diff --git a/src/main/java/org/apache/commons/collections4/list/FixedSizeList.java b/src/main/java/org/apache/commons/collections4/list/FixedSizeList.java
index 0006b15164..bd4c7de31d 100644
--- a/src/main/java/org/apache/commons/collections4/list/FixedSizeList.java
+++ b/src/main/java/org/apache/commons/collections4/list/FixedSizeList.java
@@ -172,7 +172,7 @@ public List<E> subList(final int fromIndex, final int toIndex) {
     /**
      * List iterator that only permits changes via set()
      */
-    private class FixedSizeListIterator extends AbstractListIteratorDecorator<E> {
+    private final class FixedSizeListIterator extends AbstractListIteratorDecorator<E> {
         protected FixedSizeListIterator(final ListIterator<E> iterator) {
             super(iterator);
         }
diff --git a/src/main/java/org/apache/commons/collections4/map/AbstractInputCheckedMapDecorator.java b/src/main/java/org/apache/commons/collections4/map/AbstractInputCheckedMapDecorator.java
index e568535545..3a76ecbbb7 100644
--- a/src/main/java/org/apache/commons/collections4/map/AbstractInputCheckedMapDecorator.java
+++ b/src/main/java/org/apache/commons/collections4/map/AbstractInputCheckedMapDecorator.java
@@ -107,7 +107,7 @@ public Set<Map.Entry<K, V>> entrySet() {
     /**
      * Implementation of an entry set that checks additions via setValue.
      */
-    private class EntrySet extends AbstractSetDecorator<Map.Entry<K, V>> {
+    private final class EntrySet extends AbstractSetDecorator<Map.Entry<K, V>> {
 
         /** Generated serial version ID. */
         private static final long serialVersionUID = 4354731610923110264L;
@@ -166,7 +166,7 @@ public <T> T[] toArray(final T[] array) {
     /**
      * Implementation of an entry set iterator that checks additions via setValue.
      */
-    private class EntrySetIterator extends AbstractIteratorDecorator<Map.Entry<K, V>> {
+    private final class EntrySetIterator extends AbstractIteratorDecorator<Map.Entry<K, V>> {
 
         /** The parent map */
         private final AbstractInputCheckedMapDecorator<K, V> parent;
@@ -187,7 +187,7 @@ public Map.Entry<K, V> next() {
     /**
      * Implementation of a map entry that checks additions via setValue.
      */
-    private class MapEntry extends AbstractMapEntryDecorator<K, V> {
+    private final class MapEntry extends AbstractMapEntryDecorator<K, V> {
 
         /** The parent map */
         private final AbstractInputCheckedMapDecorator<K, V> parent;
diff --git a/src/main/java/org/apache/commons/collections4/map/MultiValueMap.java b/src/main/java/org/apache/commons/collections4/map/MultiValueMap.java
index ed2474ae8f..c70fdaa904 100644
--- a/src/main/java/org/apache/commons/collections4/map/MultiValueMap.java
+++ b/src/main/java/org/apache/commons/collections4/map/MultiValueMap.java
@@ -484,7 +484,7 @@ protected Collection<V> createCollection(final int size) {
     /**
      * Inner class that provides the values view.
      */
-    private class Values extends AbstractCollection<V> {
+    private final class Values extends AbstractCollection<V> {
         @Override
         public Iterator<V> iterator() {
             final IteratorChain<V> chain = new IteratorChain<>();
@@ -508,7 +508,7 @@ public void clear() {
     /**
      * Inner class that provides the values iterator.
      */
-    private class ValuesIterator implements Iterator<V> {
+    private final class ValuesIterator implements Iterator<V> {
         private final Object key;
         private final Collection<V> values;
         private final Iterator<V> iterator;
diff --git a/src/main/java/org/apache/commons/collections4/map/StaticBucketMap.java b/src/main/java/org/apache/commons/collections4/map/StaticBucketMap.java
index e9975e5215..2f6bb707e0 100644
--- a/src/main/java/org/apache/commons/collections4/map/StaticBucketMap.java
+++ b/src/main/java/org/apache/commons/collections4/map/StaticBucketMap.java
@@ -498,7 +498,7 @@ private static final class Lock {
         public int size;
     }
 
-    private class BaseIterator {
+    class BaseIterator {
         private final ArrayList<Map.Entry<K, V>> current = new ArrayList<>();
         private int bucket;
         private Map.Entry<K, V> last;
@@ -540,7 +540,7 @@ public void remove() {
         }
     }
 
