Tidy up some collections code

Author: joegallo

modules/ingest-geoip/qa/geoip-reindexed/src/javaRestTest/java/org/elasticsearch/ingest/geoip/GeoIpReindexedIT.java

@@ -18,6 +18,7 @@
 import org.elasticsearch.client.Response;
 import org.elasticsearch.client.WarningsHandler;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.util.CollectionUtils;
 import org.elasticsearch.common.util.concurrent.ThreadContext;
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.rest.RestStatus;
@@ -34,7 +35,6 @@
 
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
-import java.util.ArrayList;
 import java.util.Base64;
 import java.util.HashSet;
 import java.util.List;
@@ -212,9 +212,8 @@ private void testCatIndices(List<String> indexNames, @Nullable List<String> addi
         String response = EntityUtils.toString(assertOK(client().performRequest(catIndices)).getEntity());
         List<String> indices = List.of(response.trim().split("\\s+"));
 
-        if (additionalIndexNames != null && additionalIndexNames.isEmpty() == false) {
-            indexNames = new ArrayList<>(indexNames); // recopy into a mutable list
-            indexNames.addAll(additionalIndexNames);
+        if (additionalIndexNames != null) {
+            indexNames = CollectionUtils.concatLists(indexNames, additionalIndexNames);
         }
 
         assertThat(new HashSet<>(indices), is(new HashSet<>(indexNames)));
@@ -240,9 +239,8 @@ private void testGetStar(List<String> indexNames, @Nullable List<String> additio
         );
         Response response = assertOK(client().performRequest(getStar));
 
-        if (additionalIndexNames != null && additionalIndexNames.isEmpty() == false) {
-            indexNames = new ArrayList<>(indexNames); // recopy into a mutable list
-            indexNames.addAll(additionalIndexNames);
+        if (additionalIndexNames != null) {
+            indexNames = CollectionUtils.concatLists(indexNames, additionalIndexNames);
         }
 
         Map<String, Object> map = responseAsMap(response);
@@ -258,9 +256,8 @@ private void testGetStarAsKibana(List<String> indexNames, @Nullable List<String>
         );
         Response response = assertOK(client().performRequest(getStar));
 
-        if (additionalIndexNames != null && additionalIndexNames.isEmpty() == false) {
-            indexNames = new ArrayList<>(indexNames); // recopy into a mutable list
-            indexNames.addAll(additionalIndexNames);
+        if (additionalIndexNames != null) {
+            indexNames = CollectionUtils.concatLists(indexNames, additionalIndexNames);
         }
 
         Map<String, Object> map = responseAsMap(response);