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)));
}