-    private class EntryIterator extends BaseIterator implements Iterator<Map.Entry<K, V>> {
+    private final class EntryIterator extends BaseIterator implements Iterator<Map.Entry<K, V>> {
 
         @Override
         public Map.Entry<K, V> next() {
@@ -549,7 +549,7 @@ public Map.Entry<K, V> next() {
 
     }
 
-    private class ValueIterator extends BaseIterator implements Iterator<V> {
+    private final class ValueIterator extends BaseIterator implements Iterator<V> {
 
         @Override
         public V next() {
@@ -558,7 +558,7 @@ public V next() {
 
     }
 
-    private class KeyIterator extends BaseIterator implements Iterator<K> {
+    private final class KeyIterator extends BaseIterator implements Iterator<K> {
 
         @Override
         public K next() {
@@ -567,7 +567,7 @@ public K next() {
 
     }
 
-    private class EntrySet extends AbstractSet<Map.Entry<K, V>> {
+    private final class EntrySet extends AbstractSet<Map.Entry<K, V>> {
 
         @Override
         public int size() {
@@ -618,7 +618,7 @@ public boolean remove(final Object obj) {
 
     }
 
-    private class KeySet extends AbstractSet<K> {
+    private final class KeySet extends AbstractSet<K> {
 
         @Override
         public int size() {
@@ -658,7 +658,7 @@ public boolean remove(final Object obj) {
     }
 
 
-    private class Values extends AbstractCollection<V> {
+    private final class Values extends AbstractCollection<V> {
 
         @Override
         public int size() {
diff --git a/src/main/java/org/apache/commons/collections4/map/UnmodifiableEntrySet.java b/src/main/java/org/apache/commons/collections4/map/UnmodifiableEntrySet.java
index 89a52dd67f..57b434ce4a 100644
--- a/src/main/java/org/apache/commons/collections4/map/UnmodifiableEntrySet.java
+++ b/src/main/java/org/apache/commons/collections4/map/UnmodifiableEntrySet.java
@@ -155,7 +155,7 @@ public <T> T[] toArray(final T[] array) {
     /**
      * Implementation of an entry set iterator.
      */
-    private class UnmodifiableEntrySetIterator extends AbstractIteratorDecorator<Map.Entry<K, V>> {
+    private final class UnmodifiableEntrySetIterator extends AbstractIteratorDecorator<Map.Entry<K, V>> {
 
         protected UnmodifiableEntrySetIterator(final Iterator<Map.Entry<K, V>> iterator) {
             super(iterator);
@@ -175,7 +175,7 @@ public void remove() {
     /**
      * Implementation of a map entry that is unmodifiable.
      */
-    private class UnmodifiableEntry extends AbstractMapEntryDecorator<K, V> {
+    private final class UnmodifiableEntry extends AbstractMapEntryDecorator<K, V> {
 
         protected UnmodifiableEntry(final Map.Entry<K, V> entry) {
             super(entry);
diff --git a/src/main/java/org/apache/commons/collections4/multimap/AbstractListValuedMap.java b/src/main/java/org/apache/commons/collections4/multimap/AbstractListValuedMap.java
index 89855e17db..03c2f80103 100644
--- a/src/main/java/org/apache/commons/collections4/multimap/AbstractListValuedMap.java
+++ b/src/main/java/org/apache/commons/collections4/multimap/AbstractListValuedMap.java
@@ -103,7 +103,7 @@ public List<V> remove(final Object key) {
     /**
      * Wrapped list to handle add and remove on the list returned by get(object)
      */
-    private class WrappedList extends WrappedCollection implements List<V> {
+    private final class WrappedList extends WrappedCollection implements List<V> {
 
         WrappedList(final K key) {
             super(key);
@@ -210,7 +210,7 @@ public int hashCode() {
     }
 
     /** Values ListIterator */
-    private class ValuesListIterator implements ListIterator<V> {
+    private final class ValuesListIterator implements ListIterator<V> {
 
         private final K key;
         private List<V> values;
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 215e1ddcd9..3749808272 100644
--- a/src/main/java/org/apache/commons/collections4/multimap/AbstractMultiValuedMap.java
+++ b/src/main/java/org/apache/commons/collections4/multimap/AbstractMultiValuedMap.java
@@ -538,7 +538,7 @@ public String toString() {
     /**
      * Inner class that provides a MultiSet<K> keys view.
      */
-    private class KeysMultiSet extends AbstractMultiSet<K> {
+    private final class KeysMultiSet extends AbstractMultiSet<K> {
 
