chore: Move java 21 code to a dedicated folder and including code sni… (#1023)

* chore: Move java 21 code to a dedicated folder and including code snippe with tab.

* chore: Update OptimizedActorWithJava21.java with new line.

Author: He-Pin

docs/src/main/java-jdk-21/docs/actors/classical/OptimizedActorWithJava21.java

@@ -0,0 +1,33 @@
+
+// #pattern-matching
+
+static class OptimizedActorWithJava21 extends UntypedAbstractActor {
+  public static class Msg1 {}
+
+  public static class Msg2 {}
+
+  public static class Msg3 {}
+
+  @Override
+  public void onReceive(Object msg) throws Exception {
+    switch(msg) {
+      case Msg1 msg -> receiveMsg1((Msg1) msg);
+      case Msg2 msg -> receiveMsg2((Msg2) msg);
+      case Msg3 msg -> receiveMsg3((Msg3) msg);
+      default _ -> unhandled(msg);
+    }
+  }
+
+  private void receiveMsg1(Msg1 msg) {
+      // actual work
+  }
+
+  private void receiveMsg2(Msg2 msg) {
+      // actual work
+  }
+
+  private void receiveMsg3(Msg3 msg) {
+      // actual work
+  }
+}
+// #pattern-matching

docs/src/main/paradox/actors.md

@@ -838,42 +838,14 @@ that the JVM can have problems optimizing and the resulting code might not be as
 untyped version. When extending `UntypedAbstractActor` each message is received as an untyped
 `Object` and you have to inspect and cast it to the actual message type in other ways, like this:
 
-@@snip [ActorDocTest.java](/docs/src/test/java/jdocs/actor/ActorDocTest.java) { #optimized }
+In addition, Java 21 introduces [powerful pattern matching for switch](https://openjdk.org/jeps/441) supporting any
+reference type. We can use `switch` instead of `if ... else ...` for conditional branches:
 
-In addition, Java 21 introduces [powerful pattern matching for switch](https://openjdk.org/jeps/441) supporting any reference type. We can use `switch` instead of `if ... else ...` for conditional branches:
-
-```java
-static class PatternMatchedActor extends UntypedAbstractActor {
-
-    public static class Msg1 {}
-
-    public static class Msg2 {}
-
-    public static class Msg3 {}
-
-    @Override
-    public void onReceive(Object msg) throws Exception {
-      switch(msg) {
-        case Msg1 msg -> receiveMsg1((Msg1) msg);
-        case Msg2 msg -> receiveMsg2((Msg2) msg);
-        case Msg3 msg -> receiveMsg3((Msg3) msg);
-        default _ -> unhandled(msg);
-      }
-    }
-
-    private void receiveMsg1(Msg1 msg) {
-      // actual work
-    }
-
-    private void receiveMsg2(Msg2 msg) {
-      // actual work
-    }
+Java
+: @@snip [ActorDocTest.java](/docs/src/test/java/jdocs/actor/ActorDocTest.java) { #optimized }
 
-    private void receiveMsg3(Msg3 msg) {
-      // actual work
-    }
-  }
-```
+Java 21
+: @@snip [OptimizedActorWithJava21.java](/docs/src/main/java-jdk-21/docs/actors/classical/OptimizedActorWithJava21.java) { #pattern-matching }
 
 @@@