diff --git a/src/conf/checkstyle.xml b/src/conf/checkstyle.xml
index e0edda7eb0..eb410f1f70 100644
--- a/src/conf/checkstyle.xml
+++ b/src/conf/checkstyle.xml
@@ -42,6 +42,7 @@ limitations under the License.
     <property name="headerFile" value="${checkstyle.header.file}"/>
   </module>
   <module name="TreeWalker">
+    <module name="ExplicitInitializationCheck" />
     <module name="AvoidStarImport"/>
     <module name="IllegalImport"/>
     <module name="RedundantImport"/>
diff --git a/src/main/java/org/apache/commons/collections4/bloomfilter/CountingLongPredicate.java b/src/main/java/org/apache/commons/collections4/bloomfilter/CountingLongPredicate.java
index c03a21900b..83d4c1a8cb 100644
--- a/src/main/java/org/apache/commons/collections4/bloomfilter/CountingLongPredicate.java
+++ b/src/main/java/org/apache/commons/collections4/bloomfilter/CountingLongPredicate.java
@@ -25,7 +25,7 @@
  * execute the @code{text} with a zero value for each remaining @{code idx} value.
  */
 class CountingLongPredicate implements LongPredicate {
-    private int idx = 0;
+    private int idx;
     private final long[] ary;
     private final LongBiPredicate func;
 
diff --git a/src/main/java/org/apache/commons/collections4/bloomfilter/IndexProducer.java b/src/main/java/org/apache/commons/collections4/bloomfilter/IndexProducer.java
index dd73a423d1..7c284de379 100644
--- a/src/main/java/org/apache/commons/collections4/bloomfilter/IndexProducer.java
+++ b/src/main/java/org/apache/commons/collections4/bloomfilter/IndexProducer.java
@@ -82,7 +82,7 @@ static IndexProducer fromBitMapProducer(final BitMapProducer producer) {
         Objects.requireNonNull(producer, "producer");
         return consumer -> {
             final LongPredicate longPredicate = new LongPredicate() {
-                int wordIdx = 0;
+                int wordIdx;
 
                 @Override
                 public boolean test(long word) {
diff --git a/src/test/java/org/apache/commons/collections4/ClosureUtilsTest.java b/src/test/java/org/apache/commons/collections4/ClosureUtilsTest.java
index f34021c340..2b4af4d7e6 100644
--- a/src/test/java/org/apache/commons/collections4/ClosureUtilsTest.java
+++ b/src/test/java/org/apache/commons/collections4/ClosureUtilsTest.java
@@ -45,7 +45,7 @@ public class ClosureUtilsTest {
     private static final Object cString = "Hello";
 
     static class MockClosure<T> implements Closure<T> {
-        int count = 0;
+        int count;
 
         @Override
         public void execute(final T object) {
@@ -58,7 +58,7 @@ public void reset() {
     }
 
     static class MockTransformer<T> implements Transformer<T, T> {
-        int count = 0;
+        int count;
 
         @Override
         public T transform(final T object) {
diff --git a/src/test/java/org/apache/commons/collections4/CollectionUtilsTest.java b/src/test/java/org/apache/commons/collections4/CollectionUtilsTest.java
index f1e6a2f8e9..a562524168 100644
--- a/src/test/java/org/apache/commons/collections4/CollectionUtilsTest.java
+++ b/src/test/java/org/apache/commons/collections4/CollectionUtilsTest.java
@@ -69,54 +69,54 @@ public class CollectionUtilsTest extends MockTestCase {
     /**
      * Collection of {@link Integer}s
      */
-    private List<Integer> collectionA = null;
+    private List<Integer> collectionA;
 
     /**
      * Collection of {@link Long}s
      */
-    private List<Long> collectionB = null;
+    private List<Long> collectionB;
 
     /**
      * Collection of {@link Integer}s that are equivalent to the Longs in
      * collectionB.
      */
-    private Collection<Integer> collectionC = null;
+    private Collection<Integer> collectionC;
 
     /**
      * Sorted Collection of {@link Integer}s
      */
-    private Collection<Integer> collectionD = null;
+    private Collection<Integer> collectionD;
 
     /**
      * Sorted Collection of {@link Integer}s
      */
-    private Collection<Integer> collectionE = null;
+    private Collection<Integer> collectionE;
 