         @Override
         public boolean contains(final Object o) {
@@ -598,7 +598,7 @@ public int getCount() {
     /**
      * Inner class that provides the Entry<K, V> view
      */
-    private class EntryValues extends AbstractCollection<Entry<K, V>> {
+    private final class EntryValues extends AbstractCollection<Entry<K, V>> {
 
         @Override
         public Iterator<Entry<K, V>> iterator() {
@@ -629,7 +629,7 @@ public int size() {
     /**
      * Inner class for MultiValuedMap Entries.
      */
-    private class MultiValuedMapEntry extends AbstractMapEntry<K, V> {
+    private final class MultiValuedMapEntry extends AbstractMapEntry<K, V> {
 
         MultiValuedMapEntry(final K key, final V value) {
             super(key, value);
@@ -645,7 +645,7 @@ public V setValue(final V value) {
     /**
      * Inner class for MapIterator.
      */
-    private class MultiValuedMapIterator implements MapIterator<K, V> {
+    private final class MultiValuedMapIterator implements MapIterator<K, V> {
 
         private final Iterator<Entry<K, V>> it;
 
@@ -700,7 +700,7 @@ public V setValue(final V value) {
     /**
      * Inner class that provides the values view.
      */
-    private class Values extends AbstractCollection<V> {
+    private final class Values extends AbstractCollection<V> {
         @Override
         public Iterator<V> iterator() {
             final IteratorChain<V> chain = new IteratorChain<>();
@@ -724,7 +724,7 @@ public void clear() {
     /**
      * Inner class that provides the values iterator.
      */
-    private class ValuesIterator implements Iterator<V> {
+    private final class ValuesIterator implements Iterator<V> {
         private final Object key;
         private final Collection<V> values;
         private final Iterator<V> iterator;
@@ -757,7 +757,7 @@ public V next() {
     /**
      * Inner class that provides the AsMap view.
      */
-    private class AsMap extends AbstractMap<K, Collection<V>> {
+    private final class AsMap extends AbstractMap<K, Collection<V>> {
         final transient Map<K, Collection<V>> decoratedMap;
 
         AsMap(final Map<K, Collection<V>> map) {
diff --git a/src/main/java/org/apache/commons/collections4/multimap/AbstractSetValuedMap.java b/src/main/java/org/apache/commons/collections4/multimap/AbstractSetValuedMap.java
index 64745cedbe..c4a4dea106 100644
--- a/src/main/java/org/apache/commons/collections4/multimap/AbstractSetValuedMap.java
+++ b/src/main/java/org/apache/commons/collections4/multimap/AbstractSetValuedMap.java
@@ -103,7 +103,7 @@ public Set<V> remove(final Object key) {
      * Wrapped set to handle add and remove on the collection returned by
      * {@code get(Object)}.
      */
-    private class WrappedSet extends WrappedCollection implements Set<V> {
+    private final class WrappedSet extends WrappedCollection implements Set<V> {
 
         WrappedSet(final K key) {
             super(key);
diff --git a/src/main/java/org/apache/commons/collections4/trie/AbstractPatriciaTrie.java b/src/main/java/org/apache/commons/collections4/trie/AbstractPatriciaTrie.java
index 35ab7fb984..62ab39f4fb 100644
--- a/src/main/java/org/apache/commons/collections4/trie/AbstractPatriciaTrie.java
+++ b/src/main/java/org/apache/commons/collections4/trie/AbstractPatriciaTrie.java
@@ -1391,7 +1391,7 @@ public String toString() {
     /**
      * This is an entry set view of the {@link org.apache.commons.collections4.Trie} as returned by {@link Map#entrySet()}.
      */
-    private class EntrySet extends AbstractSet<Map.Entry<K, V>> {
+    private final class EntrySet extends AbstractSet<Map.Entry<K, V>> {
 
