Don't initialize AtomicReference to its default value.

httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/DefaultAsyncCacheRevalidator.java

@@ -120,7 +120,7 @@ class DefaultAsyncCacheRevalidator extends CacheRevalidatorBase {
             final RevalidationCall call) {
         scheduleRevalidation(cacheKey, () -> call.execute(new AsyncExecCallback() {
 
-            private final AtomicReference<HttpResponse> responseRef = new AtomicReference<>(null);
+            private final AtomicReference<HttpResponse> responseRef = new AtomicReference<>();
 
             @Override
             public AsyncDataConsumer handleResponse(

httpclient5-testing/src/main/java/org/apache/hc/client5/testing/async/AsyncEchoHandler.java

@@ -61,7 +61,7 @@ public class AsyncEchoHandler implements AsyncServerExchangeHandler {
 
     public AsyncEchoHandler() {
         this.entityConsumer = new BasicAsyncEntityConsumer();
-        this.entityProducerRef = new AtomicReference<>(null);
+        this.entityProducerRef = new AtomicReference<>();
     }
 
     @Override

httpclient5-testing/src/main/java/org/apache/hc/client5/testing/async/AsyncRandomHandler.java

@@ -63,7 +63,7 @@ public class AsyncRandomHandler implements AsyncServerExchangeHandler {
     private final AtomicReference<AsyncEntityProducer> entityProducerRef;
 
     public AsyncRandomHandler() {
-        this.entityProducerRef = new AtomicReference<>(null);
+        this.entityProducerRef = new AtomicReference<>();
     }
 
     @Override

httpclient5-testing/src/main/java/org/apache/hc/client5/testing/async/AuthenticatingAsyncDecorator.java

@@ -65,7 +65,7 @@ public class AuthenticatingAsyncDecorator implements AsyncServerExchangeHandler
     public AuthenticatingAsyncDecorator(final AsyncServerExchangeHandler exchangeHandler, final Authenticator authenticator) {
         this.exchangeHandler = Args.notNull(exchangeHandler, "Request handler");
         this.authenticator = Args.notNull(authenticator, "Authenticator");
-        this.responseProducerRef = new AtomicReference<>(null);
+        this.responseProducerRef = new AtomicReference<>();
         this.authTokenExtractor = new BasicAuthTokenExtractor();
     }
 

httpclient5-testing/src/test/java/org/apache/hc/client5/testing/async/AbstractHttpReactiveFundamentalsTest.java

@@ -299,7 +299,7 @@ public abstract class AbstractHttpReactiveFundamentalsTest<T extends CloseableHt
             while (testCases.size() < numTestCases) {
                 final long seed = 198723L * testCaseNum++;
                 final int length = 1 + new Random(seed).nextInt(maxSize);
-                final AtomicReference<String> expectedHash = new AtomicReference<>(null);
+                final AtomicReference<String> expectedHash = new AtomicReference<>();
                 final Flowable<ByteBuffer> stream = ReactiveTestUtils.produceStream(length, expectedHash);
                 final StreamingTestCase streamingTestCase = new StreamingTestCase(length, expectedHash, stream);
                 testCases.put((long) length, streamingTestCase);

httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/H2AsyncMainClientExec.java

@@ -88,7 +88,7 @@ public class H2AsyncMainClientExec implements AsyncExecChainHandler {
 
         final AsyncClientExchangeHandler internalExchangeHandler = new AsyncClientExchangeHandler() {
 
-            private final AtomicReference<AsyncDataConsumer> entityConsumerRef = new AtomicReference<>(null);
+            private final AtomicReference<AsyncDataConsumer> entityConsumerRef = new AtomicReference<>();
 
             @Override
             public void releaseResources() {

httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/HttpAsyncMainClientExec.java

@@ -104,7 +104,7 @@ class HttpAsyncMainClientExec implements AsyncExecChainHandler {
         final AtomicInteger messageCountDown = new AtomicInteger(2);
         final AsyncClientExchangeHandler internalExchangeHandler = new AsyncClientExchangeHandler() {
 
-            private final AtomicReference<AsyncDataConsumer> entityConsumerRef = new AtomicReference<>(null);
+            private final AtomicReference<AsyncDataConsumer> entityConsumerRef = new AtomicReference<>();
 
             @Override
             public void releaseResources() {

httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/InternalH2AsyncExecRuntime.java

@@ -68,7 +68,7 @@ class InternalH2AsyncExecRuntime implements AsyncExecRuntime {
         this.log = log;
         this.connPool = connPool;
         this.pushHandlerFactory = pushHandlerFactory;
-        this.sessionRef = new AtomicReference<>(null);
+        this.sessionRef = new AtomicReference<>();
     }
 
     @Override

httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/InternalHttpAsyncExecRuntime.java

@@ -75,7 +75,7 @@ class InternalHttpAsyncExecRuntime implements AsyncExecRuntime {
         this.connectionInitiator = connectionInitiator;
         this.pushHandlerFactory = pushHandlerFactory;
         this.versionPolicy = versionPolicy;
-        this.endpointRef = new AtomicReference<>(null);
+        this.endpointRef = new AtomicReference<>();
         this.validDuration = TimeValue.NEG_ONE_MILLISECOND;
     }
 

httpclient5/src/main/java/org/apache/hc/client5/http/impl/classic/InternalExecRuntime.java

@@ -76,7 +76,7 @@ class InternalExecRuntime implements ExecRuntime, Cancellable {
         this.manager = manager;
         this.requestExecutor = requestExecutor;
         this.cancellableDependency = cancellableDependency;
-        this.endpointRef = new AtomicReference<>(null);
+        this.endpointRef = new AtomicReference<>();
         this.validDuration = TimeValue.NEG_ONE_MILLISECOND;
     }