public V remove(Object key) {
        Entry<K, V> e = removeEntryForKey(key);
        return (e == null ? null : e.value);
    }
Removes the mapping for the specified key from this map if present.

@param key
           key whose mapping is to be removed from the map
@return the previous value associated with <tt>key</tt>, or <tt>null</tt>
        if there was no mapping for <tt>key</tt>. (A <tt>null</tt> return
        can also indicate that the map previously associated
        <tt>null</tt> with <tt>key</tt>.)
src/main/java/com/alibaba/fastjson/util/AntiCollisionHashMap.java