public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = 0;
        if (key instanceof String)
            hash = hash(hashString((String) key));
        else
            hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        for (Entry<K, V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }
Associates the specified value with the specified key in this map. If the
map previously contained a mapping for the key, the old value is
replaced.

@param key
           key with which the specified value is to be associated
@param value
           value to be associated with the specified key
@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