HashMap与Hashtable的区别

//使用了同步机制，线程安全
public synchronized V put(K key, V value) {
        // Make sure the value is not null
        if (value == null) {//value不允许为null
            throw new NullPointerException();
        }

        // Makes sure the key is not already in the hashtable.
        Entry tab[] = table;
        int hash = key.hashCode();//key不能为null
        int index = (hash & 0x7FFFFFFF) % tab.length;
        @SuppressWarnings("unchecked")
        Entry entry = (Entry)tab[index];
        for(; entry != null ; entry = entry.next) {
            if ((entry.hash == hash) && entry.key.equals(key)) {
                V old = entry.value;
                entry.value = value;
                return old;
            }
        }

        addEntry(hash, key, value, index);
        return null;
    }