Use empty column definitions for insert responses

core/src/main/java/com/datastax/oss/driver/internal/core/cql/Conversions.java

@@ -237,13 +237,13 @@ static DefaultPreparedStatement toPreparedStatement(
         ImmutableMap.copyOf(request.getCustomPayload()));
   }
 
-  private static DefaultColumnDefinitions toColumnDefinitions(
+  private static ColumnDefinitions toColumnDefinitions(
       RowsMetadata metadata, InternalDriverContext context) {
     ImmutableList.Builder<ColumnDefinition> definitions = ImmutableList.builder();
     for (ColumnSpec columnSpec : metadata.columnSpecs) {
       definitions.add(new DefaultColumnDefinition(columnSpec, context));
     }
-    return new DefaultColumnDefinitions(definitions.build());
+    return DefaultColumnDefinitions.valueOf(definitions.build());
   }
 
   static Throwable toThrowable(Node node, Error errorMessage) {

core/src/main/java/com/datastax/oss/driver/internal/core/cql/DefaultAsyncResultSet.java

@@ -120,7 +120,7 @@ static AsyncResultSet empty(final ExecutionInfo executionInfo) {
     return new AsyncResultSet() {
       @Override
       public ColumnDefinitions getColumnDefinitions() {
-        return DefaultColumnDefinitions.EMPTY;
+        return EmptyColumnDefinitions.INSTANCE;
       }
 
       @Override

core/src/main/java/com/datastax/oss/driver/internal/core/cql/DefaultColumnDefinitions.java

@@ -24,16 +24,21 @@
 import java.io.ObjectInputStream;
 import java.io.Serializable;
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 
 public class DefaultColumnDefinitions implements ColumnDefinitions {
 
+  static ColumnDefinitions valueOf(List<ColumnDefinition> definitions) {
+    return definitions.isEmpty()
+        ? EmptyColumnDefinitions.INSTANCE
+        : new DefaultColumnDefinitions(definitions);
+  }
+
   private final List<ColumnDefinition> definitions;
   private final IdentifierIndex index;
 
-  public DefaultColumnDefinitions(List<ColumnDefinition> definitions) {
+  private DefaultColumnDefinitions(List<ColumnDefinition> definitions) {
     assert definitions != null && definitions.size() > 0;
     this.definitions = definitions;
     this.index = buildIndex(definitions);
@@ -120,50 +125,4 @@ private Object readResolve() {
       return new DefaultColumnDefinitions(this.definitions);
     }
   }
-
-  static final ColumnDefinitions EMPTY =
-      new ColumnDefinitions() {
-        @Override
-        public int size() {
-          return 0;
-        }
-
-        @Override
-        public ColumnDefinition get(int i) {
-          throw new ArrayIndexOutOfBoundsException();
-        }
-
-        @Override
-        public boolean contains(String name) {
-          return false;
-        }
-
-        @Override
-        public boolean contains(CqlIdentifier id) {
-          return false;
-        }
-
-        @Override
-        public int firstIndexOf(String name) {
-          return -1;
-        }
-
-        @Override
-        public int firstIndexOf(CqlIdentifier id) {
-          return -1;
-        }
-
-        @Override
-        public boolean isDetached() {
-          return false;
-        }
-
-        @Override
-        public void attach(AttachmentPoint attachmentPoint) {}
-
-        @Override
-        public Iterator<ColumnDefinition> iterator() {
-          return Collections.<ColumnDefinition>emptyList().iterator();
-        }
-      };
 }

core/src/main/java/com/datastax/oss/driver/internal/core/cql/EmptyColumnDefinitions.java

@@ -0,0 +1,74 @@
+/*
+ * Copyright (C) 2017-2017 DataStax Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.datastax.oss.driver.internal.core.cql;
+
+import com.datastax.oss.driver.api.core.CqlIdentifier;
+import com.datastax.oss.driver.api.core.cql.ColumnDefinition;
+import com.datastax.oss.driver.api.core.cql.ColumnDefinitions;
+import com.datastax.oss.driver.api.core.detach.AttachmentPoint;
+import java.util.Collections;
+import java.util.Iterator;
+
+/**
+ * The singleton that represents no column definitions (implemented as an enum which provides the
+ * serialization machinery for free).
+ */
+public enum EmptyColumnDefinitions implements ColumnDefinitions {
+  INSTANCE;
+
+  @Override
+  public int size() {
+    return 0;
+  }
+
+  @Override
+  public ColumnDefinition get(int i) {
+    throw new ArrayIndexOutOfBoundsException();
+  }
+
+  @Override
+  public boolean contains(String name) {
+    return false;
+  }
+
+  @Override
+  public boolean contains(CqlIdentifier id) {
+    return false;
+  }
+
+  @Override
+  public int firstIndexOf(String name) {
+    return -1;
+  }
+
+  @Override
+  public int firstIndexOf(CqlIdentifier id) {
+    return -1;
+  }
+
+  @Override
+  public boolean isDetached() {
+    return false;
+  }
+
+  @Override
+  public void attach(AttachmentPoint attachmentPoint) {}
+
+  @Override
+  public Iterator<ColumnDefinition> iterator() {
+    return Collections.<ColumnDefinition>emptyList().iterator();
+  }
+}