public V get(Object key) {
        int hash = hash(key.hashCode());
        return segmentFor(hash).get(key, hash);
    }
Returns the value to which the specified key is mapped,
or {@code null} if this map contains no mapping for the key.

<p>More formally, if this map contains a mapping from a key
{@code k} to a value {@code v} such that {@code key.equals(k)},
then this method returns {@code v}; otherwise it returns
{@code null}.  (There can be at most one such mapping.)

@throws NullPointerException if the specified key is null
commons-profiler/src/main/java/com/navercorp/pinpoint/common/profiler/concurrent/jsr166/ConcurrentWeakHashMap.java