HADOOP-19377: Avoid initializing useless HashMap in protocolImplMapArray.

Closes #6927

Signed-off-by: Chris Nauroth <cnauroth@apache.org>

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/RPC.java

@@ -90,7 +90,7 @@ public class RPC {
     RPC_BUILTIN ((short) 1),         // Used for built in calls by tests
     RPC_WRITABLE ((short) 2),        // Use WritableRpcEngine 
     RPC_PROTOCOL_BUFFER ((short) 3); // Use ProtobufRpcEngine
-    final static short MAX_INDEX = RPC_PROTOCOL_BUFFER.value; // used for array size
+    final static short MAX_SIZE = RPC_PROTOCOL_BUFFER.value; // used for array size
     private final short value;
 
     RpcKind(short val) {
@@ -1085,11 +1085,12 @@ public class RPC {
    }
 
    ArrayList<Map<ProtoNameVer, ProtoClassProtoImpl>> protocolImplMapArray = 
-       new ArrayList<Map<ProtoNameVer, ProtoClassProtoImpl>>(RpcKind.MAX_INDEX);
-   
+       new ArrayList<Map<ProtoNameVer, ProtoClassProtoImpl>>(RpcKind.MAX_SIZE);
+
+   @SuppressWarnings("checkstyle:Indentation")
    Map<ProtoNameVer, ProtoClassProtoImpl> getProtocolImplMap(RPC.RpcKind rpcKind) {
      if (protocolImplMapArray.size() == 0) {// initialize for all rpc kinds
-       for (int i=0; i <= RpcKind.MAX_INDEX; ++i) {
+       for (int i = 0; i < RpcKind.MAX_SIZE; ++i) {
          protocolImplMapArray.add(
              new HashMap<ProtoNameVer, ProtoClassProtoImpl>(10));
        }