Reduce API surface of Actor by avoiding subclassing

src/main/java/org/syndicate_lang/actors/Actor.java

@@ -13,7 +13,7 @@ import java.util.logging.Logger;
 /**
  * I represent the shared execution context for a collection of objects; I am roughly analogous to the E concept of a Vat.
  */
-public class Actor extends ForkJoinTask<Void> {
+public class Actor {
     private final static AtomicLong _count = new AtomicLong(0);
     private final static AtomicLong _actorId = new AtomicLong(0);
     protected final static ForkJoinPool _executor = new ForkJoinPool(
@@ -206,14 +206,21 @@ public class Actor extends ForkJoinTask<Void> {
         }
     }
 
-    @Override public final Void getRawResult() { return null; }
-    @Override public final void setRawResult(Void v) { }
-    @Override public final boolean exec() { this._runWorkItems(); return false; }
+    private static final class Task extends ForkJoinTask<Void> {
+        private Actor _actor;
+        Task(Actor a) {
+            this._actor = a;
+        }
+        @Override public final Void getRawResult() { return null; }
+        @Override public final void setRawResult(Void v) { }
+        @Override public final boolean exec() { _actor._runWorkItems(); return false; }
+    };
+    private Task _task = new Task(this);
 
     public void execute(WorkItem item) {
         tail.getAndSet(item).set(item);
         if (workItemCount.getAndIncrement() == 0) {
-            _executor.execute(this);
+            _executor.execute(this._task);
         }
     }