     /**
      * Collection of {@link Integer}s, bound as {@link Number}s
      */
-    private Collection<Number> collectionA2 = null;
+    private Collection<Number> collectionA2;
 
     /**
      * Collection of {@link Long}s, bound as {@link Number}s
      */
-    private Collection<Number> collectionB2 = null;
+    private Collection<Number> collectionB2;
 
     /**
      * Collection of {@link Integer}s (cast as {@link Number}s) that are
      * equivalent to the Longs in collectionB.
      */
-    private Collection<Number> collectionC2 = null;
+    private Collection<Number> collectionC2;
 
-    private Iterable<Integer> iterableA = null;
+    private Iterable<Integer> iterableA;
 
-    private Iterable<Long> iterableB = null;
+    private Iterable<Long> iterableB;
 
-    private Iterable<Integer> iterableC = null;
+    private Iterable<Integer> iterableC;
 
-    private Iterable<Number> iterableA2 = null;
+    private Iterable<Number> iterableA2;
 
-    private Iterable<Number> iterableB2 = null;
+    private Iterable<Number> iterableB2;
 
     private final Collection<Integer> emptyCollection = new ArrayList<>(1);
 
diff --git a/src/test/java/org/apache/commons/collections4/FactoryUtilsTest.java b/src/test/java/org/apache/commons/collections4/FactoryUtilsTest.java
index 014852b05a..3ed25c8c8e 100644
--- a/src/test/java/org/apache/commons/collections4/FactoryUtilsTest.java
+++ b/src/test/java/org/apache/commons/collections4/FactoryUtilsTest.java
@@ -168,7 +168,7 @@ public int hashCode() { // please Findbugs
     }
 
     public static class Mock3 {
-        private static int cCounter = 0;
+        private static int cCounter;
         private final int iVal;
         public Mock3() {
             iVal = cCounter++;
diff --git a/src/test/java/org/apache/commons/collections4/FluentIterableTest.java b/src/test/java/org/apache/commons/collections4/FluentIterableTest.java
index a405aa9ba2..5f4481218c 100644
--- a/src/test/java/org/apache/commons/collections4/FluentIterableTest.java
+++ b/src/test/java/org/apache/commons/collections4/FluentIterableTest.java
@@ -47,27 +47,27 @@ public class FluentIterableTest {
     /**
      * Iterable of {@link Integer}s
      */
-    private Iterable<Integer> iterableA = null;
+    private Iterable<Integer> iterableA;
 
     /**
      * Iterable of {@link Long}s
      */
-    private Iterable<Long> iterableB = null;
+    private Iterable<Long> iterableB;
 
     /**
      * Collection of even {@link Integer}s
      */
-    private Iterable<Integer> iterableEven = null;
+    private Iterable<Integer> iterableEven;
 
     /**
      * Collection of odd {@link Integer}s
      */
-    private Iterable<Integer> iterableOdd = null;
+    private Iterable<Integer> iterableOdd;
 
     /**
      * An empty Iterable.
      */
-    private Iterable<Integer> emptyIterable = null;
+    private Iterable<Integer> emptyIterable;
 
     @BeforeEach
     public void setUp() {
diff --git a/src/test/java/org/apache/commons/collections4/IterableUtilsTest.java b/src/test/java/org/apache/commons/collections4/IterableUtilsTest.java
index 4a844b36f1..028121ab44 100644
--- a/src/test/java/org/apache/commons/collections4/IterableUtilsTest.java
+++ b/src/test/java/org/apache/commons/collections4/IterableUtilsTest.java
@@ -50,17 +50,17 @@ public class IterableUtilsTest {
     /**
      * Iterable of {@link Integer}s
      */
-    private Iterable<Integer> iterableA = null;
+    private Iterable<Integer> iterableA;
 
     /**
      * Iterable of {@link Long}s
      */
-    private Iterable<Long> iterableB = null;
+    private Iterable<Long> iterableB;
 
     /**
      * An empty Iterable.
      */
-    private Iterable<Integer> emptyIterable = null;
+    private Iterable<Integer> emptyIterable;
 