         @Override
         public Iterator<Map.Entry<K, V>> iterator() {
@@ -1434,7 +1434,7 @@ public void clear() {
         /**
          * An {@link Iterator} that returns {@link Entry} Objects.
          */
-        private class EntryIterator extends AbstractTrieIterator<Map.Entry<K, V>> {
+        private final class EntryIterator extends AbstractTrieIterator<Map.Entry<K, V>> {
             @Override
             public Map.Entry<K, V> next() {
                 return nextEntry();
@@ -1445,7 +1445,7 @@ public Map.Entry<K, V> next() {
     /**
      * This is a key set view of the {@link org.apache.commons.collections4.Trie} as returned by {@link Map#keySet()}.
      */
-    private class KeySet extends AbstractSet<K> {
+    private final class KeySet extends AbstractSet<K> {
 
         @Override
         public Iterator<K> iterator() {
@@ -1477,7 +1477,7 @@ public void clear() {
         /**
          * An {@link Iterator} that returns Key Objects.
          */
-        private class KeyIterator extends AbstractTrieIterator<K> {
+        private final class KeyIterator extends AbstractTrieIterator<K> {
             @Override
             public K next() {
                 return nextEntry().getKey();
@@ -1488,7 +1488,7 @@ public K next() {
     /**
      * This is a value view of the {@link org.apache.commons.collections4.Trie} as returned by {@link Map#values()}.
      */
-    private class Values extends AbstractCollection<V> {
+    private final class Values extends AbstractCollection<V> {
 
         @Override
         public Iterator<V> iterator() {
@@ -1525,7 +1525,7 @@ public boolean remove(final Object o) {
         /**
          * An {@link Iterator} that returns Value Objects.
          */
-        private class ValueIterator extends AbstractTrieIterator<V> {
+        private final class ValueIterator extends AbstractTrieIterator<V> {
             @Override
             public V next() {
                 return nextEntry().getValue();
@@ -1609,7 +1609,7 @@ public void remove() {
     /**
      * An {@link OrderedMapIterator} for a {@link org.apache.commons.collections4.Trie}.
      */
-    private class TrieMapIterator extends AbstractTrieIterator<K> implements OrderedMapIterator<K, V> {
+    private final class TrieMapIterator extends AbstractTrieIterator<K> implements OrderedMapIterator<K, V> {
 
         protected TrieEntry<K, V> previous; // the previous node to return
 
@@ -1846,7 +1846,7 @@ protected abstract SortedMap<K, V> createRangeMap(K fromKey, boolean fromInclusi
     /**
      * A {@link AbstractRangeMap} that deals with {@link Entry}s.
      */
-    private class RangeEntryMap extends AbstractRangeMap {
+    private final class RangeEntryMap extends AbstractRangeMap {
 
         /** The key to start from, null if the beginning. */
         private final K fromKey;
@@ -2088,7 +2088,7 @@ public Map.Entry<K, V> next() {
     /**
      * A submap used for prefix views over the {@link org.apache.commons.collections4.Trie}.
      */
-    private class PrefixRangeMap extends AbstractRangeMap {
+    private final class PrefixRangeMap extends AbstractRangeMap {
 
         private final K prefix;
 
diff --git a/src/test/java/org/apache/commons/collections4/bloomfilter/ArrayHasher.java b/src/test/java/org/apache/commons/collections4/bloomfilter/ArrayHasher.java
index 753decc6e2..348cf912aa 100644
--- a/src/test/java/org/apache/commons/collections4/bloomfilter/ArrayHasher.java
+++ b/src/test/java/org/apache/commons/collections4/bloomfilter/ArrayHasher.java
@@ -37,7 +37,7 @@ public IndexProducer indices(final Shape shape) {
         return new Producer(shape);
     }
 
-    private class Producer implements IndexProducer {
+    private final class Producer implements IndexProducer {
         Shape shape;
 
         Producer(final Shape shape) {
diff --git a/src/test/java/org/apache/commons/collections4/map/PredicatedSortedMapTest.java b/src/test/java/org/apache/commons/collections4/map/PredicatedSortedMapTest.java
index dfb6c95432..7ce0d4409c 100644
--- a/src/test/java/org/apache/commons/collections4/map/PredicatedSortedMapTest.java
+++ b/src/test/java/org/apache/commons/collections4/map/PredicatedSortedMapTest.java
@@ -43,7 +43,7 @@
  */
 public class PredicatedSortedMapTest<K, V> extends AbstractSortedMapTest<K, V> {
 
-    private class ReverseStringComparator implements Comparator<K> {
+    private final class ReverseStringComparator implements Comparator<K> {
         @Override
         public int compare(final K arg0, final K arg1) {
             return ((String) arg1).compareTo((String) arg0);