remove checkstyle warnings
diff --git a/subprojects/groovy-datetime/src/main/java/org/apache/groovy/datetime/extensions/DateTimeExtensions.java b/subprojects/groovy-datetime/src/main/java/org/apache/groovy/datetime/extensions/DateTimeExtensions.java
index 85bf09f..5f6e41a 100644
--- a/subprojects/groovy-datetime/src/main/java/org/apache/groovy/datetime/extensions/DateTimeExtensions.java
+++ b/subprojects/groovy-datetime/src/main/java/org/apache/groovy/datetime/extensions/DateTimeExtensions.java
@@ -68,7 +68,7 @@
  * Date/Time API (java.time) classes inside the Groovy environment.
  * These extensions require JDK 8 or above.
  */
-public class DateTimeExtensions {
+public final class DateTimeExtensions {
 
     // Static methods only
     private DateTimeExtensions() {
diff --git a/subprojects/groovy-datetime/src/main/java/org/apache/groovy/datetime/extensions/DateTimeStaticExtensions.java b/subprojects/groovy-datetime/src/main/java/org/apache/groovy/datetime/extensions/DateTimeStaticExtensions.java
index ef9e588..d23d25c 100644
--- a/subprojects/groovy-datetime/src/main/java/org/apache/groovy/datetime/extensions/DateTimeStaticExtensions.java
+++ b/subprojects/groovy-datetime/src/main/java/org/apache/groovy/datetime/extensions/DateTimeStaticExtensions.java
@@ -37,7 +37,7 @@
  * This class defines new static extension methods which appear on normal JDK
  * Date/Time API (java.time) classes inside the Groovy environment.
  */
-public class DateTimeStaticExtensions {
+public final class DateTimeStaticExtensions {
 
     // Static methods only
     private DateTimeStaticExtensions() {