ConcurrentHashMap Put()操作示例代码

非常简练：

 private static void put(MetricKey key, float value) {
         MetricValue current;
         do {
             current = map.get(key);
         } while (current == null ? map.putIfAbsent(key, new MetricValue(value)) != null
                 : !map.replace(key, current, new MetricValue(current, value)));
 }