Fix license and checkstyle

Author: szetszwo

ratis-common/src/main/java/org/apache/ratis/util/BiWeakValueCache.java

@@ -1,3 +1,20 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 org.apache.ratis.util;
 
 import org.apache.ratis.thirdparty.com.google.common.collect.MapMaker;

ratis-server-api/src/main/java/org/apache/ratis/server/protocol/TermIndex.java

@@ -81,7 +81,9 @@ static TermIndex valueOf(long term, long index) {
    * An implementation for private use.
    * Note that this is not a public API, although this is public class.
    */
-  class Impl {
+  final class Impl {
+    private Impl() { }
+
     private static final BiWeakValueCache<Long, Long, TermIndex> CACHE
         = new BiWeakValueCache<>("term", "index", Impl::newTermIndex);