     @BeforeEach
     public void setUp() {
diff --git a/src/test/java/org/apache/commons/collections4/IteratorUtilsTest.java b/src/test/java/org/apache/commons/collections4/IteratorUtilsTest.java
index fca963a1d0..aafbacaf12 100644
--- a/src/test/java/org/apache/commons/collections4/IteratorUtilsTest.java
+++ b/src/test/java/org/apache/commons/collections4/IteratorUtilsTest.java
@@ -70,21 +70,21 @@ public class IteratorUtilsTest {
     /**
      * Collection of {@link Integer}s
      */
-    private List<Integer> collectionA = null;
+    private List<Integer> collectionA;
 
     /**
      * Collection of even {@link Integer}s
      */
-    private List<Integer> collectionEven = null;
+    private List<Integer> collectionEven;
 
     /**
      * Collection of odd {@link Integer}s
      */
-    private List<Integer> collectionOdd = null;
+    private List<Integer> collectionOdd;
 
     private final Collection<Integer> emptyCollection = new ArrayList<>(1);
 
-    private Iterable<Integer> iterableA = null;
+    private Iterable<Integer> iterableA;
 
     /**
      * Creates a NodeList containing the specified nodes.
diff --git a/src/test/java/org/apache/commons/collections4/bidimap/AbstractOrderedBidiMapDecoratorTest.java b/src/test/java/org/apache/commons/collections4/bidimap/AbstractOrderedBidiMapDecoratorTest.java
index 48048b1342..644b42f0cb 100644
--- a/src/test/java/org/apache/commons/collections4/bidimap/AbstractOrderedBidiMapDecoratorTest.java
+++ b/src/test/java/org/apache/commons/collections4/bidimap/AbstractOrderedBidiMapDecoratorTest.java
@@ -67,7 +67,7 @@ public boolean isSetValueSupported() {
      */
     private static final class TestOrderedBidiMap<K, V> extends AbstractOrderedBidiMapDecorator<K, V> {
 
-        private TestOrderedBidiMap<V, K> inverse = null;
+        private TestOrderedBidiMap<V, K> inverse;
 
         TestOrderedBidiMap() {
             super(new DualTreeBidiMap<>());
diff --git a/src/test/java/org/apache/commons/collections4/iterators/CollatingIteratorTest.java b/src/test/java/org/apache/commons/collections4/iterators/CollatingIteratorTest.java
index a1a112830a..bfc30467e0 100644
--- a/src/test/java/org/apache/commons/collections4/iterators/CollatingIteratorTest.java
+++ b/src/test/java/org/apache/commons/collections4/iterators/CollatingIteratorTest.java
@@ -45,10 +45,10 @@ public CollatingIteratorTest() {
 
     //--------------------------------------------------------------- Lifecycle
 
-    private Comparator<Integer> comparator = null;
-    private ArrayList<Integer> evens = null;
-    private ArrayList<Integer> odds = null;
-    private ArrayList<Integer> fib = null;
+    private Comparator<Integer> comparator;
+    private ArrayList<Integer> evens;
+    private ArrayList<Integer> odds;
+    private ArrayList<Integer> fib;
 
     @BeforeEach
     public void setUp() throws Exception {
diff --git a/src/test/java/org/apache/commons/collections4/iterators/FilterListIteratorTest.java b/src/test/java/org/apache/commons/collections4/iterators/FilterListIteratorTest.java
index 9ac73be842..7181bf4b36 100644
--- a/src/test/java/org/apache/commons/collections4/iterators/FilterListIteratorTest.java
+++ b/src/test/java/org/apache/commons/collections4/iterators/FilterListIteratorTest.java
@@ -39,18 +39,18 @@
 @SuppressWarnings("boxing")
 public class FilterListIteratorTest {
 
-    private ArrayList<Integer> list = null;
-    private ArrayList<Integer> odds = null;
-    private ArrayList<Integer> evens = null;
-    private ArrayList<Integer> threes = null;
-    private ArrayList<Integer> fours = null;
-    private ArrayList<Integer> sixes = null;
-    private Predicate<Integer> truePred = null;
-    private Predicate<Integer> falsePred = null;
-    private Predicate<Integer> evenPred = null;
-    private Predicate<Integer> oddPred = null;
-    private Predicate<Integer> threePred = null;
-    private Predicate<Integer> fourPred = null;
+    private ArrayList<Integer> list;
+    private ArrayList<Integer> odds;
+    private ArrayList<Integer> evens;
+    private ArrayList<Integer> threes;
+    private ArrayList<Integer> fours;
+    private ArrayList<Integer> sixes;
+    private Predicate<Integer> truePred;
+    private Predicate<Integer> falsePred;
+    private Predicate<Integer> evenPred;
+    private Predicate<Integer> oddPred;
+    private Predicate<Integer> threePred;
+    private Predicate<Integer> fourPred;
     private final Random random = new Random();
 
