[apache/helix] -- Updated Java 9 functions with Java-8 complaint func…

…tions (#2689)

We will releasing the Open source Helix on Java 11, but we are updating the codebase to be build compatible with Java-8, so that we can internally release it on Java-8.

helix-core/src/main/java/org/apache/helix/model/ResourceConfig.java

@@ -28,6 +28,7 @@
 
 import com.fasterxml.jackson.core.type.TypeReference;
 import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.common.collect.ImmutableMap;
 import org.apache.helix.HelixProperty;
 import org.apache.helix.api.config.HelixConfigProperty;
 import org.apache.helix.api.config.RebalanceConfig;
@@ -448,7 +449,7 @@ public void setPartitionCapacityMap(Map<String, Map<String, Integer>> partitionC
             String.format("Capacity Data contains a negative value:%s", capacities.toString()));
       }
       newCapacityRecord.put(partition, _objectMapper.writeValueAsString(capacities));
-      newDeserializedPartitionCapacityMap.put(partition, Map.copyOf(capacities));
+      newDeserializedPartitionCapacityMap.put(partition, ImmutableMap.copyOf(capacities));
     }
 
     _record.setMapField(ResourceConfigProperty.PARTITION_CAPACITY_MAP.name(), newCapacityRecord);

meta-client/src/main/java/org/apache/helix/metaclient/impl/zk/ZkMetaClientCache.java

@@ -19,6 +19,7 @@
  * under the License.
  */
 
+import com.google.common.collect.ImmutableList;
 import org.apache.helix.metaclient.api.ChildChangeListener;
 import org.apache.helix.metaclient.api.MetaClientCacheInterface;
 import org.apache.helix.metaclient.exception.MetaClientException;
@@ -118,7 +119,7 @@ public List<String> getDirectChildrenKeys(final String key) {
                 LOG.debug("Children not found in cache for key: {}. This could be because the cache is still being populated.", key);
                 return null;
             }
-            return List.copyOf(node.getChildren().keySet());
+            return ImmutableList.copyOf(node.getChildren().keySet());
         }
         return super.getDirectChildrenKeys(key);
     }