diff --git a/src/main/java/org/apache/commons/collections4/ArrayUtils.java b/src/main/java/org/apache/commons/collections4/ArrayUtils.java
index 8f2871ea6d..61cbb4821d 100644
--- a/src/main/java/org/apache/commons/collections4/ArrayUtils.java
+++ b/src/main/java/org/apache/commons/collections4/ArrayUtils.java
@@ -35,7 +35,7 @@
  *
  * @since 4.2 (Copied from Apache Commons Lang.)
  */
-class ArrayUtils {
+final class ArrayUtils {
 
 
     /**
diff --git a/src/main/java/org/apache/commons/collections4/bag/SynchronizedBag.java b/src/main/java/org/apache/commons/collections4/bag/SynchronizedBag.java
index ba469ccaad..3420f4ddfe 100644
--- a/src/main/java/org/apache/commons/collections4/bag/SynchronizedBag.java
+++ b/src/main/java/org/apache/commons/collections4/bag/SynchronizedBag.java
@@ -133,7 +133,7 @@ public int getCount(final Object object) {
     /**
      * Synchronized Set for the Bag class.
      */
-    class SynchronizedBagSet extends SynchronizedCollection<E> implements Set<E> {
+    final class SynchronizedBagSet extends SynchronizedCollection<E> implements Set<E> {
         /** Serialization version */
         private static final long serialVersionUID = 2990565892366827855L;
 
diff --git a/src/main/java/org/apache/commons/collections4/bidimap/TreeBidiMap.java b/src/main/java/org/apache/commons/collections4/bidimap/TreeBidiMap.java
index 0eb3a8adf0..c6e46fe5e1 100644
--- a/src/main/java/org/apache/commons/collections4/bidimap/TreeBidiMap.java
+++ b/src/main/java/org/apache/commons/collections4/bidimap/TreeBidiMap.java
@@ -1496,7 +1496,7 @@ public void clear() {
         }
     }
 
-    class KeyView extends AbstractView<K> {
+    final class KeyView extends AbstractView<K> {
 
         /**
          * Creates a new TreeBidiMap.KeyView.
@@ -1523,7 +1523,7 @@ public boolean remove(final Object o) {
 
     }
 
-    class ValueView extends AbstractView<V> {
+    final class ValueView extends AbstractView<V> {
 
         /**
          * Creates a new TreeBidiMap.ValueView.
@@ -1553,7 +1553,7 @@ public boolean remove(final Object o) {
     /**
      * A view of this map.
      */
-    class EntryView extends AbstractView<Map.Entry<K, V>> {
+    final class EntryView extends AbstractView<Map.Entry<K, V>> {
 
         EntryView() {
             super(KEY);
@@ -1594,7 +1594,7 @@ public Iterator<Map.Entry<K, V>> iterator() {
     /**
      * A view of this map.
      */
-    class InverseEntryView extends AbstractView<Map.Entry<V, K>> {
+    final class InverseEntryView extends AbstractView<Map.Entry<V, K>> {
 
         InverseEntryView() {
             super(VALUE);
@@ -1718,7 +1718,7 @@ public final void remove() {
     /**
      * An iterator over the map.
      */
-    class ViewMapIterator extends AbstractViewIterator implements OrderedMapIterator<K, V> {
+    final class ViewMapIterator extends AbstractViewIterator implements OrderedMapIterator<K, V> {
 
         /**
          * Constructor.
@@ -1764,7 +1764,7 @@ public K previous() {
     /**
      * An iterator over the map.
      */
-    class InverseViewMapIterator extends AbstractViewIterator implements OrderedMapIterator<V, K> {
+    final class InverseViewMapIterator extends AbstractViewIterator implements OrderedMapIterator<V, K> {
 
         /**
          * Creates a new TreeBidiMap.InverseViewMapIterator.
@@ -1810,7 +1810,7 @@ public V previous() {
     /**
      * An iterator over the map entries.
      */
-    class ViewMapEntryIterator extends AbstractViewIterator implements OrderedIterator<Map.Entry<K, V>> {
+    final class ViewMapEntryIterator extends AbstractViewIterator implements OrderedIterator<Map.Entry<K, V>> {
 
         /**
          * Constructor.
@@ -1833,7 +1833,7 @@ public Map.Entry<K, V> previous() {
     /**
      * An iterator over the inverse map entries.
      */
-    class InverseViewMapEntryIterator extends AbstractViewIterator implements OrderedIterator<Map.Entry<V, K>> {
+    final class InverseViewMapEntryIterator extends AbstractViewIterator implements OrderedIterator<Map.Entry<V, K>> {
 
