BAEL-5180 improvement on the article about sealed class

move code to java 17 and use getPermittedSubClasses instead of permittedSubClasses

core-java-modules/core-java-15/README.md

@@ -5,4 +5,3 @@ This module contains articles about Java 15.
 ### Relevant articles
 
 - [Hidden Classes in Java 15](https://www.baeldung.com/java-hidden-classes)
-- [Sealed Classes and Interfaces in Java 15](https://www.baeldung.com/java-sealed-classes-interfaces)

core-java-modules/core-java-17/README.md

@@ -5,3 +5,4 @@
 - [Introduction to HexFormat in Java 17](https://www.baeldung.com/java-hexformat)
 - [New Features in Java 17](https://www.baeldung.com/java-17-new-features)
 - [Random Number Generators in Java 17](https://www.baeldung.com/java-17-random-number-generators)
+- [Sealed Classes and Interfaces in Java 17](https://www.baeldung.com/java-sealed-classes-interfaces)

core-java-modules/core-java-17/src/test/java/com/baeldung/sealed/classes/VehicleUnitTest.java

@@ -21,7 +21,7 @@ public class VehicleUnitTest {
     public void givenCar_whenUsingReflectionAPI_thenSuperClassIsSealed() {
         Assertions.assertThat(car.getClass().isSealed()).isEqualTo(false);
         Assertions.assertThat(car.getClass().getSuperclass().isSealed()).isEqualTo(true);
-        Assertions.assertThat(car.getClass().getSuperclass().permittedSubclasses())
+        Assertions.assertThat(car.getClass().getSuperclass().getPermittedSubclasses())
                 .contains(ClassDesc.of(car.getClass().getCanonicalName()));
     }
 
@@ -29,7 +29,7 @@ public class VehicleUnitTest {
     public void givenTruck_whenUsingReflectionAPI_thenSuperClassIsSealed() {
         Assertions.assertThat(truck.getClass().isSealed()).isEqualTo(false);
         Assertions.assertThat(truck.getClass().getSuperclass().isSealed()).isEqualTo(true);
-        Assertions.assertThat(truck.getClass().getSuperclass().permittedSubclasses())
+        Assertions.assertThat(truck.getClass().getSuperclass().getPermittedSubclasses())
                 .contains(ClassDesc.of(truck.getClass().getCanonicalName()));
     }