diff --git a/src/ImmediatelyThrowableBackOff.php b/src/ImmediatelyThrowableBackOff.php
index 39798e6..a809966 100644
--- a/src/ImmediatelyThrowableBackOff.php
+++ b/src/ImmediatelyThrowableBackOff.php
@@ -4,7 +4,7 @@
 
 namespace Orangesoft\BackOff;
 
-class ImmediatelyThrowableBackOff implements BackOffInterface
+final class ImmediatelyThrowableBackOff implements BackOffInterface
 {
     public function backOff(int $attempt, \Throwable $throwable): void
     {
diff --git a/src/Jitter/NullJitter.php b/src/Jitter/NullJitter.php
index 3ca4523..f863a02 100644
--- a/src/Jitter/NullJitter.php
+++ b/src/Jitter/NullJitter.php
@@ -6,7 +6,7 @@
 
 use Orangesoft\BackOff\Duration\DurationInterface;
 
-class NullJitter implements JitterInterface
+final class NullJitter implements JitterInterface
 {
     public function jitter(DurationInterface $duration): DurationInterface
     {