	 * This is an asynchronous service which computes arithmetic sum
-	 */
-	class ArithmeticSumService extends AsynchronousService<Long, Long> {
+	class ArithmeticSumTask implements AsyncTask<Long> {
+		private long n;
+
+		public ArithmeticSumTask(long n) {
+			this.n = n;
+		}
+		
+		@Override
+		public Long call() throws Exception {
+			return ap(n);
+		}
 
 		@Override
-		protected Long doInBackground(Long n) {
-			return (n) * (n + 1) / 2;
+		public void preExecute() {
+			if (n < 0) {
+				throw new IllegalArgumentException("n is less than 0");
+			}
+		}
+
+		@Override
+		public void onResult(Long result) {
+			assertEquals(ap(n), result.longValue());
+		}
+
+		@Override
+		public void onError(Throwable throwable) {
+			fail("Should not occur");
 		}
 	}
-
-	private long ap(int i) {
+	
+	private long ap(long i) {
 		long out = (i) * (i + 1) / 2;
-		System.out.println(out);
 		return out;
 	}
 }