Make private class final

src/main/java/org/apache/commons/lang3/concurrent/BackgroundInitializer.java

@@ -122,7 +122,7 @@ public class BackgroundInitializer<T> extends AbstractConcurrentInitializer<T, E
 
     }
 
-    private class InitializationTask implements Callable<T> {
+    private final class InitializationTask implements Callable<T> {
         /** Stores the executor service to be destroyed at the end. */
         private final ExecutorService execFinally;
 

src/main/java/org/apache/commons/lang3/time/FastDateParser.java

@@ -418,7 +418,7 @@ public class FastDateParser implements DateParser, Serializable {
     /**
      * Parse format into Strategies
      */
-    private class StrategyParser {
+    private final class StrategyParser {
         private final Calendar definingCalendar;
         private int currentIdx;
 

src/test/java/org/apache/commons/lang3/ArrayUtilsTest.java

@@ -45,7 +45,7 @@ import org.junit.jupiter.api.Test;
 @SuppressWarnings("deprecation") // deliberate use of deprecated code
 public class ArrayUtilsTest extends AbstractLangTest {
 
-    private class TestClass {
+    private final class TestClass {
         // empty
     }
 

src/test/java/org/apache/commons/lang3/ClassUtilsTest.java

@@ -86,7 +86,7 @@ public class ClassUtilsTest extends AbstractLangTest {
     }
 
     private static final class Inner {
-        private class DeeplyNested {
+        private final class DeeplyNested {
             // empty
         }
     }