         /**
          * Constructor.
@@ -2083,7 +2083,7 @@ public int hashCode() {
     /**
      * The inverse map implementation.
      */
-    class Inverse implements OrderedBidiMap<V, K> {
+    final class Inverse implements OrderedBidiMap<V, K> {
 
         /** Store the keySet once created. */
         private Set<V> inverseKeySet;
diff --git a/src/main/java/org/apache/commons/collections4/functors/FunctorUtils.java b/src/main/java/org/apache/commons/collections4/functors/FunctorUtils.java
index 0b1998dbe9..251edfa6cb 100644
--- a/src/main/java/org/apache/commons/collections4/functors/FunctorUtils.java
+++ b/src/main/java/org/apache/commons/collections4/functors/FunctorUtils.java
@@ -28,7 +28,7 @@
  *
  * @since 3.0
  */
-class FunctorUtils {
+final class FunctorUtils {
 
     /**
      * Restricted constructor.
diff --git a/src/main/java/org/apache/commons/collections4/multimap/AbstractMultiValuedMap.java b/src/main/java/org/apache/commons/collections4/multimap/AbstractMultiValuedMap.java
index 10f101d5f1..215e1ddcd9 100644
--- a/src/main/java/org/apache/commons/collections4/multimap/AbstractMultiValuedMap.java
+++ b/src/main/java/org/apache/commons/collections4/multimap/AbstractMultiValuedMap.java
@@ -828,7 +828,7 @@ public void clear() {
             AbstractMultiValuedMap.this.clear();
         }
 
-        class AsMapEntrySet extends AbstractSet<Map.Entry<K, Collection<V>>> {
+        final class AsMapEntrySet extends AbstractSet<Map.Entry<K, Collection<V>>> {
 
             @Override
             public Iterator<Map.Entry<K, Collection<V>>> iterator() {
@@ -864,7 +864,7 @@ public boolean remove(final Object o) {
         /**
          * EntrySet iterator for the asMap view.
          */
-        class AsMapEntrySetIterator extends AbstractIteratorDecorator<Map.Entry<K, Collection<V>>> {
+        final class AsMapEntrySetIterator extends AbstractIteratorDecorator<Map.Entry<K, Collection<V>>> {
 
             AsMapEntrySetIterator(final Iterator<Map.Entry<K, Collection<V>>> iterator) {
                 super(iterator);
diff --git a/src/test/java/org/apache/commons/collections4/list/SetUniqueListTest.java b/src/test/java/org/apache/commons/collections4/list/SetUniqueListTest.java
index 3da6f6d720..ee22daf49f 100644
--- a/src/test/java/org/apache/commons/collections4/list/SetUniqueListTest.java
+++ b/src/test/java/org/apache/commons/collections4/list/SetUniqueListTest.java
@@ -42,7 +42,7 @@
  */
 public class SetUniqueListTest<E> extends AbstractListTest<E> {
 
-    class SetUniqueList307 extends SetUniqueList<E> {
+    final class SetUniqueList307 extends SetUniqueList<E> {
         /**
          * Generated serial version ID.
          */
diff --git a/src/test/java/org/apache/commons/collections4/map/EmptyMapMutator.java b/src/test/java/org/apache/commons/collections4/map/EmptyMapMutator.java
index 57955ec51b..04418ea4a3 100644
--- a/src/test/java/org/apache/commons/collections4/map/EmptyMapMutator.java
+++ b/src/test/java/org/apache/commons/collections4/map/EmptyMapMutator.java
@@ -24,7 +24,7 @@
  * the class has to be separate of TestCompositeMap, else the test
  * class also has to be serialized.
  */
-class EmptyMapMutator<K, V> implements CompositeMap.MapMutator<K, V> {
+final class EmptyMapMutator<K, V> implements CompositeMap.MapMutator<K, V> {
     /** Serialization version */
     private static final long serialVersionUID = -2729718980002476794L;
 
diff --git a/src/test/java/org/apache/commons/collections4/set/EmptySetMutator.java b/src/test/java/org/apache/commons/collections4/set/EmptySetMutator.java
index ff790a15be..4c531f4653 100644
--- a/src/test/java/org/apache/commons/collections4/set/EmptySetMutator.java
+++ b/src/test/java/org/apache/commons/collections4/set/EmptySetMutator.java
@@ -25,7 +25,7 @@
  * the class has to be separate of CompositeSetTest, else the test
  * class also has to be serialized.
  */
-class EmptySetMutator<E> implements CompositeSet.SetMutator<E> {
+final class EmptySetMutator<E> implements CompositeSet.SetMutator<E> {
 
     /** Serialization version */
     private static final long serialVersionUID = 5321193666420238910L;