Add static import convenience methods to Done and NotUsed (#25174)

* Add Done.done() convenience for static import
* Add NotUsed.notUsed() convenience for static import

akka-actor-testkit-typed/src/test/java/akka/actor/testkit/typed/javadsl/ActorTestKitTest.java

@@ -13,6 +13,7 @@ import org.scalatest.junit.JUnitSuite;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.TimeUnit;
 
+import static akka.Done.done;
 import static org.junit.Assert.assertEquals;
 
 /**
@@ -32,7 +33,7 @@ public class ActorTestKitTest extends JUnitSuite {
   public void testKitShouldSpawnActor() throws Exception {
     final CompletableFuture<Done> started = new CompletableFuture<>();
     testKit.spawn(Behaviors.setup((ctx) -> {
-      started.complete(Done.getInstance());
+      started.complete(done());
       return Behaviors.same();
     }));
     started.get(3, TimeUnit.SECONDS);

akka-actor-typed-tests/src/test/java/akka/actor/typed/javadsl/WatchTest.java

@@ -17,6 +17,7 @@ import org.junit.Test;
 
 import akka.actor.typed.*;
 
+import static akka.Done.done;
 import static akka.actor.typed.javadsl.Behaviors.*;
 
 public class WatchTest extends JUnitSuite {
@@ -53,7 +54,7 @@ public class WatchTest extends JUnitSuite {
       (ctx, msg) -> unhandled(),
       (ctx, sig) -> {
         if (sig instanceof Terminated) {
-          replyWhenTerminated.tell(Done.getInstance());
+          replyWhenTerminated.tell(done());
         }
         return same();
       }
@@ -64,7 +65,7 @@ public class WatchTest extends JUnitSuite {
     return receive(
       (ctx, msg) -> {
         if (msg instanceof CustomTerminationMessage) {
-          replyWhenReceived.tell(Done.getInstance());
+          replyWhenReceived.tell(done());
           return same();
         } else {
           return unhandled();

akka-actor/src/main/scala/akka/Done.scala

@@ -19,4 +19,11 @@ case object Done extends Done {
    * Java API: the singleton instance
    */
   def getInstance(): Done = this
+
+  /**
+   * Java API: the singleton instance
+   *
+   * This is equivalent to [[Done#getInstance()]], but can be used with static import.
+   */
+  def done(): Done = this
 }

akka-actor/src/main/scala/akka/NotUsed.scala

@@ -17,4 +17,11 @@ case object NotUsed extends NotUsed {
    * Java API: the singleton instance
    */
   def getInstance(): NotUsed = this
+
+  /**
+   * Java API: the singleton instance
+   *
+   * This is equivalent to [[NotUsed#getInstance()]], but can be used with static import.
+   */
+  def notUsed(): NotUsed = this
 }

akka-stream-tests/src/test/java/akka/stream/javadsl/FlowTest.java

@@ -34,6 +34,7 @@ import java.util.concurrent.TimeoutException;
 import java.util.stream.Stream;
 import java.time.Duration;
 
+import static akka.Done.done;
 import static akka.stream.testkit.StreamTestKit.PublisherProbeSubscription;
 import static org.junit.Assert.*;
 
@@ -595,7 +596,7 @@ public class FlowTest extends StreamTest {
             .watchTermination(Keep.right())
             .to(Sink.ignore()).run(materializer);
 
-    assertEquals(Done.getInstance(), future.toCompletableFuture().get(3, TimeUnit.SECONDS));
+    assertEquals(done(), future.toCompletableFuture().get(3, TimeUnit.SECONDS));
   }
 
   @Test

akka-stream-tests/src/test/java/akka/stream/javadsl/KillSwitchTest.java

@@ -17,6 +17,7 @@ import java.util.concurrent.CompletionStage;
 import java.util.concurrent.TimeUnit;
 import akka.testkit.AkkaJUnitActorSystemResource;
 
+import static akka.Done.done;
 import static org.junit.Assert.*;
 
 public class KillSwitchTest extends StreamTest {
@@ -55,7 +56,7 @@ public class KillSwitchTest extends StreamTest {
     upstream.expectCancellation();
     downstream.expectComplete();
 
-    assertEquals(completionStage.toCompletableFuture().get(3, TimeUnit.SECONDS), Done.getInstance());
+    assertEquals(completionStage.toCompletableFuture().get(3, TimeUnit.SECONDS), done());
   }
 
   @Test

akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java

@@ -33,6 +33,7 @@ import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 import java.util.stream.Stream;
 
+import static akka.NotUsed.notUsed;
 import static akka.stream.testkit.StreamTestKit.PublisherProbeSubscription;
 import static akka.stream.testkit.TestPublisher.ManualProbe;
 import static org.junit.Assert.*;
@@ -451,7 +452,7 @@ public class SourceTest extends StreamTest {
   @SuppressWarnings("unused")
   public void mustCompileMethodsWithJavaDuration() {
     Source<NotUsed, Cancellable> tickSource = Source.tick(Duration.ofSeconds(1),
-            Duration.ofMillis(500), NotUsed.getInstance());
+            Duration.ofMillis(500), notUsed());
   }
 
   @Test