+act,sam,doc #3940 Added receive setter for Java Lambda actors

* Added a setter for Java lambda actors to "hide" the not so nice looking type signature of the "receive" method.
* Updated docs to reflect the changes.
* Converted samples to use the new setter.

akka-samples/akka-sample-supervision-java-lambda/src/main/java/supervision/ArithmeticService.java

@@ -61,15 +61,15 @@ class ArithmeticService extends AbstractLoggingActor {
     }
   }
 
-  @Override
-  public PartialFunction<Object, BoxedUnit> receive() {
-    return ReceiveBuilder.
+  ArithmeticService() {
+    receive(ReceiveBuilder.
       match(Expression.class, expr -> {
         // We delegate the dangerous task of calculation to a worker, passing the
         // expression as a constructor argument to the actor.
         ActorRef worker = context().actorOf(FlakyExpressionCalculator.props(expr, Left));
         pendingWorkers.put(worker, sender());
       }).
-      match(Result.class, r -> notifyConsumerSuccess(sender(), r.getValue())).build();
+      match(Result.class, r -> notifyConsumerSuccess(sender(), r.getValue())).build()
+    );
   }
-}
+}

akka-samples/akka-sample-supervision-java-lambda/src/main/java/supervision/FlakyExpressionCalculator.java

@@ -67,11 +67,6 @@ public class FlakyExpressionCalculator extends AbstractLoggingActor {
   private final Expression expr;
   private final Position myPosition;
 
-  public FlakyExpressionCalculator(Expression expr, Position myPosition) {
-    this.expr = expr;
-    this.myPosition = myPosition;
-  }
-
   private Expression getExpr() {
     return expr;
   }
@@ -107,9 +102,11 @@ public class FlakyExpressionCalculator extends AbstractLoggingActor {
     }
   }
 
-  @Override
-  public PartialFunction<Object, BoxedUnit> receive() {
-    return ReceiveBuilder.
+  public FlakyExpressionCalculator(Expression expr, Position myPosition) {
+    this.expr = expr;
+    this.myPosition = myPosition;
+
+    receive(ReceiveBuilder.
       match(Result.class, r -> expected.contains(r.getPosition()), r -> {
         expected.remove(r.getPosition());
         results.put(r.getPosition(), r.getValue());
@@ -126,7 +123,7 @@ public class FlakyExpressionCalculator extends AbstractLoggingActor {
       throw new IllegalStateException("Expected results for positions " +
         expected.stream().map(Object::toString).collect(Collectors.joining(", ")) +
         " but got position " + r.getPosition());
-    }).build();
+    }).build());
   }
 
   private Integer evaluate(Expression expr, Integer left, Integer right) {