     @BeforeEach
diff --git a/src/test/java/org/apache/commons/collections4/iterators/IteratorChainTest.java b/src/test/java/org/apache/commons/collections4/iterators/IteratorChainTest.java
index 3f1e42a85c..88e33542bb 100644
--- a/src/test/java/org/apache/commons/collections4/iterators/IteratorChainTest.java
+++ b/src/test/java/org/apache/commons/collections4/iterators/IteratorChainTest.java
@@ -41,9 +41,9 @@ public class IteratorChainTest extends AbstractIteratorTest<String> {
         "One", "Two", "Three", "Four", "Five", "Six"
     };
 
-    protected List<String> list1 = null;
-    protected List<String> list2 = null;
-    protected List<String> list3 = null;
+    protected List<String> list1;
+    protected List<String> list2;
+    protected List<String> list3;
 
     public IteratorChainTest() {
         super(IteratorChainTest.class.getSimpleName());
diff --git a/src/test/java/org/apache/commons/collections4/iterators/LazyIteratorChainTest.java b/src/test/java/org/apache/commons/collections4/iterators/LazyIteratorChainTest.java
index ff1a8e454a..1f97d27f08 100644
--- a/src/test/java/org/apache/commons/collections4/iterators/LazyIteratorChainTest.java
+++ b/src/test/java/org/apache/commons/collections4/iterators/LazyIteratorChainTest.java
@@ -41,9 +41,9 @@ public class LazyIteratorChainTest extends AbstractIteratorTest<String> {
         "One", "Two", "Three", "Four", "Five", "Six"
     };
 
-    protected List<String> list1 = null;
-    protected List<String> list2 = null;
-    protected List<String> list3 = null;
+    protected List<String> list1;
+    protected List<String> list2;
+    protected List<String> list3;
 
     public LazyIteratorChainTest() {
         super(LazyIteratorChainTest.class.getSimpleName());
diff --git a/src/test/java/org/apache/commons/collections4/iterators/ListIteratorWrapper2Test.java b/src/test/java/org/apache/commons/collections4/iterators/ListIteratorWrapper2Test.java
index c931246452..6fd8f115c8 100644
--- a/src/test/java/org/apache/commons/collections4/iterators/ListIteratorWrapper2Test.java
+++ b/src/test/java/org/apache/commons/collections4/iterators/ListIteratorWrapper2Test.java
@@ -38,7 +38,7 @@ public class ListIteratorWrapper2Test<E> extends AbstractIteratorTest<E> {
         "One", "Two", "Three", "Four", "Five", "Six"
     };
 
-    protected List<E> list1 = null;
+    protected List<E> list1;
 
     public ListIteratorWrapper2Test() {
         super(ListIteratorWrapper2Test.class.getSimpleName());
diff --git a/src/test/java/org/apache/commons/collections4/iterators/ListIteratorWrapperTest.java b/src/test/java/org/apache/commons/collections4/iterators/ListIteratorWrapperTest.java
index 97c5a589eb..2d41458fed 100644
--- a/src/test/java/org/apache/commons/collections4/iterators/ListIteratorWrapperTest.java
+++ b/src/test/java/org/apache/commons/collections4/iterators/ListIteratorWrapperTest.java
@@ -39,7 +39,7 @@ public class ListIteratorWrapperTest<E> extends AbstractIteratorTest<E> {
         "One", "Two", "Three", "Four", "Five", "Six"
     };
 
-    protected List<E> list1 = null;
+    protected List<E> list1;
 
     public ListIteratorWrapperTest() {
         super(ListIteratorWrapperTest.class.getSimpleName());
diff --git a/src/test/java/org/apache/commons/collections4/iterators/ObjectGraphIteratorTest.java b/src/test/java/org/apache/commons/collections4/iterators/ObjectGraphIteratorTest.java
index b01021b96f..7befde8a58 100644
--- a/src/test/java/org/apache/commons/collections4/iterators/ObjectGraphIteratorTest.java
+++ b/src/test/java/org/apache/commons/collections4/iterators/ObjectGraphIteratorTest.java
@@ -39,10 +39,10 @@ public class ObjectGraphIteratorTest extends AbstractIteratorTest<Object> {
 
     protected String[] testArray = { "One", "Two", "Three", "Four", "Five", "Six" };
 
-    protected List<String> list1 = null;
-    protected List<String> list2 = null;
-    protected List<String> list3 = null;
-    protected List<Iterator<String>> iteratorList = null;
+    protected List<String> list1;
+    protected List<String> list2;
+    protected List<String> list3;
+    protected List<Iterator<String>> iteratorList;
 
     public ObjectGraphIteratorTest() {
         super(ObjectGraphIteratorTest.class.getSimpleName());
diff --git a/src/test/java/org/apache/commons/collections4/iterators/UniqueFilterIteratorTest.java b/src/test/java/org/apache/commons/collections4/iterators/UniqueFilterIteratorTest.java
index a5e2180ae3..9ae28ca38b 100644
--- a/src/test/java/org/apache/commons/collections4/iterators/UniqueFilterIteratorTest.java
+++ b/src/test/java/org/apache/commons/collections4/iterators/UniqueFilterIteratorTest.java
@@ -36,7 +36,7 @@ public class UniqueFilterIteratorTest<E> extends AbstractIteratorTest<E> {
         "One", "Two", "Three", "Four", "Five", "Six"
     };
 
-    protected List<E> list1 = null;
+    protected List<E> list1;
 
     public UniqueFilterIteratorTest() {
         super(UniqueFilterIteratorTest.class.getSimpleName());
diff --git a/src/test/java/org/apache/commons/collections4/iterators/ZippingIteratorTest.java b/src/test/java/org/apache/commons/collections4/iterators/ZippingIteratorTest.java
index 899be2bb8c..4c7e2e34be 100644
--- a/src/test/java/org/apache/commons/collections4/iterators/ZippingIteratorTest.java
+++ b/src/test/java/org/apache/commons/collections4/iterators/ZippingIteratorTest.java
@@ -40,9 +40,9 @@ public ZippingIteratorTest() {
 
     //--------------------------------------------------------------- Lifecycle
 
-    private ArrayList<Integer> evens = null;
-    private ArrayList<Integer> odds = null;
-    private ArrayList<Integer> fib = null;
+    private ArrayList<Integer> evens;
+    private ArrayList<Integer> odds;
+    private ArrayList<Integer> fib;
 
     @BeforeEach
     public void setUp() throws Exception {
diff --git a/src/test/java/org/apache/commons/collections4/set/UnmodifiableNavigableSetTest.java b/src/test/java/org/apache/commons/collections4/set/UnmodifiableNavigableSetTest.java
index 2b5c7812a9..c628025920 100644
--- a/src/test/java/org/apache/commons/collections4/set/UnmodifiableNavigableSetTest.java
+++ b/src/test/java/org/apache/commons/collections4/set/UnmodifiableNavigableSetTest.java
@@ -36,8 +36,8 @@
  * @since 4.1
  */
 public class UnmodifiableNavigableSetTest<E> extends AbstractNavigableSetTest<E> {
-    protected UnmodifiableNavigableSet<E> set = null;
-    protected ArrayList<E> array = null;
+    protected UnmodifiableNavigableSet<E> set;
+    protected ArrayList<E> array;
 
     public UnmodifiableNavigableSetTest() {
         super(UnmodifiableNavigableSetTest.class.getSimpleName());
diff --git a/src/test/java/org/apache/commons/collections4/set/UnmodifiableSortedSetTest.java b/src/test/java/org/apache/commons/collections4/set/UnmodifiableSortedSetTest.java
index 258e210977..574cb4292d 100644
--- a/src/test/java/org/apache/commons/collections4/set/UnmodifiableSortedSetTest.java
+++ b/src/test/java/org/apache/commons/collections4/set/UnmodifiableSortedSetTest.java
@@ -37,8 +37,8 @@
  * @since 3.0
  */
 public class UnmodifiableSortedSetTest<E> extends AbstractSortedSetTest<E> {
-    protected UnmodifiableSortedSet<E> set = null;
-    protected ArrayList<E> array = null;
+    protected UnmodifiableSortedSet<E> set;
+    protected ArrayList<E> array;
 
     public UnmodifiableSortedSetTest() {
         super(UnmodifiableSortedSetTest.class.getSimpleName());